INORGANIC CHEMISTRY.
Inorganic chemistry is the study of the synthesis and behavior of inorganic and organometallic compounds. This field covers all chemical compounds except the myriad organic compounds (carbon based compounds, usually containing C-H bonds), which are the subjects of organic chemistry.
The distinction between the two disciplines is far from absolute, and
there is much overlap, most importantly in the sub-discipline of organometallic chemistry.
It has applications in every aspect of the chemical industry–including
catalysis, materials science, pigments, surfactants, coatings, medicine,
fuel, and agriculture. [1]
Key concepts
Many inorganic compounds are ionic compounds, consisting of cations and anions joined by ionic bonding. Examples of salts (which are ionic compounds) are magnesium chloride MgCl2, which consists of magnesium cations Mg2+ and chloride anions Cl−; or sodium oxide Na2O, which consists of sodium cations Na+ and oxide anions O2−.
In any salt, the proportions of the ions are such that the electric
charges cancel out, so that the bulk compound is electrically neutral.
The ions are described by their oxidation state and their ease of formation can be inferred from the ionization potential (for cations) or from the electron affinity (anions) of the parent elements.
Important classes of inorganic salts are the oxides, the carbonates, the sulfates and the halides. Many inorganic compounds are characterized by high melting points. Inorganic salts typically are poor conductors in the solid state. Other important features include their solubility in water(see: solubility chart) and ease of crystallization. Where some salts (e.g., NaCl) are very soluble in water, others (e.g., SiO2) are not.
The simplest inorganic reaction is double displacement when in mixing of two salts the ions are swapped without a change in oxidation state. In redox reactions one reactant, the oxidant, lowers its oxidation state and another reactant, the reductant, has its oxidation state increased. The net result is an exchange of electrons. Electron exchange can occur indirectly as well, e.g., in batteries, a key concept in electrochemistry.
When one reactant contains hydrogen atoms, a reaction can take place by exchanging protons in acid-base chemistry. In a more general definition, an acid can be any chemical species capable of binding to electron pairs is called a Lewis acid; conversely any molecule that tends to donate an electron pair is referred to as a Lewis base. As a refinement of acid-base interactions, the HSAB theory takes into account polarizability and size of ions.
Inorganic compounds are found in nature as minerals. Soil may contain iron sulfide as pyrite or calcium sulfate as gypsum. Inorganic compounds are also found multitasking as biomolecules: as electrolytes (sodium chloride), in energy storage (ATP) or in construction (the polyphosphate backbone in DNA).
The first important man-made inorganic compound was ammonium nitrate for soil fertilization through the Haber process. Inorganic compounds are synthesized for use as catalysts such as vanadium(V) oxide and titanium(III) chloride, or as reagents in organic chemistry such as lithium aluminium hydride.
Subdivisions of inorganic chemistry are organometallic chemistry, cluster chemistry and bioinorganic chemistry. These fields are active areas of research in inorganic chemistry, aimed toward new catalysts, superconductors, and therapies.
Industrial inorganic chemistry
Inorganic chemistry is a highly practical area of science.
Traditionally, the scale of a nation's economy could be evaluated by
their productivity of sulfuric acid. The top 20 inorganic chemicals
manufactured in Canada, China, Europe, India, Japan, and the US (2005
data):[2] aluminium sulfate, ammonia, ammonium nitrate, ammonium sulfate, carbon black, chlorine, hydrochloric acid, hydrogen, hydrogen peroxide, nitric acid, nitrogen, oxygen, phosphoric acid, sodium carbonate, sodium chlorate, sodium hydroxide, sodium silicate, sodium sulfate, sulfuric acid, and titanium dioxide.
The manufacturing of fertilizers is another practical application of industrial inorganic chemistry.
Descriptive inorganic chemistry
Descriptive inorganic chemistry focuses on the classification of
compounds based on their properties. Partly the classification focuses
on the position in the periodic table of the heaviest element (the
element with the highest atomic weight) in the compound, partly by
grouping compounds by their structural similarities. When studying
inorganic compounds, one often encounters parts of the different classes
of inorganic chemistry (an organometallic compound is characterized by
its coordination chemistry, and may show interesting solid state
properties).
Different classifications are:
Coordination compounds
Classical coordination compounds feature metals bound to "lone pairs" of electrons residing on the main group atoms of ligands such as H2O, NH3, Cl−, and CN−.
In modern coordination compounds almost all organic and inorganic
compounds can be used as ligands. The "metal" usually is a metal from
the groups 3-13, as well as the trans-lanthanides and trans-actinides, but from a certain perspective, all chemical compounds can be described as coordination complexes.
The stereochemistry of coordination complexes can be quite rich, as hinted at by Werner's separation of two enantiomers of [Co((OH)2Co(NH3)4)3]6+,
an early demonstration that chirality is not inherent to organic
compounds. A topical theme within this specialization is supramolecular
coordination chemistry.[3]
Main group compounds
These species feature elements from groups 1, 2 and 13-18 (excluding hydrogen) of the periodic table. Due to their often similar reactivity, the elements in group 3 (Sc, Y, and La) and group 12 (Zn, Cd, and Hg) are also generally included.[4]
Main group compounds have been known since the beginnings of chemistry, e.g., elemental sulfur and the distillable white phosphorus. Experiments on oxygen, O2, by Lavoisier and Priestley not only identified an important diatomic gas, but opened the way for describing compounds and reactions according to stoichiometric ratios. The discovery of a practical synthesis of ammonia using iron catalysts by Carl Bosch and Fritz Haber
in the early 1900s deeply impacted mankind, demonstrating the
significance of inorganic chemical synthesis. Typical main group
compounds are SiO2, SnCl4, and N2O. Many main group compounds can also be classed as “organometallic”, as they contain organic groups, e.g., B(CH3)3). Main group compounds also occur in nature, e.g., phosphate in DNA,
and therefore may be classed as bioinorganic. Conversely, organic
compounds lacking (many) hydrogen ligands can be classed as “inorganic”,
such as the fullerenes, buckytubes and binary carbon oxides.
Transition metal compounds
Compounds containing metals from group 4 to 11 are considered
transition metal compounds. Compounds with a metal from group 3 or 12
are sometimes also incorporated into this group, but also often
classified as main group compounds.
Transition metal compounds show a rich coordination chemistry, varying from tetrahedral for titanium (e.g., TiCl4)
to square planar for some nickel complexes to octahedral for
coordination complexes of cobalt. A range of transition metals can be
found in biologically important compounds, such as iron in hemoglobin.
Organometallic compounds
Usually, organometallic compounds are considered to contain the M-C-H group.[5]
The metal (M) in these species can either be a main group element or a
transition metal. Operationally, the definition of an organometallic
compound is more relaxed to include also highly lipophilic complexes such as metal carbonyls and even metal alkoxides.
Organometallic compounds are mainly considered a special category
because organic ligands are often sensitive to hydrolysis or oxidation,
necessitating that organometallic chemistry employs more specialized
preparative methods than was traditional in Werner-type complexes.
Synthetic methodology, especially the ability to manipulate complexes in
solvents of low coordinating power, enabled the exploration of very
weakly coordinating ligands such as hydrocarbons, H2, and N2.
Because the ligands are petrochemicals in some sense, the area of
organometallic chemistry has greatly benefited from its relevance to
industry.
Cluster compounds
Clusters can be found in all classes of chemical compounds.
According to the commonly accepted definition, a cluster consists
minimally of a triangular set of atoms that are directly bonded to each
other. But metal-metal bonded dimetallic complexes are highly relevant
to the area. Clusters occur in "pure" inorganic systems, organometallic
chemistry, main group chemistry, and bioinorganic chemistry. The
distinction between very large clusters and bulk solids is increasingly
blurred. This interface is the chemical basis of nanoscience or nanotechnology and specifically arise from the study of quantum size effects in cadmium selenide
clusters. Thus, large clusters can be described as an array of bound
atoms intermediate in character between a molecule and a solid.
Bioinorganic compounds
By definition, these compounds occur in nature, but the subfield includes anthropogenic species, such as pollutants (e.g., methylmercury) and drugs (e.g., Cisplatin).[6]
The field, which incorporates many aspects of biochemistry, includes
many kinds of compounds, e.g., the phosphates in DNA, and also metal
complexes containing ligands that range from biological macromolecules,
commonly peptides, to ill-defined species such as humic acid, and to water (e.g., coordinated to gadolinium complexes employed for MRI).
Traditionally bioinorganic chemistry focuses on electron- and
energy-transfer in proteins relevant to respiration. Medicinal inorganic
chemistry includes the study of both non-essential and essential elements with applications to diagnosis and therapies.
Solid state compounds
This important area focuses on structure,[7] bonding, and the physical properties of materials. In practice, solid state inorganic chemistry uses techniques such as crystallography
to gain an understanding of the properties that result from collective
interactions between the subunits of the solid. Included in solid state
chemistry are metals and their alloys or intermetallic derivatives. Related fields are condensed matter physics, mineralogy, and materials science.
Theoretical inorganic chemistry
An alternative perspective on the area of inorganic chemistry begins with the Bohr model of the atom and, using the tools and models of theoretical chemistry and computational chemistry,
expands into bonding in simple and then more complex molecules. Precise
quantum mechanical descriptions for multielectron species, the province
of inorganic chemistry, is difficult. This challenge has spawned many
semi-quantitative or semi-empirical approaches including molecular orbital theory and ligand field theory, In parallel with these theoretical descriptions, approximate methodologies are employed, including density functional theory.
Exceptions to theories, qualitative and quantitative, are extremely important in the development of the field. For example, CuII2(OAc)4(H2O)2
is almost diamagnetic below room temperature whereas Crystal Field
Theory predicts that the molecule would have two unpaired electrons. The
disagreement between qualitative theory (paramagnetic) and observation
(diamagnetic) led to the development of models for "magnetic coupling."
These improved models led to the development of new magnetic materials
and new technologies.
Qualitative theories
Inorganic chemistry has greatly benefited from qualitative theories.
Such theories are easier to learn as they require little background in
quantum theory. Within main group compounds, VSEPR theory powerfully predicts, or at least rationalizes, the structures of main group compounds, such as an explanation for why NH3 is pyramidal whereas ClF3 is T-shaped. For the transition metals, crystal field theory allows one to understand the magnetism of many simple complexes, such as why [FeIII(CN)6]3− has only one unpaired electron, whereas [FeIII(H2O)6]3+
has five. A particularly powerful qualitative approach to assessing the
structure and reactivity begins with classifying molecules according to
electron counting, focusing on the numbers of valence electrons, usually at the central atom in a molecule.
Molecular symmetry group theory
A central construct in inorganic chemistry is the theory of molecular symmetry.[8] Mathematical group theory provides the language to describe the shapes of molecules according to their point group symmetry. Group theory also enables factoring and simplification of theoretical calculations.
Spectroscopic features are analyzed and described with respect to the symmetry properties of the, inter alia,
vibrational or electronic states. Knowledge of the symmetry properties
of the ground and excited states allows one to predict the numbers and
intensities of absorptions in vibrational and electronic spectra. A
classic application of group theory is the prediction of the number of
C-O vibrations in substituted metal carbonyl complexes. The most common
applications of symmetry to spectroscopy involve vibrational and
electronic spectra.
As an instructional tool, group theory highlights commonalities and
differences in the bonding of otherwise disparate species, such as WF6 and Mo(CO)6 or CO2 and NO2.
Thermodynamics and inorganic chemistry
An alternative quantitative approach to inorganic chemistry focuses
on energies of reactions. This approach is highly traditional and empirical, but it is also useful. Broad concepts that are couched in thermodynamic terms include redox potential, acidity, phase changes. A classic concept in inorganic thermodynamics is the Born-Haber cycle, which is used for assessing the energies of elementary processes such as electron affinity, some of which cannot be observed directly.
Mechanistic inorganic chemistry
An important and increasingly popular aspect of inorganic chemistry
focuses on reaction pathways. The mechanisms of reactions are discussed
differently for different classes of compounds.
Main group elements and lanthanides
The mechanisms of main group compounds of groups 13-18 are usually
discussed in the context of organic chemistry (organic compounds are
main group compounds, after all). Elements heavier than C, N, O, and F
often form compounds with more electrons than predicted by the octet rule, as explained in the article on hypervalent molecules. The mechanisms of their reactions differ from organic compounds for this reason. Elements lighter than carbon (B, Be, Li) as well as Al and Mg often form electron-deficient structures that are electronically akin to carbocations.
Such electron-deficient species tend to react via associative pathways.
The chemistry of the lanthanides mirrors many aspects of chemistry seen
for aluminium.
Transition metal complexes
Mechanisms for the reactions of transition metals are discussed differently from main group compounds.[9]
The important role of d-orbitals in bonding strongly influences the
pathways and rates of ligand substitution and dissociation. These themes
are covered in articles on coordination chemistry and ligand. Both associative and dissociative pathways are observed.
An overarching aspect of mechanistic transition metal chemistry is
the kinetic lability of the complex illustrated by the exchange of free
and bound water in the prototypical complexes [M(H2O)6]n+:
- [M(H2O)6]n+ + 6 H2O* → [M(H2O*)6]n+ + 6 H2O
- where H2O* denotes isotopically enriched water, e.g., H217O
The rates of water exchange varies by 20 orders of magnitude across
the periodic table, with lanthanide complexes at one extreme and Ir(III)
species being the slowest.
Redox reactions
Redox reactions are prevalent for the transition elements. Two
classes of redox reaction are considered: atom-transfer reactions, such
as oxidative addition/reductive elimination, and electron-transfer. A fundamental redox reaction is "self-exchange", which involves the degenerate reaction between an oxidant and a reductant. For example, permanganate and its one-electron reduced relative manganate exchange one electron:
- [MnO4]− + [Mn*O4]2− → [MnO4]2− + [Mn*O4]−
Reactions at ligands
Coordinated ligands display reactivity distinct from the free ligands. For example, the acidity of the ammonia ligands in [Co(NH3)6]3+ is elevated relative to NH3
itself. Alkenes bound to metal cations are reactive toward nucleophiles
whereas alkenes normally are not. The large and industrially important
area of catalysis hinges on the ability of metals to modify the reactivity of organic ligands. Homogeneous catalysis occurs in solution and heterogeneous catalysis occurs when gaseous or dissolved substrates interact with surfaces of solids. Traditionally homogeneous catalysis is considered part of organometallic chemistry and heterogeneous catalysis is discussed in the context of surface science,
a subfield of solid state chemistry. But the basic inorganic chemical
principles are the same. Transition metals, almost uniquely, react with
small molecules such as CO, H2, O2, and C2H4.
The industrial significance of these feedstocks drives the active area
of catalysis. Ligands can also undergo ligand transfer reactions such as
transmetalation.
Characterization of inorganic compounds
Because of the diverse range of elements and the correspondingly
diverse properties of the resulting derivatives, inorganic chemistry is
closely associated with many methods of analysis. Older methods tended
to examine bulk properties such as the electrical conductivity of
solutions, melting points, solubility, and acidity. With the advent of quantum theory
and the corresponding expansion of electronic apparatus, new tools have
been introduced to probe the electronic properties of inorganic
molecules and solids. Often these measurements provide insights relevant
to theoretical models. For example, measurements on the photoelectron spectrum of methane demonstrated that describing the bonding by the two-center, two-electron bonds predicted between the carbon and hydrogen using Valence Bond Theory is not appropriate for describing ionisation processes in a simple way. Such insights led to the popularization of molecular orbital theory as fully delocalised orbitals are a more appropriate simple description of electron removal and electron excitation.
Commonly encountered techniques are:
Synthetic inorganic chemistry
Although some inorganic species can be obtained in pure form from
nature, most are synthesized in chemical plants and in the laboratory.
Inorganic synthetic methods can be classified roughly according the volatility or solubility of the component reactants.[10] Soluble inorganic compounds are prepared using methods of organic synthesis. For metal-containing compounds that are reactive toward air, Schlenk line and glove box
techniques are followed. Volatile compounds and gases are manipulated
in “vacuum manifolds” consisting of glass piping interconnected through
valves, the entirety of which can be evacuated to 0.001 mm Hg or less.
Compounds are condensed using liquid nitrogen (b.p. 78K) or other cryogens.
Solids are typically prepared using tube furnaces, the reactants and
products being sealed in containers, often made of fused silica
(amorphous SiO2) but sometimes more specialized materials
such as welded Ta tubes or Pt “boats”. Products and reactants are
transported between temperature zones to drive reactions.
ORGANIC CHEMISTRY.
Structure of the organic methane molecule, the simplest hydrocarbon compound
Organic chemistry is a chemistry subdiscipline involving the scientific study of the structure, properties, and reactions of organic compounds and organic materials, i.e., matter in its various forms that contain carbon atoms.[1][2] Study of structure includes using spectroscopy and other physical and chemical methods to determine the chemical composition and constitution of organic compounds and materials.[3] Study of properties includes both physical properties and chemical properties, and uses similar methods as well as methods to evaluate chemical reactivity,
with the aim to understand the behavior of the organic matter in its
pure form (when possible), but also in solutions, mixtures, and
fabricated forms. The study of organic reactions includes both their preparation—by synthesis or by other means—as well as their subsequent reactivities, both in the laboratory and via theoretical (in silico) study.
The range of chemicals studied in organic chemistry include hydrocarbons, compounds containing only carbon and hydrogen, as well as compositions based on carbon but containing other elements.[2][4][5][6][7] Organic chemistry overlaps with many areas including medicinal chemistry, biochemistry, organometallic chemistry, and polymer chemistry, as well as many aspects of materials science.[2]
Organic compounds form the basis of all earthly life.
They are structurally diverse. The range of application of organic
compounds is enormous. They either form the basis of, or are important
constituents of, many products including plastics, drugs, petrochemicals, food, explosive material, and paints.
History
Before the nineteenth century, chemists generally believed that
compounds obtained from living organisms were endowed with a vital force
that distinguished them from inorganic compounds. According to the concept of vitalism (vital force theory), organic matter was endowed with a "vital force".[8]
During the first half of the nineteenth century, some of the first
systematic studies of organic compounds were reported. Around 1816 Michel Chevreul started a study of soaps made from various fats and alkalis.
He separated the different acids that, in combination with the alkali,
produced the soap. Since these were all individual compounds, he
demonstrated that it was possible to make a chemical change in various
fats (which traditionally come from organic sources), producing new
compounds, without "vital force". In 1828 Friedrich Wöhler produced the organic chemical urea (carbamide), a constituent of urine, from the inorganic ammonium cyanate NH4CNO, in what is now called the Wöhler synthesis.
Although Wöhler was always cautious about claiming that he had
disproved the theory of vital force, this event has often been thought
of as a turning point.[8]
In 1856 William Henry Perkin, while trying to manufacture quinine, accidentally manufactured the organic dye now known as Perkin's mauve. Through its great financial success, this discovery greatly increased interest in organic chemistry.[9]
The crucial breakthrough for organic chemistry was the concept of
chemical structure, developed independently and simultaneously by Friedrich August Kekulé and Archibald Scott Couper in 1858.[10] Both men suggested that tetravalent
carbon atoms could link to each other to form a carbon lattice, and
that the detailed patterns of atomic bonding could be discerned by
skillful interpretations of appropriate chemical reactions.
The pharmaceutical
industry began in the last decade of the 19th century when the
manufacturing of acetylsalicylic acid (more commonly referred to as aspirin) in Germany was started by Bayer.[11] The first time a drug was systematically improved was with arsphenamine (Salvarsan). Though numerous derivatives of the dangerous toxic atoxyl were examined by Paul Ehrlich and his group, the compound with best effectiveness and toxicity characteristics was selected for production.[citation needed]
Early examples of organic reactions and applications were often serendipitous.
The latter half of the 19th century however witnessed systematic
studies of organic compounds, Illustrative is the development of
synthetic indigo. The production of indigo from plant sources dropped
from 19,000 tons in 1897 to 1,000 tons by 1914 thanks to the synthetic
methods developed by Adolf von Baeyer. In 2002, 17,000 tons of synthetic indigo were produced from petrochemicals.[12]
In the early part of the 20th Century, polymers and enzymes were
shown to be large organic molecules, and petroleum was shown to be of
biological origin.
The multistep synthesis of complex organic compounds is called total synthesis. Total synthesis of complex natural compounds increased in complexity to glucose and terpineol. For example, cholesterol-related
compounds have opened ways to synthesize complex human hormones and
their modified derivatives. Since the start of the 20th century,
complexity of total syntheses has been increased to include molecules of
high complexity such as lysergic acid and vitamin B12.[13]
The total synthesis of vitamin B12 marked a major achievement in organic chemistry.
The development of organic chemistry benefited from the discovery of petroleum
and the development of the petrochemical industry. The conversion of
individual compounds obtained from petroleum into different compound
types by various chemical processes led to the birth of the petrochemical industry, which successfully manufactured artificial rubbers, various organic adhesives, property-modifying petroleum additives, and plastics.
The majority of chemical compounds occurring in biological organisms
are in fact carbon compounds, so the association between organic
chemistry and biochemistry is so close that biochemistry might be regarded as in essence a branch of organic chemistry. Although the history of biochemistry
might be taken to span some four centuries, fundamental understanding
of the field only began to develop in the late 19th century and the
actual term biochemistry was coined around the start of 20th
century. Research in the field increased throughout the twentieth
century, without any indication of slackening in the rate of increase,
as may be verified by inspection of abstraction and indexing services
such as BIOSIS Previews and Biological Abstracts,
which began in the 1920s as a single annual volume, but has grown so
drastically that by the end of the 20th century it was only available to
the everyday user as an online electronic database.[14]
Characterization
Since organic compounds often exist as mixtures, a variety of techniques have also been developed to assess purity, especially important being chromatography techniques such as HPLC and gas chromatography. Traditional methods of separation include distillation, crystallization, and solvent extraction.
Organic compounds were traditionally characterized by a variety of
chemical tests, called "wet methods", but such tests have been largely
displaced by spectroscopic or other computer-intensive methods of
analysis.[15] Listed in approximate order of utility, the chief analytical methods are:
- Nuclear magnetic resonance (NMR) spectroscopy is the most commonly used technique, often permitting complete assignment of atom connectivity and even stereochemistry using correlation spectroscopy.
The principal constituent atoms of organic chemistry - hydrogen and
carbon - exist naturally with NMR-responsive isotopes, respectively 1H and 13C.
- Elemental analysis: A destructive method used to determine the elemental composition of a molecule. See also mass spectrometry, below.
- Mass spectrometry indicates the molecular weight of a compound and, from the fragmentation patterns,
its structure. High resolution mass spectrometry can usually identify
the exact formula of a compound and is used in lieu of elemental
analysis. In former times, mass spectrometry was restricted to neutral
molecules exhibiting some volatility, but advanced ionization techniques
allow one to obtain the "mass spec" of virtually any organic compound.
- Crystallography is an unambiguous method for determining molecular geometry,
the proviso being that single crystals of the material must be
available and the crystal must be representative of the sample. Highly
automated software allows a structure to be determined within hours of
obtaining a suitable crystal.
Traditional spectroscopic methods such as infrared spectroscopy, optical rotation, UV/VIS spectroscopy provide relatively nonspecific structural information but remain in use for specific classes of compounds.
Properties
Physical properties of organic compounds typically of interest
include both quantitative and qualitative features. Quantitative
information includes melting point, boiling point, and index of
refraction. Qualitative properties include odor, consistency,
solubility, and color.
Melting and boiling properties
In contrast to many inorganic materials, organic compounds typically
melt and many boil. In earlier times, the melting point (m.p.) and
boiling point (b.p.) provided crucial information on the purity and
identity of organic compounds. The melting and boiling points correlate
with the polarity of the molecules and their molecular weight. Some
organic compounds, especially symmetrical ones, sublime, that is they
evaporate without melting. A well-known example of a sublimable organic
compound is para-dichlorobenzene,
the odiferous constituent of modern mothballs. Organic compounds are
usually not very stable at temperatures above 300 °C, although some
exceptions exist.
Solubility
Neutral organic compounds tend to be hydrophobic; that is, they are less soluble in water than in organic solvents. Exceptions include organic compounds that contain ionizable groups as well as low molecular weight alcohols, amines, and carboxylic acids where hydrogen bonding occurs. Organic compounds tend to dissolve in organic solvents. Solvents can be either pure substances like ether or ethyl alcohol, or mixtures, such as the paraffinic solvents such as the various petroleum ethers and white spirits, or the range of pure or mixed aromatic solvents obtained from petroleum or tar fractions
by physical separation or by chemical conversion. Solubility in the
different solvents depends upon the solvent type and on the functional groups if present.
Solid state properties
Various specialized properties of molecular crystals and organic polymers with conjugated systems are of interest depending on applications, e.g. thermo-mechanical and electro-mechanical such as piezoelectricity, electrical conductivity (see conductive polymers and organic semiconductors), and electro-optical (e.g. non-linear optics) properties. For historical reasons, such properties are mainly the subjects of the areas of polymer science and materials science.
Nomenclature
Various names and depictions for one organic compound.
The names of organic compounds are either systematic, following
logically from a set of rules, or nonsystematic, following various
traditions. Systematic nomenclature is stipulated by specifications from
IUPAC. Systematic nomenclature starts with the name for a parent structure
within the molecule of interest. This parent name is then modified by
prefixes, suffixes, and numbers to unambiguously convey the structure.
Given that millions of organic compounds are known, rigorous use of
systematic names can be cumbersome. Thus, IUPAC recommendations are more
closely followed for simple compounds, but not complex molecules. To
use the systematic naming, one must know the structures and names of the
parent structures. Parent structures include unsubstituted
hydrocarbons, heterocycles, and monofunctionalized derivatives thereof.
Nonsystematic nomenclature is simpler and unambiguous, at least to
organic chemists. Nonsystematic names do not indicate the structure of
the compound. They are common for complex molecules, which includes most
natural products. Thus, the informally named lysergic acid diethylamide is systematically named (6aR,9R)-N,N-diethyl-7-methyl-4,6,6a,7,8,9-hexahydroindolo-[4,3-fg] quinoline-9-carboxamide.
With the increased use of computing, other naming methods have
evolved that are intended to be interpreted by machines. Two popular
formats are SMILES and InChI.
Structural drawings
Organic molecules are described more commonly by drawings or structural formulas, combinations of drawings and chemical symbols. The line-angle formula
is simple and unambiguous. In this system, the endpoints and
intersections of each line represent one carbon, and hydrogen atoms can
either be notated explicitly or assumed to be present as implied by
tetravalent carbon. The depiction of organic compounds with drawings is
greatly simplified by the fact that carbon in almost all organic
compounds has four bonds, nitrogen three, oxygen two, and hydrogen one.
Classification of organic compounds
Functional groups
The concept of functional groups is central in organic chemistry,
both as a means to classify structures and for predicting properties. A
functional group is a molecular module, and the reactivity of that
functional group is assumed, within limits, to be the same in a variety
of molecules. Functional groups can have decisive influence on the
chemical and physical properties of organic compounds. Molecules are
classified on the basis of their functional groups. Alcohols, for
example, all have the subunit C-O-H. All alcohols tend to be somewhat hydrophilic, usually form esters, and usually can be converted to the corresponding halides.
Most functional groups feature heteroatoms (atoms other than C and H).
Organic compounds are classified according to functional groups,
alcohols, carboxylic acids, amines, etc.
Aliphatic compounds
The aliphatic hydrocarbons are subdivided into three groups of homologous series according to their state of saturation:
- paraffins, which are alkanes without any double or triple bonds,
- olefins or alkenes which contain one or more double bonds, i.e. di-olefins (dienes) or poly-olefins.
- alkynes, which have one or more triple bonds.
The rest of the group is classed according to the functional groups
present. Such compounds can be "straight-chain", branched-chain or
cyclic. The degree of branching affects characteristics, such as the octane number or cetane number in petroleum chemistry.
Both saturated (alicyclic)
compounds and unsaturated compounds exist as cyclic derivatives. The
most stable rings contain five or six carbon atoms, but large rings
(macrocycles) and smaller rings are common. The smallest cycloalkane
family is the three-membered cyclopropane ((CH2)3).
Saturated cyclic compounds contain single bonds only, whereas aromatic
rings have an alternating (or conjugated) double bond. Cycloalkanes do not contain multiple bonds, whereas the cycloalkenes and the cycloalkynes do.
Aromatic compounds
Benzene is one of the best-known aromatic compounds as it is one of the simplest and most stable aromatics.
Aromatic hydrocarbons contain conjugated
double bonds. This means that every carbon atom in the ring is sp2
hybridized, allowing for added stability. The most important example is benzene, the structure of which was formulated by Kekulé who first proposed the delocalization or resonance
principle for explaining its structure. For "conventional" cyclic
compounds, aromaticity is conferred by the presence of 4n + 2
delocalized pi electrons, where n is an integer. Particular instability (antiaromaticity) is conferred by the presence of 4n conjugated pi electrons.
Heterocyclic compounds
The characteristics of the cyclic hydrocarbons are again altered if
heteroatoms are present, which can exist as either substituents attached
externally to the ring (exocyclic) or as a member of the ring itself
(endocyclic). In the case of the latter, the ring is termed a heterocycle. Pyridine and furan are examples of aromatic heterocycles while piperidine and tetrahydrofuran
are the corresponding alicyclic heterocycles. The heteroatom of
heterocyclic molecules is generally oxygen, sulfur, or nitrogen, with
the latter being particularly common in biochemical systems.
Examples of groups among the heterocyclics are the aniline dyes, the
great majority of the compounds discussed in biochemistry such as
alkaloids, many compounds related to vitamins, steroids, nucleic acids
(e.g. DNA, RNA) and also numerous medicines. Heterocyclics with
relatively simple structures are pyrrole (5-membered) and indole
(6-membered carbon ring).
Rings can fuse with other rings on an edge to give polycyclic compounds. The purine
nucleoside bases are notable polycyclic aromatic heterocycles. Rings
can also fuse on a "corner" such that one atom (almost always carbon)
has two bonds going to one ring and two to another. Such compounds are
termed spiro and are important in a number of natural products.
Polymers
This swimming board is made of polystyrene, an example of a polymer.
One important property of carbon is that it readily forms chains, or
networks, that are linked by carbon-carbon (carbon to carbon) bonds. The
linking process is called polymerization, while the chains, or networks, are called polymers. The source compound is called a monomer.
Two main groups of polymers exist: synthetic polymers and biopolymers. Synthetic polymers are artificially manufactured, and are commonly referred to as industrial polymers.[16] Biopolymers occur within a respectfully natural environment, or without human intervention.
Since the invention of the first synthetic polymer product, bakelite, synthetic polymer products have frequently been invented.[citation needed]
Common synthetic organic polymers are polyethylene (polythene), polypropylene, nylon, teflon (PTFE), polystyrene, polyesters, polymethylmethacrylate (called perspex and plexiglas), and polyvinylchloride (PVC).[citation needed]
Both synthetic and natural rubber are polymers.[citation needed]
Varieties of each synthetic polymer product may exist, for purposes
of a specific use. Changing the conditions of polymerization alters the
chemical composition of the product and its properties. These
alterations include the chain length, or branching, or the tacticity.[citation needed]
With a single monomer as a start, the product is a homopolymer.[citation needed]
Secondary component(s) may be added to create a heteropolymer (co-polymer) and the degree of clustering of the different components can also be controlled.[citation needed]
Physical characteristics, such as hardness, density, mechanical or tensile strength, abrasion resistance, heat resistance, transparency, colour, etc. will depend on the final composition.[citation needed]
Biomolecules
Biomolecular chemistry is a major category within organic chemistry which is frequently studied by biochemists. Many complex multi-functional group molecules are important in living organisms. Some are long-chain biopolymers, and these include peptides, DNA, RNA and the polysaccharides such as starches in animals and celluloses in plants. The other main classes are amino acids (monomer building blocks of peptides and proteins), carbohydrates (which includes the polysaccharides), the nucleic acids (which include DNA and RNA as polymers), and the lipids. In addition, animal biochemistry contains many small molecule intermediates which assist in energy production through the Krebs cycle, and produces isoprene, the most common hydrocarbon in animals. Isoprenes in animals form the important steroid structural (cholesterol) and steroid hormone compounds; and in plants form terpenes, terpenoids, some alkaloids, and a class of hydrocarbons called biopolymer polyisoprenoids present in the latex of various species of plants, which is the basis for making rubber.
- Peptide Synthesis
-
- See also Peptide synthesis
- Oligonucleotide Synthesis
-
- See also Oligonucleotide synthesis
- Carbohydrate Synthesis
-
- See also Carbohydrate synthesis
Small molecules
In pharmacology, an important group of organic compounds is small molecules,
also referred to as 'small organic compounds'. In this context, a small
molecule is a small organic compound that is biologically active, but
is not a polymer. In practice, small molecules have a molar mass less than approximately 1000 g/mol.
Fullerenes
Fullerenes and carbon nanotubes, carbon compounds with spheroidal and tubular structures, have stimulated much research into the related field of materials science.
Others
Organic compounds containing bonds of carbon to nitrogen, oxygen and
the halogens are not normally grouped separately. Others are sometimes
put into major groups within organic chemistry and discussed under
titles such as organosulfur chemistry, organometallic chemistry, organophosphorus chemistry and organosilicon chemistry.
Organic synthesis
Synthetic organic chemistry is an applied science as it borders engineering,
the "design, analysis, and/or construction of works for practical
purposes". Organic synthesis of a novel compound is a problem solving
task, where a synthesis is designed for a target molecule by selecting
optimal reactions from optimal starting materials. Complex compounds can
have tens of reaction steps that sequentially build the desired
molecule. The synthesis proceeds by utilizing the reactivity of the
functional groups in the molecule. For example, a carbonyl compound can be used as a nucleophile by converting it into an enolate, or as an electrophile; the combination of the two is called the aldol reaction.
Designing practically useful syntheses always requires conducting the
actual synthesis in the laboratory. The scientific practice of creating
novel synthetic routes for complex molecules is called total synthesis.
There are several strategies to design a synthesis. The modern method of retrosynthesis,
developed by E.J. Corey, starts with the target molecule and splices it
to pieces according to known reactions. The pieces, or the proposed
precursors, receive the same treatment, until available and ideally
inexpensive starting materials are reached. Then, the retrosynthesis is
written in the opposite direction to give the synthesis. A "synthetic
tree" can be constructed, because each compound and also each precursor
has multiple syntheses.
Organic reactions
Organic reactions are chemical reactions involving organic compounds. While pure hydrocarbons
undergo certain limited classes of reactions, many more reactions which
organic compounds undergo are largely determined by functional groups.
The general theory of these reactions involves careful analysis of such
properties as the electron affinity of key atoms, bond strengths and steric hindrance. These issues can determine the relative stability of short-lived reactive intermediates, which usually directly determine the path of the reaction.
The basic reaction types are: addition reactions, elimination
reactions, substitution reactions, pericyclic reactions, rearrangement
reactions and redox reactions. An example of a common reaction is a substitution reaction written as:
- Nu− + C-X → C-Nu + X−
where X is some functional group and Nu is a nucleophile.
The number of possible organic reactions is basically infinite.
However, certain general patterns are observed that can be used to
describe many common or useful reactions. Each reaction has a stepwise
reaction mechanism that explains how it happens in sequence—although the
detailed description of steps is not always clear from a list of
reactants alone.
The stepwise course of any given reaction mechanism can be represented using arrow pushing
techniques in which curved arrows are used to track the movement of
electrons as starting materials transition through intermediates to
final products.
See also
PHYSICAL CHEMISTRY.
Physical chemistry is the study of macroscopic, atomic, subatomic, and particulate phenomena in chemical systems in terms of laws and concepts of physics. It applies the principles, practices and concepts of physics such as motion, energy, force, time, thermodynamics, quantum chemistry, statistical mechanics and dynamics, equilibrium.
Physical chemistry, in contrast to chemical physics,
is predominantly (but not always) a macroscopic or supra-molecular
science, as the majority of the principles on which physical chemistry
was founded, are concepts related to the bulk rather than on
molecular/atomic structure alone. For example, chemical equilibrium, and colloids.
Some of the relationships that physical chemistry strives to resolve include the effects of:
- Intermolecular forces that act upon the physical properties of materials (plasticity, tensile strength, surface tension in liquids).
- Reaction kinetics on the rate of a reaction.
- The identity of ions and the electrical conductivity of materials.
- Surface chemistry and electrochemistry of membranes.[1]
- Interaction of one body with another in terms of quantities of heat and work called thermodynamics.
- Transfer of heat between a chemical system and its surroundings during change of phase or chemical reaction taking place called thermochemistry
- Study of colligative properties of number of species present in solution.
- Number of phases, number of components and degree of freedom (or variance) can be correlated with one another with help of phase rule.
- Reactions of electrochemical cells.
Key concepts
The key concepts of physical chemistry are the ways in which pure physics is applied to chemical problems.
One of the key concepts in classical chemistry is that all chemical compounds can be described as groups of atoms bonded together and chemical reactions
can be described as the making and breaking of those bonds. Predicting
the properties of chemical compounds from a description of atoms and how
they bond is one of the major goals of physical chemistry. To describe
the atoms and bonds precisely, it is necessary to know both where the nuclei of the atoms are, and how electrons are distributed around them.[2]
Quantum chemistry, a subfield of physical chemistry especially concerned with the application of quantum mechanics to chemical problems, provides tools to determine how strong and what shape bonds are,[2] how nuclei move, and how light can be absorbed or emitted by a chemical compound.[3] Spectroscopy is the related sub-discipline of physical chemistry which is specifically concerned with the interaction of electromagnetic radiation with matter.
Another set of important questions in chemistry concerns what kind of
reactions can happen spontaneously and which properties are possible
for a given chemical mixture. This is studied in chemical thermodynamics, which sets limits on quantities like how far a reaction can proceed, or how much energy can be converted into work in an internal combustion engine, and which provides links between properties like the thermal expansion coefficient and rate of change of entropy with pressure for a gas or a liquid.[4]
It can frequently be used to assess whether a reactor or engine design
is feasible, or to check the validity of experimental data. To a limited
extent, quasi-equilibrium and non-equilibrium thermodynamics can describe irreversible changes.[5] However, classical thermodynamics is mostly concerned with systems in equilibrium and reversible changes and not what actually does happen, or how fast, away from equilibrium.
Which reactions do occur and how fast is the subject of chemical kinetics, another branch of physical chemistry. A key idea in chemical kinetics is that for reactants to react and form products, most chemical species must go through transition states which are higher in energy than either the reactants or the products and serve as a barrier to reaction.[6] In general, the higher the barrier, the slower the reaction. A second is that most chemical reactions occur as a sequence of elementary reactions,[7]
each with its own transition state. Key questions in kinetics include
how the rate of reaction depends on temperature and on the
concentrations of reactants and catalysts in the reaction mixture, as well as how catalysts and reaction conditions can be engineered to optimize the reaction rate.
The fact that how fast reactions occur can often be specified with
just a few concentrations and a temperature, instead of needing to know
all the positions and speeds of every molecule in a mixture, is a
special case of another key concept in physical chemistry, which is that
to the extent an engineer needs to know, everything going on in a
mixture of very large numbers (perhaps of the order of the Avogadro constant, 6 x 1023)
of particles can often be described by just a few variables like
pressure, temperature, and concentration. The precise reasons for this
are described in statistical mechanics,[8]
a specialty within physical chemistry which is also shared with
physics. Statistical mechanics also provides ways to predict the
properties we see in everyday life from molecular properties without
relying on empirical correlations based on chemical similarities.[5]
History
Fragment of M. Lomonosov's manuscript 'Physical Chemistry' (1752)
The term "physical chemistry" was coined by Mikhail Lomonosov
in 1752, when he presented a lecture course entitled "A Course in True
Physical Chemistry" (Russian: «Курс истинной физической химии») before
the students of Petersburg University.[9]
Modern physical chemistry originated in the 1860s to 1880s with work on chemical thermodynamics, electrolytes in solutions, chemical kinetics and other subjects. One milestone was the publication in 1876 by Josiah Willard Gibbs of his paper, On the Equilibrium of Heterogeneous Substances. This paper introduced several of the cornerstones of physical chemistry, such as Gibbs energy, chemical potentials, Gibbs phase rule.[10] Other milestones include the subsequent naming and accreditation of enthalpy to Heike Kamerlingh Onnes and to macromolecular processes.[citation needed]
The first scientific journal specifically in the field of physical chemistry was the German journal, Zeitschrift für Physikalische Chemie, founded in 1887 by Wilhelm Ostwald and Jacobus Henricus van 't Hoff. Together with Svante August Arrhenius,[11]
these were the leading figures in physical chemistry in the late 19th
century and early 20th century. All three were awarded with the Nobel Prize in Chemistry between 1901-1909.
Developments in the following decades include the application of statistical mechanics to chemical systems and work on colloids and surface chemistry, where Irving Langmuir made many contributions. Another important step was the development of quantum mechanics into quantum chemistry from the 1930s, where Linus Pauling
was one of the leading names. Theoretical developments have gone hand
in hand with developments in experimental methods, where the use of
different forms of spectroscopy, such as infrared spectroscopy, microwave spectroscopy, EPR spectroscopy and NMR spectroscopy, is probably the most important 20th century development.
Further development in physical chemistry may be attributed to discoveries in nuclear chemistry, especially in isotope separation (before and during World War II), more recent discoveries in astrochemistry,[12]
as well as the development of calculation algorithms in the field of
"additive physicochemical properties" (practically all physicochemical
properties, such as boiling point, critical point, surface tension,
vapor pressure, etc. - more than 20 in all - can be precisely calculated
from chemical structure alone, even if the chemical molecule remains
unsynthesized),[citation needed] and in this area is concentrated practical importance of contemporary physical chemistry.
