Question

Would 2-chloropropane or 1-chloro-2,2-dimethylpropane undergo substitution faster with `"HC≡C"^-"`, `"Na"^+`?

Answer 1

2-Chloropropane will undergo substitution faster than 1-chloro-2,2-dimethylpropane.

The acetylide anion, HC≡C⁻, is the conjugate base of the extremely weak acid, acetylene.

So it is both a powerful nucleophile and an extremely strong base — about a billion times as strong as hydroxide ion.

It undergoes `"S"_"N"2"` reactions with methyl and primary alkyl halides.

But acetylide ion is such a strong base that it undergoes mainly `"E2"` eliminations with 2° alkyl halides.

And it undergoes only `"E2"` elimination with 3° alkyl halides.

An `"S"_"N"2"` reaction goes by backside attack at the α carbon atom.

1-Chloro-2,2-dimethylpropane is completely hindered to backside attack.

The only product is the elimination product.

2-Chloropropane is less hindered.

The acetylide ion can attack more readily, so its substitution reaction is faster than on the 3° halide.