(A) The reaction of $CH_3-CH_2-CH=CH-CH_3$ (pent$-2-$ene) with $HCl$ is an electrophilic addition reaction.
The reaction proceeds via the formation of carbocation intermediates.
$1$. Protonation of the double bond by $H^+$ leads to two possible carbocations:
- $CH_3-CH_2-CH^+-CH_2-CH_3$ (a secondary carbocation).
- $CH_3-CH_2-CH_2-CH^+-CH_3$ (also a secondary carbocation,but more stable due to the inductive effect of the propyl group).
$2$. Attack by the chloride ion $(Cl^-)$ on these carbocations yields the products:
- Attack on the more stable carbocation gives $CH_3-CH_2-CH_2-CHCl-CH_3$ ($2$-chloropentane),which is the major product $(A)$.
- Attack on the other carbocation gives $CH_3-CH_2-CHCl-CH_2-CH_3$ ($3$-chloropentane),which is the minor product $(B)$.