ANALYTICAL CHEMISTRY.
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials.[1] Qualitative analysis gives an indication of the identity of the chemical species in the sample, and quantitative analysis
determines the amount of certain components in the substance. The
separation of components is often performed prior to analysis.
Analytical methods can be separated into classical and instrumental.[2] Classical methods (also known as wet chemistry methods) use separations such as precipitation, extraction, and distillation
and qualitative analysis by color, odor, or melting point. Classical
quantitative analysis is achieved by measurement of weight or volume.
Instrumental methods use an apparatus to measure physical quantities of
the analyte such as light absorption, fluorescence, or conductivity. The separation of materials is accomplished using chromatography, electrophoresis or Field Flow Fractionation methods.
Analytical chemistry is also focused on improvements in experimental design, chemometrics, and the creation of new measurement tools to provide better chemical information. Analytical chemistry has applications in forensics, bioanalysis, clinical analysis, environmental analysis, and materials analysis.
History
Gustav Kirchhoff (left) and Robert Bunsen (right)
Analytical chemistry has been important since the early days of
chemistry, providing methods for determining which elements and
chemicals are present in the object in question. During this period
significant analytical contributions to chemistry include the
development of systematic elemental analysis by Justus von Liebig and systematized organic analysis based on the specific reactions of functional groups.
The first instrumental analysis was flame emissive spectrometry developed by Robert Bunsen and Gustav Kirchhoff who discovered rubidium (Rb) and caesium (Cs) in 1860.[3]
Most of the major developments in analytical chemistry take place
after 1900. During this period instrumental analysis becomes
progressively dominant in the field. In particular many of the basic
spectroscopic and spectrometric techniques were discovered in the early
20th century and refined in the late 20th century.[4]
The separation sciences follow a similar time line of development and also become increasingly transformed into high performance instruments.[5] In the 1970s many of these techniques began to be used together to achieve a complete characterization of samples.
Starting in approximately the 1970s into the present day analytical
chemistry has progressively become more inclusive of biological
questions (bioanalytical chemistry), whereas it had previously been
largely focused on inorganic or small organic molecules.
Lasers have been increasingly used in chemistry as probes and even to
start and influence a wide variety of reactions. The late 20th century
also saw an expansion of the application of analytical chemistry from
somewhat academic chemical questions to forensic, environmental, industrial and medical questions, such as in histology.[6]
Modern analytical chemistry is dominated by instrumental analysis.
Many analytical chemists focus on a single type of instrument. Academics
tend to either focus on new applications and discoveries or on new
methods of analysis. The discovery of a chemical present in blood that
increases the risk of cancer would be a discovery that an analytical
chemist might be involved in. An effort to develop a new method might
involve the use of a tunable laser
to increase the specificity and sensitivity of a spectrometric method.
Many methods, once developed, are kept purposely static so that data can
be compared over long periods of time. This is particularly true in
industrial quality assurance
(QA), forensic and environmental applications. Analytical chemistry
plays an increasingly important role in the pharmaceutical industry
where, aside from QA, it is used in discovery of new drug candidates and
in clinical applications where understanding the interactions between
the drug and the patient are critical.
Classical methods
The presence of copper in this qualitative analysis is indicated by the bluish-green color of the flame.
Although modern analytical chemistry is dominated by sophisticated
instrumentation, the roots of analytical chemistry and some of the
principles used in modern instruments are from traditional techniques
many of which are still used today. These techniques also tend to form
the backbone of most undergraduate analytical chemistry educational
labs.
Qualitative analysis
A qualitative analysis determines the presence or absence of a
particular compound, but not the mass or concentration. By definition,
qualitative analyses do not measure quantity.
Chemical tests
There are numerous qualitative chemical tests, for example, the acid test for gold and the Kastle-Meyer test for the presence of blood.
Flame test
Inorganic qualitative analysis generally refers to a systematic
scheme to confirm the presence of certain, usually aqueous, ions or
elements by performing a series of reactions that eliminate ranges of
possibilities and then confirms suspected ions with a confirming test.
Sometimes small carbon containing ions are included in such schemes.
With modern instrumentation these tests are rarely used but can be
useful for educational purposes and in field work or other situations
where access to state-of-the-art instruments are not available or
expedient.
Quantitative analysis
Gravimetric analysis
Gravimetric analysis involves determining the amount of material
present by weighing the sample before and/or after some transformation. A
common example used in undergraduate education is the determination of
the amount of water in a hydrate by heating the sample to remove the
water such that the difference in weight is due to the loss of water.
Volumetric analysis
For more details on this topic, see Titration.
Titration involves the addition of a reactant to a solution being
analyzed until some equivalence point is reached. Often the amount of
material in the solution being analyzed may be determined. Most familiar
to those who have taken chemistry during secondary education is the
acid-base titration involving a color changing indicator. There are many
other types of titrations, for example potentiometric titrations. These
titrations may use different types of indicators to reach some
equivalence point.
Instrumental methods
Block diagram of an analytical instrument showing the stimulus and measurement of response
Spectroscopy
Spectroscopy measures the interaction of the molecules with electromagnetic radiation. Spectroscopy consists of many different applications such as atomic absorption spectroscopy, atomic emission spectroscopy, ultraviolet-visible spectroscopy, x-ray fluorescence spectroscopy, infrared spectroscopy, Raman spectroscopy, dual polarisation interferometry, nuclear magnetic resonance spectroscopy, photoemission spectroscopy, Mössbauer spectroscopy and so on.
Mass spectrometry
Mass spectrometry measures mass-to-charge ratio of molecules using electric and magnetic fields. There are several ionization methods: electron impact, chemical ionization,
electrospray, fast atom bombardment, matrix assisted laser desorption
ionization, and others. Also, mass spectrometry is categorized by
approaches of mass analyzers: magnetic-sector, quadrupole mass analyzer, quadrupole ion trap, time-of-flight, Fourier transform ion cyclotron resonance, and so on.
Electrochemical analysis
Electroanalytical methods measure the potential (volts) and/or current (amps) in an electrochemical cell containing the analyte.[7][8]
These methods can be categorized according to which aspects of the cell
are controlled and which are measured. The three main categories are potentiometry (the difference in electrode potentials is measured), coulometry (the cell's current is measured over time), and voltammetry (the cell's current is measured while actively altering the cell's potential).
Thermal analysis
Calorimetry and thermogravimetric analysis measure the interaction of a material and heat.
Separation
Separation processes are used to decrease the complexity of material mixtures. Chromatography, electrophoresis and Field Flow Fractionation are representative of this field.
Hybrid techniques
Combinations of the above techniques produce a "hybrid" or "hyphenated" technique.[9][10][11][12][13] Several examples are in popular use today and new hybrid techniques are under development. For example, gas chromatography-mass spectrometry, gas chromatography-infrared spectroscopy, liquid chromatography-mass spectrometry, liquid chromatography-NMR spectroscopy. liquid chromagraphy-infrared spectroscopy and capillary electrophoresis-mass spectrometry.
Hyphenated separation techniques refers to a combination of two (or
more) techniques to detect and separate chemicals from solutions. Most
often the other technique is some form of chromatography. Hyphenated techniques are widely used in chemistry and biochemistry. A slash is sometimes used instead of hyphen, especially if the name of one of the methods contains a hyphen itself.
Microscopy
The visualization of single molecules, single cells, biological tissues and nanomaterials
is an important and attractive approach in analytical science. Also,
hybridization with other traditional analytical tools is revolutionizing
analytical science. Microscopy can be categorized into three different fields: optical microscopy, electron microscopy, and scanning probe microscopy. Recently, this field is rapidly progressing because of the rapid development of the computer and camera industries.
Lab-on-a-chip
Devices that integrate (multiple) laboratory functions on a single
chip of only millimeters to a few square centimeters in size and that
are capable of handling extremely small fluid volumes down to less than
picoliters.
Standards
Standard curve
A general method for analysis of concentration involves the creation of a calibration curve.
This allows for determination of the amount of a chemical in a material
by comparing the results of unknown sample to those of a series of
known standards. If the concentration of element or compound in a sample
is too high for the detection range of the technique, it can simply be
diluted in a pure solvent. If the amount in the sample is below an
instrument's range of measurement, the method of addition can be used.
In this method a known quantity of the element or compound under study
is added, and the difference between the concentration added, and the
concentration observed is the amount actually in the sample.
Internal standards
Sometimes an internal standard
is added at a known concentration directly to an analytical sample to
aid in quantitation. The amount of analyte present is then determined
relative to the internal standard as a calibrant. An ideal internal
standard is isotopically-enriched analyte which gives rise to the method
of isotope dilution.
Standard addition
The method of standard addition is used in instrumental analysis to determine concentration of a substance (analyte) in an unknown sample by comparison to a set of samples of known concentration, similar to using a calibration curve. Standard addition can be applied to most analytical techniques and is used instead of a calibration curve to solve the matrix effect problem.
Signals and noise
One of the most important components of analytical chemistry is maximizing the desired signal while minimizing the associated noise.[15] The analytical figure of merit is known as the signal-to-noise ratio (S/N or SNR).
Noise can arise from environmental factors as well as from fundamental physical processes.
Thermal noise
Thermal noise results from the motion of charge carriers (usually
electrons) in an electrical circuit generated by their thermal motion.
Thermal noise is white noise meaning that the power spectral density is constant throughout the frequency spectrum.
The root mean square value of the thermal noise in a resistor is given by[15]
where kB is Boltzmann's constant, T is the temperature, R is the resistance, and 
is the bandwidth of the frequency 
.
Shot noise
Shot noise is a type of electronic noise that occurs when the finite number of particles (such as electrons in an electronic circuit or photons in an optical device) is small enough to give rise to statistical fluctuations in a signal.
Shot noise is a Poisson process and the charge carriers that make up the current follow a Poisson distribution. The root mean square current fluctuation is given by[15]
where e is the elementary charge and I is the average current. Shot noise is white noise.
Flicker noise
Main article: flicker noise
Flicker noise is electronic noise with a 1/ƒ frequency spectrum; as f
increases, the noise decreases. Flicker noise arises from a variety of
sources, such as impurities in a conductive channel, generation and recombination noise in a transistor due to base current, and so on. This noise can be avoided by modulation of the signal at a higher frequency, for example through the use of a lock-in amplifier.
Environmental noise
Noise in a thermogravimetric analysis; lower noise in the middle of the plot results from less human activity (and environmental noise) at night.
Environmental noise arises from the surroundings of the analytical instrument. Sources of electromagnetic noise are power lines, radio and television stations, wireless devices, Compact fluorescent lamps[16] and electric motors. Many of these noise sources are narrow bandwidth and therefore can be avoided. Temperature and vibration isolation may be required for some instruments.
Noise reduction
Noise reduction can be accomplished either in computer hardware or software. Examples of hardware noise reduction are the use of shielded cable, analog filtering, and signal modulation. Examples of software noise reduction are digital filtering, ensemble average, boxcar average, and correlation methods.[15]
Applications
Analytical chemistry research is largely driven by performance (sensitivity, selectivity, robustness, linear range,
accuracy, precision, and speed), and cost (purchase, operation,
training, time, and space). Among the main branches of contemporary
analytical atomic spectrometry, the most widespread and universal are
optical and mass spectrometry.[17] In the direct elemental analysis of solid samples, the new leaders are laser-induced breakdown and laser ablation mass spectrometry, and the related techniques with transfer of the laser ablation products into inductively coupled plasma.
Advances in design of diode lasers and optical parametric oscillators
promote developments in fluorescence and ionization spectrometry and
also in absorption techniques where uses of optical cavities for
increased effective absorption pathlength are expected to expand. The
use of plasma- and laser-based methods is increasing. An interest
towards absolute (standardless) analysis has revived, particularly in
emission spectrometry.[citation needed]
great effort is put in shrinking the analysis techniques to chip
size. Although there are few examples of such systems competitive with
traditional analysis techniques, potential advantages include
size/portability, speed, and cost. (micro Total Analysis System (µTAS) or Lab-on-a-chip). Microscale chemistry reduces the amounts of chemicals used.
Many developments improve the analysis of biological systems. Examples of rapidly expanding fields in this area are:
- Genomics - DNA sequencing and its related research. Genetic fingerprinting and DNA microarray are important tools and research fields.
- Proteomics
- the analysis of protein concentrations and modifications, especially
in response to various stressors, at various developmental stages, or in
various parts of the body.
- Metabolomics - similar to proteomics, but dealing with metabolites.
- Transcriptomics - mRNA and its associated field
- Lipidomics - lipids and its associated field
- Peptidomics - peptides and its associated field
- Metalomics - similar to proteomics and metabolomics, but dealing
with metal concentrations and especially with their binding to proteins
and other molecules.
Analytical chemistry has played critical roles in the understanding
of basic science to a variety of practical applications, such as
biomedical applications, environmental monitoring, quality control of
industrial manufacturing, forensic science and so on.
The recent developments of computer automation and information
technologies have extended analytical chemistry into a number of new
biological fields. For example, automated DNA sequencing machines were
the basis to complete human genome projects leading to the birth of genomics. Protein identification and peptide sequencing by mass spectrometry opened a new field of proteomics.
Analytical chemistry has been an indispensable area in the development of nanotechnology. Surface characterization instruments, electron microscopes and scanning probe microscopes enables scientists to visualize atomic structures with chemical characterizations.
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.
