Which of the following alkynes is insoluble in conc. `H_2SO_4`?

Options

A. `CH_3C-=C CH_3`
B. `CH_3C-=CH`
C. `CH-=CH`
D. `CH_3CH_2C-=CH`

Answer

C

Explanation

Unlike ethylene, acetylene does not dissolve in conc. `H_2SO_4` because ethylene gives an alkyl carbocation salt, `H_3Coverset(+)(C)H_2HSO_4^(-)`, while if `CH-=CH` were to dissolve in `H_2SO_4`, a bisulphate salt of a vinyl carbocation, `CH_2=overset(+)(C)HHSO_4^(-)` would be formed. The more the s character in the positively charged C, the less stable is the carbocation and the less likely it is to form. On the other hand, vinyl carbocations from other alkynes have electron-repelling R substituents:
`CH_3tooverset(+)(C)=CH-CH_3`
and thus, they are more stable than the vinyl carbocation from acetylene.