Alkanes are extremely unreactive. Carbon-carbon and carbon-hydrogen bonds are among the most stable bonds in chemistry, so alkanes seldom undergo reactions. Thus, alkanes alone are not typically used in reactions in organic chemistry. Instead, they serve as a backbone or template on which unreactive carbon or hydrogen atoms can be replaced by substituents consisting of more reactive atoms or groups of atoms.
A substituent consisting of an atom or group of atoms other than carbons and hydrogens is called a functional group. Functional groups are significant because they are the part of the molecule that undergoes reactions. One functional group may change into another one, or a functional group might react with a separate molecule to build up a larger structure. Functional groups are the essential "reacting units" in organic chemistry.
We have already encountered two functional groups. The double bonds in alkenes and the triple bonds in alkynes are able to undergo reactions that the single bonds in alkanes cannot. Thus, double and triple bonds are the first and simplest examples of functional groups. There are several other significant functional groups, summarized in the table below. Note that in organic chemistry, the letter "R" represents any alkane, and the letter "X" represents any halogen.
One of the simplest functional groups is the alkyl halide. In an alkyl halide, one of the hydrogen atoms in an alkane has been replaced by a halogen. Alkyl halides are easy to name. The name of the alkane is preceded by the number of the carbon on which the halogen is substituted and the name of the halogen, modified so that -ine is replaced by -o (e.g. 2-bromopropane). Alkyl halides are treated the same as alkyl substituents; if a molecule contains both, they are ranked to give the lowest possible number to a substituent and then listed in alphabetical order. If a molecule contains an alkyl halide and a multiple bond, numbers are assigned to give the lowest number to the first functional group. In the event of a tie, the lowest number goes to the multiple bond.
The alcohol is a very common functional group and a very easy one to name. The molecule is named as if it were an alkane (or alkene or alkyne), except that the suffix -ane is replaced by -ol and the number of the carbon atom on which the -OH group is located is placed before the name of the compound (e.g. 2-butanol). The alcohol functional group takes precedence over alkyl substituents, multiple bonds, and halides and always gets the lowest number.
An ether is a molecule consisting of two alkyl groups connected to an oxygen atom. Ethers are named by considering one alkyl group (the shorter one) plus the oxygen atom to be a substituent and the other alkyl group (the longer one) to be an alkane. The alkyl group plus oxygen atom is called an "alkoxy" substituent and is named by replacing -ane suffix from the alkane with -oxy (e.g. methane becomes methoxy). The allkoxy substituent gets priority over alkyl and halide substituents, but not over alcohols, which will get the lower number.
An amine is a derivatives of the molecule ammonia, NH3, in which one or more of the hydrogens has been replaced by an alkyl substituent (R group). Amines are named by replacing the suffix -ane with -amine, like in alcohols. If there are further substituents attached to the nitrogen atom, they are preceded by N-. The amine gets the lowest number.
|3. Naming functional groups
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