Chemistry - A Molecular Science

Chapter 13 Organic Chemistry

Example 13.4

Draw the constitutional isomers of the alkane C

H 6

. 14

Our strategy is to start with the six-carbon continuous chain isomer and then successively reduce the length of the longest continuous c

hain by one carbon atom, while adding alkyl

groups to maintain six carbons. We do this un

til the branching chains are longer than our

continuous chain, at which poi

nt we are done. In the following, the first number is the

length of the longest chain, and the subse

quent number(s) are the lengths of the alkyl

groups. 6 + 0

: Isomer a

in the margin is the six-carbon chain with no side chains.

5 + 1

: One methyl group must be attached to a five-carbon chain. The methyl group

cannot be added to a terminal carbon (positions 1 or 5) because that would simply lengthen the chain and produce Isomer a. In addition, the 2 and 4 positions of a five-carbon chain are identical. Therefore, there are two isomers that have a five-carbon chain and one methyl group.

Isomer b

places the methyl group at position 2, and

Isomer c

places it at position 3. 4 + 2

: An ethyl group cannot be added to a four-carbon chain without lengthening the

chain, so there is no 4 + 2 isomer. If the ethyl gr

oup is placed at positions 1 or 4, the result

is Isomer a. If it is placed at either pos

ition 2 or position 3,

Isomer c is produced.

4 + 1 + 1

: The methyl groups cannot be added to the terminal carbons (positions 1 and

4), which leaves only positions 2 and 3. Adding one methyl to each position produces Isomer d,

and adding both methyl groups to the same position yields

Isomer e

. Note that

the 2 and 3 positions are identical, so there is no 3,3-dimethlybutane. 3 + 2 + 1

: The three-carbon chain is shown in blue in Figure (f) in the margin. Adding an

ethyl group to position 2 lengthens the chain

to four atoms to produce Isomer e.

3 + 3

: As shown in (g), adding a propyl group (three carbon alkyl group) to position 2 of a

three-carbon chain (blue bonds) lengthens the chain to five carbons. The resulting isomer is identical to Isomer b. We conclude that there

are no isomers of C

H 6

that have longest chains of less than four 14

atoms and that there are five constitutional isomers of C

H 6

. 14

STEREOISOMERS Stereoisomers have the same connectivity but differ in the

spatial

arrangement of their

atoms. We will consider two types of stereoisomers:

geometric isomers

and

enantiomers.

(a) hexane

(b) 2-methylpentane

(c) 3-methylpentane

(d) 2,3-dimethylbutane

(e) 2,2-dimethylbutane

(f) 2,2-dimethylbutane

Isomer (e)

(g) 2-methylpentane

Isomer (b)

3 2

4

3 2

5 4

1

1

© by

North

Carolina

State

University