Question

Which of the following is more reactive towards addition of $HCl$?

• A.
• B.
• C.
• D.

Answer 1

The correct option is D

Addition of HX on unsymmetrical alkynes takes place according to Markovnikov’s rule.
The intermediate carbocation is formed by markovnikov addition of HCl to the give compounds.
In compound (d), the intermediate carbocation is stabilized by hyper-conjugation exerted by $-C{H}_3$ group.
In compound (b) and (c), the electron withdrawing group at para position withdraw the electron density toward itself and destabilise the intermediate form.
In (a), the intermediate formed is less stable than (d) because it lack electron donating group.
Thus compound (d) is more reactive toward addition of HCl.

and it is more easily formed.