Organic chemistry is that part of chemistry concerned with the composition, structure, properties, reactions and synthesis of organic compounds. The nexus is carbon, which is almost unique among the elements of the periodic table in that it can form long molecular chains and rings which also include nitrogen, oxygen, halogens, phosphorus, sulfur, and a variety of other atomic species. Carbon atoms usually participate in four covalent bonds, with a tetrahedral angle of 109.5 degrees in the "saturated" alkanes. "Unsaturated" double bonds are common (alkenes), and triple bonds also occur (alkynes).
Because of their unique properties, multi-carbon compounds exhibit enormous variety and are the basis for plastics and drugs, and for life itself. The different shapes and chemical reactivities of the substituents provide an astonishing variety of functions, including those of enzyme catalysts. Because of its special properties, it is likely that life on other star systems will be found to be carbon-based, in spite of speculations about the possibility of substituting silicon, which lies just below carbon in the periodic table.
Trends in organic chemistry include chiral synthesis, green chemistry, microwave chemistry and microwave spectroscopy — which has identified dozens of organic molecules in interstellar space — and fullerenes.
History
Organic chemistry as a science is generally agreed to have started in 1828 with Friedrich Woehler's synthesis of the biologically significant compound urea by accidentally evaporating an aqueous solution of ammonium cyanate NH4OCN, now called the Wöhler synthesis. The name "organic" comes from the earlier mistaken assumption that carbon chains, and even such smaller carbon compounds, could only be produced biologically. A large part of organic chemistry still coincides with biochemistry, the study of the molecules in living organisms.
Characteristics of organic substances
Organic compounds are generally covalently bonded. This allows for unique structures such as long carbon chains and rings. The reason carbon is excellent at forming unique structures and that there are so many carbon compounds is that carbon atoms form very stable covalent bonds with one another (catenation). In contrast to inorganic materials, organic compounds typically melt, boil, sublimate, or decompose below 300°C. Neutral organic compounds tend to be less soluble in water compared to many inorganic salts, with the exception of certain compounds such as ionic organic compounds and low molecular weight alcohols and carboxylic acids where hydrogen bonding occurs. Organic compounds tend to be much more soluble in organic solvents such as ether or alcohol, but the solubility in each solute depends upon the functional groups present and on the overall structure. Like inorganic salts, organic compounds form crystals. Another unique property of carbon in organic compounds is the ease of formation of carbon carbon double bonds and triple bonds. When these bonds are arranged in a special way it gives rise to conjugated systems and aromaticity.
Categories of organic substances
Because so very many compounds exist, a clear, unambiguous naming system is necessary. Organic nomenclature is the system established for naming and grouping organic compounds. Organic substances are classified by their molecular structural arrangement, and by what other atoms are present: hydrogen is implicitly assumed. Other atoms such as O, N, or Cl almost always bond in certain relative ways, forming functional groups. In chemistry, structure is quite synonymous with function, and so the structural categories double as categories of property or activity. The main organizational categories are aliphatic compounds such as alkanes, aromatic compounds such as benzene, and heterocyclic compounds such as pyrrole and indole.
Examples of functional group-based categories are alcohols, aldehydes, ketones, amides, amines, carboxylic acids, ethers and esters.
Organic compounds containing bonds of carbon to nitrogen, oxygen and the halogens are considered part of regular organic chemistry. Those with carbon connected to other elements are often treated separately such as in organosulfur chemistry, organophosphorus chemistry and organosilicon chemistry.
Polymers
Polymers consist of long chains of repeating segments of smaller molecular units, the monomers. When the segments are all the same the molecule is called a homopolymer; when the segments vary in chemical structure the molecule is called a heteropolymer. Common synthetic organic polymers include polyethylene, polypropylene, nylon, polyesters, and polymethyl methacrylate. There are also a few common inorganic polymers such as the silicones.
Biomolecules
Biomolecular chemistry is a major category within organic chemistry. Many complex multi-functional group molecules are important in living organisms. Some are long-chain biopolymers. The main classes are carbohydrates, amino acids and proteins, polysaccharides, lipids, and nucleic acids.
Molecular structure of an organic compound
Organic compounds are generally made from the building blocks of carbon atoms, hydrogen atoms, and functional groups. The valence of carbon is 4, and hydrogen is 1, functional groups are generally 1. Many, but not all structures can be envisioned by the simple valence rule that there will be one bond for each valence number. The knowledge of the chemical formula for an organic compound is not sufficient information because many isomers can exist. Organic compounds often exist as mixtures. Because many organic compound have relatively low boiling points and/or dissolve easily in organic solvents there exist many methods for separating mixtures into pure constituents that are specific to organic chemistry such as distillation, crystallization and chromatography techniques. Currently, there exist several methods for deducing the structure an organic compound. In general usage are (in alphabetical order):
Additional methods are provided by analytical chemistry.
Organic reactions
Organic reactions are chemical reactions involving organic compounds. While pure hydrocarbons undergo certain classes of reactions, many more reactions which organic compounds undergo is largely determined by functional groups. The general theory of these reactions involves careful analysis of such properties as the electron affinity of key atoms, and bond strengths. These issues can determine the relative stability of short-lived reactive intermediates, which usually directly determine the path of the reaction. A common reaction is generically written here as an example:
- R-F + X-Y → R-Y + X-F
where F is some functional group such as the hydroxyl or -OH group . It is presumed that functional group F is bonded to one of the carbon atoms in R. R is often one of the hydrocarbon categories mentioned previously. The example above is a substitution reaction, since Y is substituted for F.
Important concerns of a reaction include whether it will occur spontaneously determined by the Gibbs free energy, what heat is produced or needed in terms of Enthalpy and what unintended products are formed as well.
See also
References
- Robert T. Morrison, Robert N. Boyd, and Robert K. Boyd, Organic Chemistry, 6th edition (Benjamin Cummings, 1992, ISBN 0136436692) - this is "Morrison and Boyd", a classic textbook
- Richard F. and Sally J. Daley, Organic Chemistry, www.ochem4free.com, Online organic chemistry textbook.
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