Organic Chemistry

254 CHAPTER 6 Reactions of Alkynes • Introduction to Multistep Synthesis

PROBLEM 20 SOLVED

A chemist wants to synthesize 4-decyne but cannot find any 1-pentyne, the starting mater-

ial used in the synthesis just described. How else can 4-decyne be synthesized?

SOLUTION The spcarbons of 4-decyne are bonded to a propyl group and to a pentyl

group. Therefore to obtain 4-decyne, the acetylide ion of 1-pentyne can react with a pentyl

halide or the acetylide ion of 1-heptyne can react with a propyl halide. Since 1-pentyne is

not available, the chemist should use 1-heptyne and a propyl halide.

6.11 Designing a Synthesis I:

An Introduction to Multistep Synthesis

For each reaction we have studied so far, we have seen whythe reaction occurs,howit

occurs, and the products that are formed. A good way to review these reactions is to

design syntheses, because in doing so, you have to be able to recall many of the reac-

tions you have learned.

Synthetic chemists consider time, cost, and yield in designing syntheses. In the in-

terest of time, a well-designed synthesis should require as few steps (sequential reac-

tions) as possible, and those steps should each involve a reaction that is easy to carry

out. If two chemists in a pharmaceutical company were each asked to prepare a new

drug, and one synthesized the drug in three simple steps while the other used 20 diffi-

cult steps, which chemist would not get a raise? In addition, each step in the synthesis

should provide the greatest possible yield of the desired product, and the cost of the

starting materials must be considered. The more reactant needed to synthesize one

gram of product, the more expensive it is to produce. Sometimes it is preferable to de-

sign a synthesis involving several steps if the starting materials are inexpensive, the re-

actions are easy to carry out, and the yield of each step is high. This would be better

than designing a synthesis with fewer steps that require expensive starting materials

and reactions that are more difficult or give lower yields. At this point you don’t know

how much chemicals cost or how difficult it is to carry out certain reactions. So, for the

time being, when you design a synthesis, just try to find the route with the fewest

steps.

The following examples will give you an idea of the type of thinking required for

the design of a successful synthesis. This kind of problem will appear repeatedly

throughout this book because working such problems is a good way to learn organic

chemistry.

Example 1.Starting with 1-butyne, how could you make the following ketone? You

can use any organic and inorganic reagents.

Many chemists find that the easiest way to design a synthesis is to work backward. In-

stead of looking at the starting material and deciding how to do the first step of the

synthesis, look at the product and decide how to do the last step. The product is a ke-

tone. At this point the only reaction you know that forms a ketone is the addition of

water (in the presence of a catalyst) to an alkyne. If the alkyne used in the reaction has

identical substituents on each of the spcarbons, only one ketone will be obtained.

Thus, 3-hexyne is the best alkyne to use for the synthesis of the desired ketone.

CH 3 CH 2 CCHCH 3 CH 2 CCH 2 CH 2 CH 3

O

?

1-butyne

1-heptyne 4-decyne

1. NaNH 2
   CH 3 CH 2 CH 2 CH 2 CH 2 C CH 2. CH 3 CH 2 CH 2 Cl CH 3 CH 2 CH 2 C CCH 2 CH 2 CH 2 CH 2 CH 3

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