20.4: Prepare a Substituted Acetylene
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A monosubstituted acetylene can be prepared from acetylene by a combination of acid-base reaction followed by an Sn2 displacement. A disubstituted acetylene can be prepared from a monosubstituted acetylene by a similar reaction sequence. Before proceeding, make sure you understand this nomenclature by consulting the relevant section in the chapter on alkynes.
EXAMPLE 5. From acetylene prepare methylacetylene.
My first task is to remove the acidic proton from acetylene to convert it into its conjugate base. I can then use this conjugate base as a nucleophile in an Sn2 reaction with the relevant alkyl halide to yield the desired product.
Something to note here: NaNH2 is an ionic salt. The actual base is NH2– (amide ion). Sodium remains a spectator ion and as such does not directly participate in the process. Second thing to note is that to perform the first step efficiently, a base whose pKa is higher than the pKa of acetylene must be used. Otherwise equilibrium would not be favorable to formation of the conjugate base. In this case, acetylene has a pKa of 25, and NH2– has a pKa of about 38 (as measured by the pKa of its conjugate acid, ammonia). Because 38 is greater than 25, NH2– can be effectively used to deprotonate acetylene.
Disubstituted acetylenes follow a similar reaction sequence, but the deprotonation steps must be conducted separately. Trying to remove both protons from acetylene simultaneously would not work very well because the potential energy of the resulting species is too high. That is to say, it’s not very stable.
Possible but impractical. This species is high energy, therefore very unstable.
EXAMPLE 6. From acetylene prepare methylethylacetylene.
The synthetic sequence has four steps total.