Removing a hydrogen from an alkane results in an alkyl substituent (or an alkyl group). Alkyl substituents are named by replacing the “ane” ending of the alkane with
“yl.” The letter “R” is used to indicate any alkyl group.
If a hydrogen of an alkane is replaced by an OH, the compound becomes an alcohol; if it is replaced by an NH2, the compound becomes an amine; if it is replaced by a halogen, the compound becomes an alkyl halide; and if it is replaced by an OR, the compound becomes an ether.
The alkyl group name followed by the name of the class of the compound (alcohol, amine, and so on) yields the common name of the compound. The two alkyl groups in ethers are stated in alphabetical order. The following examples show how alkyl group names are used to build common names:
CH3OH methyl alcohol
CH3CH2NH2 ethylamine
CH3CH2CH2Br propyl bromide
CH3CH2CH2CH2Cl butyl chloride CH3I
methyl iodide
CH3CH2OH ethyl alcohol
CH3CH2CH2NH2 propylamine
CH3CH2OCH3 ethyl methyl ether
R OH
an alcohol
R O R
an ether R NH2
an amine
R X X = F, Cl, Br, or I an alkyl halide
CH3 a methyl group
CH3CH2 an ethyl group
CH3CH2CH2 a propyl group
CH3CH2CH2CH2 a butyl group R
any alkyl group CH3CH2CH2CH2CH2
a pentyl group
Notice that there is a space between the name of the alkyl group and the name of the class of compound, except in the case of amines where the entire name is written as one word.
3-D Molecule:
Isopropyl chloride
PROBLEM 1♦
Name the following:
a. b. c.
Two alkyl groups—the propyl group and the isopropyl group—have three carbons.
A propyl group is obtained when a hydrogen is removed from a primary carbon of propane. A primary carbon is a carbon bonded to only one other carbon. An iso- propyl group is obtained when a hydrogen is removed from the secondary carbon of propane. A secondary carbon is a carbon bonded to two other carbons. Notice that an isopropyl group, as its name indicates, has its three carbons arranged as an iso structur- al unit.
Molecular structures can be drawn in different ways. Isopropyl chloride, for ex- ample, is drawn here in two ways. Both representations depict the same compound, although, at first glance, the two-dimensional representations appear to be differ- ent: the methyl groups are placed at opposite ends in one structure and at right an- gles in the other. The structures are identical, however, because carbon is tetrahedral. The four groups bonded to the central carbon—a hydrogen, a chlorine, and two methyl groups—point to the corners of a tetrahedron. (If you visit the Molecule Gallery in Chapter 3 of the website (www.chemplace.com), you will be able to actually rotate isopropyl chloride to prove that the two molecules are identical.)
CH3CHCH3
Cl
CH3CHCl CH3
isopropyl chloride isopropyl chloride two different ways to draw isopropyl chloride CH3CH2CH2
a propyl group
CH3CHCH3 an isopropyl group a primary carbon a secondary carbon
CH3CH2CH2Cl propyl chloride
CH3CHCH3 isopropyl chloride
Cl
Mentally rotate the model on the right clockwise until it looks the same as the model on the left.
There are four alkyl groups that contain four carbons. Two of them, the butyl and isobutyl groups, have a hydrogen removed from a primary carbon. A sec-butyl group has a hydrogen removed from a secondary carbon (sec-, often abbreviated s-, stands for secondary), and a tert-butyl group has a hydrogen removed from a tertiary carbon (tert-, sometimes abbreviated t-, stands for tertiary). A tertiary carbon is a carbon that is bonded to three other carbons. Notice that the isobutyl group is the only group with an iso structural unit.
The names of straight-chain alkyl groups sometimes have the prefix “n” (for “nor- mal”), to emphasize that its carbons are in an unbranched chain. If the name does not have a prefix such as “n” or “iso,” we assume that the carbons are in an unbranched chain.
butyl bromide pentyl fluoride
or or
n-butyl bromide n-pentyl fluoride
A chemical name must specify one compound only. The prefix “sec,” therefore, can be used only with sec-butyl. The name “sec-pentyl,” for example, cannot be used because pentane has two different secondary carbons, which means that remov- ing a hydrogen from a secondary carbon of pentane produces one of two different alkyl groups, depending on which hydrogen is removed. As a result, “sec-pentyl chloride” would specify two different alkyl chlorides, and, therefore, it is not a correct name.
If you examine the following structures, you will see that whenever the prefix “iso”
is used, the iso structural unit will be at one end of the molecule and any group replac- ing a hydrogen will be at the other end:
Alkyl group names are used so frequently that you should learn them. Some of the most common alkyl group names are compiled in Table 2 for your convenience.
CH3CHCH2CH2CH2Cl CH3
isohexyl chloride CH3CHCH2CH2OH
CH3
isopentyl alcohol
CH3CHCH2Br CH3 isobutyl bromide
CH3CHCH2NH2
CH3 isobutylamine
CH3CHBr CH3 isopropyl bromide CH3CHCH2CH2OH
CH3
isopentyl alcohol CH3CHCH2CH2CH3
Cl
Both alkyl halides have five carbon atoms with a chlorine attached to a secondary carbon, so both compounds would be named sec-pentyl chloride.
CH3CH2CHCH2CH3
Cl
CH3CH2CH2CH2CH2F CH3CH2CH2CH2Br
CH3CH2CH2CH2 a butyl group
CH3CHCH2
CH3 an isobutyl group
CH3CH2CH CH3 a sec-butyl group
CH3C CH3
CH3
a tert-butyl group a secondary carbon a tertiary carbon
a primary carbon a primary carbon
A primary carbon is bonded to one carbon, a secondary carbon is bonded to two carbons, and a tertiary carbon is bonded to three carbons.
Tutorials:
Common names of alkyl groups
PROBLEM 3♦
Write a structure for each of the following:
First, determine the number of carbons in the longest continuous chain.
Table 2 Names of Some Alkyl Groups
CH3C CH3
CH3
CH3CH2CH2CH2CH2 pentyl
CH3CHCH2CH2 isopentyl
CH3
CH3 CH3CH2CH sec-butyl
tert-butyl CH3
CH3
methyl
CH3CH2 ethyl
CH3CH2CH2
propyl
CH3CH isopropyl
CH3 CH3CHCH2 isobutyl
CH3CH2CH2CH2 butyl
CH3CH2CH2CH2CH2CH2
hexyl
CH3CHCH2CH2CH2
isohexyl
CH3
a. isopropyl alcohol b. isopentyl fluoride
c. ethyl propyl ether d. sec-butyl iodide
e. tert-butylamine f. n-octyl bromide
PROBLEM 4♦
Name the following:
c. CH3CH2CHNH2 CH3
e. CH3CHCH2Br CH3
f. CH3CH2CHCl CH3 a. CH3OCH2CH3
b. CH3OCH2CH2CH3 d. CH3CH2CH2CH2OH
PROBLEM 2
Draw the structures and name the four constitutional isomers with molecular formula C4H9Br.
PROBLEM 5♦
Draw the structure and give the systematic name of a compound with molecular formula C5H12that has
a. one tertiary carbon. c. no secondary or tertiary carbons.
b. three secondary carbons.