Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Elimination Reactions of Organic Chemistry
12.5 Applications of Elimination Reactions to Synthesis

Now we need to concentrate on the application of elimination reactions to synthesize specific target molecules that have double bonds. The same approach should be used as discussed in previous sections; that is, identify the functional group in the target molecule, in this case the double bond, and determine the best type of reaction and needed molecule that can be used to synthesize the target molecule that contains the double bond functionality. This concept is illustrated below.

The best approach in solving synthesis problems is to work backward by looking carefully at the target molecule and making some important observations. For example, identify the functional group present and then determine how best to make that functional group based on the reactions that we have discussed. The strategy that can be used is that an appropriate starting material to make the alkene functionality would be an alkyl halide, with of course the same carbon skeleton. An elimination reaction involving an alkyl halide that should produce the desired product is shown in Reaction (12-49).

(12-49)

You will readily recognize that there is another possibility as shown below in Reaction 12-50.

(12-50)

To determine the possible reactions that can be used, appropriate reagents and reactions conditions for an elimination reaction are needed, as shown for the reactions discussed in section 12.3 and given in Reactions (12-51) and (12-52) (note that the reactions are shown in reverse).

(12-51)

(12-52)

We will see in Chapter 14 that one of these reactions is a better choice, compared to the other based on the ease in the synthesis of the starting compound.

Problem 12.8

Starting with any alkyl halide, show how to synthesize the following compounds.

Let us now turn our attention to the combination of reactions studied so far for the synthesis of a target molecule starting from a specific starting compound (multistep synthesis), as shown in Reaction (12-53).

(12-53)

Using the strategy described earlier in this section shows that an oxidation of an alcohol could be used to synthesize the target molecule, which means that the intermediate molecule would have to be an alkene. We learned in this chapter that elimination reaction of alkyl halides results in the formation of alkenes. Thus, the strategy outlined below could be used for this transformation.

Once the strategy is worked out, we now need to select appropriate reactions and reaction conditions to accomplish the multistep synthesis, a possibility is outlined in Reaction (12-54).

(12-54)

Problem 12.9

Using a multi-step approach, show how to carry out the following transformations.