Working with the Concepts: Planning a Synthesis
Suggest starting materials for the preparation (synthesis) of \(\mathrm{CH}_{3}
\mathrm{CH}_{2} \mathrm{SCH}_{3}\). Strategy The question does not specify a
method for preparing this molecule, but it makes sense to use our newest
reaction, nucleophilic substitution. A powerful method for designing synthetic
preparations involves working backward from the structure of the target
molecule and is called retrosynthetic analysis. We demonstrate the idea here
and will return to it in Section 8-9. Begin by rephrasing the question as
"What substances must react by nucleophilic substitution to give the desired
product?" Write the structure in full, so as to see clearly all the bonds it
contains, and identify one that might be formed in the course of nucleophilic
substitution. Solution Example 5 from Table 6-3 gives us a model for a
reaction that forms a sulfur compound with two \(\mathrm{C}-\mathrm{S}\) bonds.
Proceed in the same way, even though the problem does not tell us which halide
leaving group to displace by the sulfur nucleophile. We may choose any one
that will work, namely, chloride, bromide, or iodide:
We finish by writing the preparation in the forward direction, the way we
would actually carry it out:
\(\mathrm{CH}_{3} \ddot{\mathrm{S}}:^{-}+\mathrm{CH}_{3} \mathrm{CH}_{2}
\mathrm{Br}: \longrightarrow \mathrm{CH}_{3} \ddot{\mathrm{S}} \mathrm{CH}_{2}
\mathrm{CH}_{3}+: \ddot{\mathrm{Br}}:^{-}\)
Notice that we could have just as easily conducted our reverse analysis by
deleting the bond between the sulfur and the methyl carbon, rather than the
ethyl. That would give us a second, equally correct method of preparation:
\(\mathrm{CH}_{3} \ddot{\mathrm{I}}: \stackrel{-}{*} \ddot{\mathrm{S}}
\mathrm{CH}_{2} \mathrm{CH}_{3} \longrightarrow \mathrm{CH}_{3}
\ddot{\mathrm{S}} \mathrm{CH}_{2} \mathrm{CH}_{3}+\ddot{\mathrm{I}}:\)
As before, the choice of halide leaving group is immaterial.
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