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The root word for the name organic is the same as for multiple Greek words like organoo I arrange, or organotheteo I set forth in a table. This is the same word that forms the root for English words like organism and organize. Thus the name organic does not mean "associated with life." It does, however, mean "having an organized structure." And no compounds not containing carbon are known to have the intricate structures of some organic compounds.
A set of numeric infixes is available to tell the chemist exactly how many carbon atoms a given string or ring of atoms contains. The infixes are an infinite series beginning with these entries:
|Infix||Number of atoms|
Families of Organic Compounds
Organic compounds exist in multiple families, each of which has certain distinguishing properties that derive from the basic structures of these compounds.
These have mostly single covalent bonds between successive carbon atoms, though some of the bonds might be double or even triple (but never quadruple). They may exist as strings or rings.
- Alkanes have only single bonds between carbon atoms. They are named by adding the suffix -ane to the numeric infix--thus, methane, ethane, propane, and butane for the alkanes having one, two, three, or four carbon atoms. (If no other element is listed as binding to any of these atoms, then hydrogen is assumed to be the other element.)
- Alkenes have occasional double bonds but never any triple bonds. Of necessity, the number of carbon atoms must be two or more--that is, no such compound as "methene" can exist. Alkenes are named by adding the suffix -ene to the numeric infix. Where any doubt might exist as to where the double bond might be located, a numeral giving the position of the bond is prefixed to the name. Thus, while ethene and propene can have only one possible form, at least two forms of butene exist, depending on whether the double bond is at the end of the four-carbon string or in the middle. The two forms would be called 1-butene and 2-butene.
- Alkynes have at least one triple bond.
- An aliphatic residue is any group of carbon atoms that might bind to the middle of another chain to form a branch. Such residues are called alkyl, alkenyl, or alkynyl groups. By convention, one prefixes the name of the group to the name of the base string or ring that it binds to, with a numeral added to indicate the position of the binding.
- An aliphatic string having three or more carbon atoms might bind the last string in the chain back to the first. The result is a ring, and its name is formed by adding the prefix cyclo- to the rest of the name. Thus, cyclohexane is a six-member alkane in which the sixth carbon atom is bound back to the first, forming a ring. One of the earliest anesthetics used was cyclopropane, (CH2)3.
The chief feature of aromatic hydrocarbons is a special type of ring structure called an aromatic ring. It has six carbon atoms in it, but can bind only one extra atom at each corner instead of the two one would normally expect. It is as if every other bond in the ring is a double bond--but that is not accurate, either, as the ring is a perfect, planar hexagon. Thus the aromatic ring is an instance of a resonance structure.
An organic compound is aromatic if and only if it has at least one aromatic ring structure as part of it. The most basic aromatic hydrocarbon is benzene, which has the simple formula C6H6.
The association of carbon with oxygen gives rise to another set of organic-compound families:
An alcohol has at least one hydroxyl (-OH) group. By convention, alcohols have names ending in -ol. The general case of an alcohol has the generic graphic formula R-OH, where R is the rest, or residue, of the molecule.
Aldehydes have an aldehyde group consisting of an atom of carbon double-bonded to oxygen, with an organic residue on one end of this group and a hydrogen atom on the other. Aldehydes have names ending in -al and have the general graphic formula R-CHO. (Note: the first four aldehydes have slightly different traditional names: formaldehyde, acetaldehyde, propionaldehyde, and butyraldehyde.)
- Main Article: Carboxylic acid
At the highest oxidation state is the carboxylic acid. The hallmark of such an acid is the carboxylic group, which has a carbon atom having a double-bonded oxygen atom and a hydroxyl group bonded to it, with one space left over to bind to the rest of the molecule. The general formula for a carboxylic acid is R-COOH. Carboxylic acids are water-soluble, and when they dissolve, the hydroxyl group yields its proton to a water molecule; the result is a hydronium ion and a monovalent organic anion called a carboxylate ion. The formula now is R-COO-, and its structure undergoes a key change: instead of having one double-bonded oxygen and one single-bonded oxygen, the carbon atom and the two oxygen atoms form a resonance structure.
A carboxylic acid might be liquid at 25 degrees Celsius, in which case its pure form is often called the glacial form. The pure form of the acid is named by adding the suffix -anoic to the numeric infix and following this word with the word acid--hence, methanoic acid, ethanoic acid, propanoic acid, butanoic acid, pentanoic acid, and so forth. The name of the dissociated, or anionic, form has the terminal -ic syllable changed to -ate. (Note: the first four carboxylic acids have the traditional names formic acid, acetic acid, propionic acid, and butyric acid.)
An ether has two aliphatic residues bound one on each side to an atom of oxygen. The general graphic formula is R-O-R' where R and R' represent two different residues (which could still be identical). The first-ever anesthetic used in surgery was diethyl ether, CH3CH2OCH2CH3, the "ether" of common medical-historical and cultural parlance.
An ester has two aliphatic residues bound one on each side to a carbon atom with a double-bonded oxygen atom bound to it. It is as if an aldehyde group had two residues bound to it instead of only one. Its general formula is R-CO-R'.
Nitrogen adds a few properties of its own. The most common nitrogen-containing families are:
- Main Article: Amino compound
An amino compound has an amino group (NH2) bound to its chain at any point. The amino acids are a key subset of the amino family, having the general graphic formula NH2CHRCOOH (where R can also be H; the result is glycine, the simplest of the amino acids).
These compounds have at least two nitrogen atoms joined by a double bond.
These compounds have at least one group consisting of a carbon atom triply bound to a nitrogen atom. The general graphic formula is RCN. If R = H, the result is hydrogen cyanide. More typically, CN- exists as a monovalent anion associated with any of a number of monovalent cations. (Warning. The cyanides are some of the deadliest poisons known to man.)
Alternative Molecular Forms
- Main Article: Isomer
Organic compounds having more than a minimum number of atoms often form isomers. An isomer (Greek isos the same and meros a part) of a compound is any other compound having an identical simple formula (that is to say, ratio of constituent elements) but a different structure. Different isomers have different properties and thus cannot be considered to be the same substance.
Some simple compounds, typically the alkenes, have the uncanny ability to join together to form chains of almost infinite length. Such a chain is called a polymer. A polymer is typically named by adding the prefix poly- (from the Greek polys much or many) to the name of the simpler compound, called the monomer, from which it is formed. Thus, polyethylene is an almost infinitely long single-chain alkane made from multiple molecules of ethylene, or ethene, that have joined together. Other common polymers include polypropylene and polyvinyl chloride.
In addition, carbon itself is known to exist in two distinct multi-atomic forms. They are:
- Diamond, which consists of carbon atoms binding to one another in an infinite three-dimensional aliphatic ring structure.
- Graphite, which consists of an infinite structure consisting of fused aromatic rings.
Applications of Organic Chemistry
Organic compounds are ubiquitous in nature and form several of the key constituents of life. These include:
- The amino acids, and the proteins formed from them.
- The nucleic acids DNA and RNA, and all their constituent compounds.
In addition, all fossil fuels are organic compounds.