Organic Chemistry

256 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

Example 3.How could 2,6-dimethylheptane be prepared from an alkyne and an alkyl

halide? (The prime on signifies that R and are different alkyl groups.)

2,6-Dimethyl-3-heptyne is the only alkyne that will form the desired alkane upon hy-

drogenation. This alkyne can be dissected in two different ways. In one case the alkyne

could be prepared from the reaction of an acetylide ion with a primary alkyl halide

(isobutyl bromide); in the other case the acetylide ion would have to react with a sec-

ondary alkyl halide (isopropyl bromide).

Because we know that the reaction of an acetylide ion with an alkyl halide works best

with primary alkyl halides and methyl halides, we know how to proceed:

Example 4.How could you carry out the following synthesis using the given starting

material?

An alcohol can be prepared from an alkene, and an alkene can be prepared from an

alkyne.

You can use either of the two methods you know to convert an alkyne into an alkene,

because this alkene does not have cis–trans isomers. Hydroboration–oxidation must be

used to convert the alkene into the desired alcohol, because acid-catalyzed addition of

water would not form the desired alcohol.

Example 5.How could you prepare (E)-2-pentene from ethyne?

CC

CH 3 CH 2

HCH 3

H

HC CH

?

(E)-2-pentene

CH 2 CH 2 OH CH CH 2 CCH

retrosynthetic analysis

?

C CH CH 2 CH 2 OH

CH 3 CHCH 2 CH 2 CH 2 CHCH 3 CH 3 CHCH 2 C CCHCH 3

CH 3 CH 3 CH 3

CH 3 CHCH 2 Br + HC CCHCH 3 or

CH 3 CH 3

CH 3 CHBr + HC CCH 2 CHCH 3

CH 3 CH 3

CH 3

retrosynthetic analysis

RC CH

CH 3 CH 3

R′Br CH 3 CHCH 2 CH 2 CH 2 CHCH 3

?

+

2,6-dimethylheptane

R¿ R¿

Lindlar

catalyst

or Na/NH 3 (liq)

synthesis

1. BH 3

CCH CH CH (^2) 2. HO−, H 2 O 2 , H 2 O CH 2 CH 2 OH
H 2

1. NaNH 2


2. CH 3 CHCH 2 Br Pd/C

H 2

CH 3 CH 3

CH 3 CHC CH

CH 3 CH 3

CH 3 CHCH 2 C

CH 3 CH 3

CCHCH 3 CH 3 CHCH 2 CH 2 CH 2 CHCH 3

synthesis

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