Organic chemistry is the study of the compounds of carbon. It is the single largest branch of chemistry, and the one with the most direct impact on the daily lives of most people in the world. The number of organic compounds is well over a million, and thousands of new compounds are created or discovered every year. In addition to the practical uses of organic compounds as drugs, fuels, and industrial chemicals, the study of organic compounds provides new information about chemical bonding, reactions, and other processes unavailable from the study of other types of compounds.
Why is carbon at the center of this vast field of study, rather than, say, sodium? The answer is that carbon's electron configuration allows it to form four bonds, and its size and electronegativity mean these bonds will be primarily covalent or polar covalent, rather than ionic. The ability to form multiple bonds means carbon can form chains, allowing the creation of large and complex carbon backbones, as it does in molecules such as proteins, DNA, plastics, and other polymers. In addition, carbon normally has no lone pairs of electrons in its compounds, preventing the kind of lone-pair repulsion that makes nitrogen or oxygen chains unstable.
The covalent character of carbon's bonds, even with such electronegative elements as oxygen, means that its bonds continue to be highly directional, unlike the ionic bonds of sodium, for instance, which are equally strong in all directions. This directionality gives the individual molecule its identity separate from other molecules, allowing it to participate as a unit in the highly complex series of reactions found in living organisms or in the modern chemistry laboratory.
The first organic compounds, and still the most complex, are those from living organisms. Most organisms share perhaps a thousand or more similar or identical compounds, such as amino acids, sugars, and nucleotides. Many organisms, however, make unique compounds, which function in signaling, metabolic pathways, or defense. Such compounds have proven to be a potent source of new drugs and enzymes. Examples include taxol from the Pacific yew, curare from a group of South American shrubs, and aspirin, from the bark of willow trees. While many useful compounds originally isolated from living organisms are now made synthetically, others are either too complex or too expensive to synthesize, and continue to be harvested from the natural source.
The other source of new organic compounds is from deliberate modification of existing ones in the laboratory. Changing the types of bonds or side chains can often have significant effects on the properties of the compound, and may give it important new characteristics.
While carbon's bonding characteristics form the theoretical basis of organic chemistry, the subject (and, indeed, the world) would be rather dull if carbon only bonded to itself.Diamond, graphite, and the fullerenes would be the only carbon compounds. The vast range of organic compounds arises from the addition of different atoms onto a carbon skeleton. Hydrocarbons are compounds in which hydrogen is the only other atom. The simplest is methane, CH4. Larger hydrocarbons include all the compounds used as gasoline and heating fuels, as well as petrochemical-based waxes.
Substitution of a halogen such as chlorine for a hydrogen on a hydrocarbon creates a new class of compounds, the alkyl halides. The halogen is called the functional group of this class, meaning it is the group that gives the class its characteristic properties.
The study of functional groups provide the theoretical framework for understanding the reactions and behavior of the various classes of organic compounds. For instance, a halide is an electronegative atom, which withdraws electrons from the carbon it is bonded to. This leaves carbon with a partial positive charge, which will, in turn, serve to attract negative groups during reactions. The electron- withdrawing nature of the halides, then, strongly influences the chemical behavior of the parent compound. Compounds may have more than one functional group, which influence the behavior in different ways.
Other important functional groups include: OH, the hydroxyl group, which makes the parent compound an alcohol; O, which makes the compound an ether; C=O, the carbonyl group, which makes the compound an aldehyde if the group is on a terminal carbon, or a ketone if it is between two carbons; COOH, the carboxylic acid group, which makes the compound an acid; NH2, the amine group, which makes the compound an amine; NH, the amide group, which makes the compound an amide; C6H5, the phenyl group; and SH, the thiol group.
In addition, the chemistry of a compound is strongly affected by whether it is a straight-chain molecule or a ring compound, and whether that ring is aliphatic or aromatic.
The study of organic compounds includes the determination of the identity and structure of existing compounds, the synthesis of new ones, and the determination of the step-by-step mechanisms and other parameters of reactions.
Analysis is concerned with determining the structure and identity of a compound. It requires the isolation and purification of a sample of the compound, followed by a series of physical and chemical tests. Simple tests include determination of melting and boiling point.Molecular weight was once commonly determined through colligative property analysis, but is now more likely to be done with a mass spectrometer. Determination of the atomic makeup of the compound can be approached through combustion analysis combined with a variety of qualitative tests for various functional groups.
Structure determination is most often performed with one or another type of spectroscopy.Infrared spectroscopy is a principal tool for functional group analysis, while Nuclear Magentic Resonance (NMR) spectroscopy gives detailed information on the position of hydrogens and other atoms. X-ray crystallography can solve the three-dimensional structure, especially important for larger compounds that could exist in any one of many different conformations. Proposed structures are often confirmed by creation of derivative compounds, whose successful formation from the parent compound depends on the predicted structure. Structure determination is a prerequisite for synthesizing the compound, as would be needed for manufacturing a compound isolated from a natural source.
Synthesis means creation of a compound from simpler or more readily available starting materials. For many syntheses, this means using such compounds as hydrocarbons and alcohols, plus inorganic acids, metals, halogens, or other compounds. The synthesis of urea from hydrogen cyanide and ammonia by Friedrich Wohler (1800-1882) in 1828 is often said to mark the beginning of organic chemistry as a separate discipline.
Working out the series of reactions for an organic synthesis is often like solving a logic puzzle or a maze, in that the starting point and ultimate goal are known, but the intermediate steps in between are unknown. Techniques for planning organic syntheses have been highly refined by Elias Corey, among others, who was awarded the 1990 Nobel Prize for his work.
Reactions in a multi-step synthesis must be chosen for their ability to create intermediates that can go on to react in later steps, and which will not engage in undesired reactions instead. "Blocking groups" may need to be added to functional groups to protect them from such undesired reactions, while preserving them for reaction later in the synthesis.
Intermediates must be formed in high yield, so that by the end of the synthesis, a meaningful amount of final product is available. One of the principal barriers to high yield is the creation of both enantiomers of a chiral compound, when only one is desired. Under most conditions, these mirror-image molecules will be formed in equal amounts, which must then be separated and half discarded before proceeding. The development of enantiomer-selective catalysts for "asymmetric synthesis" is one of the most intensely studied areas in organic chemical synthesis today.
To fully understand a reaction, the step-by-step mechanism by which products become reactants must be worked out. A mechanism may involve only a single step and a single molecular collision, but more often involves several steps, in which very short-lived intermediates are formed and consumed in the course of forming products. Reaction mechanisms can be used to determine the best conditions under which to run the reaction, or to devise reactants with slight structural changes that alter the reactivity in desired ways. In addition, the determination of a mechanism can be a purely theoretical challenge, whose results can shed light on the nature of the matter and its interactions.
Reaction mechanisms are determined through a variety of means, most importantly spectroscopy, which is used to determine the structure of unstable intermediates. Computer modeling may also play a part, allowing the calculation of the quantum mechanical changes of proposed intermediates. Simple kinetics experiments play their part as well by varying the concentration of starting materials, it is possible to determine which is involved in the rate-determining step.
While organic chemistry is more than a century old, it remains one of the most prolific branches of chemistry, with cutting-edge research constantly pushing into new areas. The frontiers of organic chemistry include many different areas. Catalysis has been expanded greatly with the use of asymmetric catalysts for organic syntheses. Organometallic chemistry is the study of carbon-metal compounds, such as the organolithiums. These compounds are of both theoretical and practical interest, for they, too, may allow enantioselectivity in reactions. Polymers continue to be of great importance in chemistry, and new polymers are being synthesized for specific uses. Electrically conductive polymers are finding uses in the electronics industry, while thin films that change their transparency with temperature are being developed as coatings from windows.
The border between organic chemistry and biochemistry continues to be one of the most fruitful areas of research, especially as it concerns use of biomolecules as organic reagents or catalysts. Enzymes are especially important in this regard because of their strict enantioselectivity. Reactions that exploit these natural catalysts are increasingly being used in synthetic pathways. Finally, drug discovery and design is an important application of organic chemistry to medicine, and is one of the most profitable areas of research.
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