RADICALS AND FUNCTIONAL GROUPS

18.6. RADICALS AND FUNCTIONAL GROUPS

The millions of organic compounds other than hydrocarbons can be regarded as derivatives of hydrocarbons, where one (or more) of the hydrogen atoms on the parent molecule is replaced by another kind of atom or group of atoms. For example, if one hydrogen atom in a molecule of methane, CH 4 , is replaced by an —OH group, the resulting compound is methanol, also called methyl alcohol, CH 3 OH. (This replacement is often not easy to perform in the laboratory and is meant in the context used here as a mental exercise.) In most cases, the compound is named in a manner that designates the hydrocarbon parent from which it was derived. Thus, the word methyl is derived from the word methane. The hydrocarbon part of the molecule is often called the radical. To name a radical, change the ending of the parent hydrocarbon name from -ane to -yl. The names of some common radicals

are listed in Table 18-3. Note that the radical derived from benzene, C 6 H 5 —, is called the phenyl radical. Some other radicals are also given names that are not derived from the names of the parent hydrocarbons, but these other cases will not be discussed here. Since, in many reactions, the hydrocarbon part of the organic compound is not changed and does not affect the nature of the reaction, it is useful to generalize many reactions by using

the symbol R— to denote any radical. Thus, the compounds CH 3 Cl, CH 3 CH 2 Cl, CH 3 CH 2 CH 2 Cl, and so forth, all of which undergo similar chemical reactions, can be reprersented by the formula RCl. If the radical is derived

ORGANIC CHEMISTRY

[ CHAP. 18

Table 18-3 Some Common Radicals

Parent Hydrocarbon

Radical

Name

Line Formula

Name

Line Formula

Structural Formula

Methane

CH 4 Methyl

CH 3 —

H H Ethane

CH 3 CH 3 Ethyl

CH 3 CH 2 —

H H H Propane

CH 3 CH 2 CH 3 Propyl

CH 3 CH 2 CH 2 —

H H H Benzene

C 6 H 6 Phenyl

from an alkane, the radical is called an alkyl radical; and if it is derived from an aromatic hydrocarbon, it is called an aryl radical.

The names of alkanes with radical branches use the name from Table 18-2 for the longest continuous chain of carbon atoms. The radicals are named using the -yl ending presented in Table 18-3, and their positions along the carbon chain are denoted by a number. For example,

CH 3 CH 2 CH CH 2 CH 2 CH 3 CH 3

is called 3-methylhexane. The longest continuous carbon chain is six carbon atoms long, therefore it is a hexane. The single-carbon branch is the methyl group, which is on the third carbon atom from the left end of the chain. We actually start counting from the end of the chain that gives the lower number . In this case, the branch is on the fourth carbon atom from the right, so the smaller number is used. Be sure to use the longest continuous chain of carbon atoms; they are not always presented on a horizontal line .

If there are two or more branches, each is named. If there are two or more identical branches, the prefixes from Table 6-2 are used. Thus the following names:

CH 3 CH CH CH 2 CH 2 CH 3 CH 3 CH CH CH 2 CH 2 CH 3

CH 3 CH 2 CH 3 CH 3 CH 3

2-methyl-3-ethylhexane 2,3-dimethylhexane When a hydrogen atom of an alkane or aromatic hydrocarbon molecule is replaced by an atom of another

element or group of atoms containing another element, the hydrocarbon-like part of the molecule is relatively inert, like these hydrocarbons themselves. Therefore, the resulting compound will have properties characteristic of the substituting group. Specific groups of atoms responsible for the characteristic properties of the com- pound are called functional groups. For the most part, organic compounds can be classified according to the

CHAP. 18]

ORGANIC CHEMISTRY

Table 18-4 Formulas for Functional Groups

Characteristic

Type

Functional Group

Example

Alcohol

OH

R OH or ROH

Ether

R O R ′ or ROR ′

Aldehyde

R C O or RCHO

Ketone

C R C R ′ or RCOR ′

Acid

C OH

R C OH or RCO 2 H

Ester

R C OR ′ or RCO 2 R ′

Amine

NH 2 , NH,

R NH 2 , RNHR ′, RR′R′′N

Amide

R C N R ′ or RCONRR ′

O R ′′

The radicals labeled R ′ or R ′′ may be the same as or different from the radicals labeled R in the same compounds.

functional group they contain. The most important classes of such compounds include (1) alcohols, (2) ethers, (3) aldehydes, (4) ketones, (5) acids, (6) amines, (7) esters, and (8) amides. The general formulas for these classes are given in Table 18-4, where the symbol for a radical, R, is written for the hydrocarbon part(s) of the molecule.

Molecules of some compounds contain more than one functional group, and there may be even more than one kind of functional group in each molecule. The purpose of this discussion is merely to describe some of the possible compounds. Therefore, the methods of preparation of the various classes of compounds will not be given here for every class, nor will more than a few of their properties be described.