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$H_2C=CH-CH(CH_3)-CH_3 + HBr \rightarrow (X)$
Here (X) (predominantly) is :
Here (X) (predominantly) is :
A
$CH_3-CH(Br)-CH(CH_3)-CH_3$
B
$CH_3-CH(CH_3)-CH(Br)-CH_3$
C
$CH_3-CH(CH_3)-CH_2-CH_2Br$
D
$CH_3-C(Br)(CH_3)-CH_2CH_3$
Detailed Solution
The alkene is 3-methylbut-1-ene. The addition of HBr is an electrophilic addition that proceeds through a carbocation.
Step 1: $H^+$ adds to the terminal $CH_2$ carbon (Markovnikov's rule) to give a secondary carbocation.
$CH_2=CH-CH(CH_3)-CH_3 + H^+ \rightarrow CH_3-\overset{+}{C}H-CH(CH_3)-CH_3$ ($2^\circ$ carbocation, less stable)
Step 2: a 1,2-hydride shift from the adjacent carbon converts it into a more stable tertiary carbocation.
$CH_3-\overset{+}{C}H-CH(CH_3)-CH_3 \xrightarrow{1,2\text{-hydride shift}} CH_3-CH_2-\overset{+}{C}(CH_3)-CH_3$ ($3^\circ$ carbocation, more stable)
Step 3: $Br^-$ attacks the tertiary carbocation.
$CH_3-CH_2-\overset{+}{C}(CH_3)_2 + Br^- \rightarrow CH_3-C(Br)(CH_3)-CH_2CH_3$
The major product (X) is 2-bromo-2-methylbutane, $CH_3-C(Br)(CH_3)-CH_2CH_3$.
Step 1: $H^+$ adds to the terminal $CH_2$ carbon (Markovnikov's rule) to give a secondary carbocation.
$CH_2=CH-CH(CH_3)-CH_3 + H^+ \rightarrow CH_3-\overset{+}{C}H-CH(CH_3)-CH_3$ ($2^\circ$ carbocation, less stable)
Step 2: a 1,2-hydride shift from the adjacent carbon converts it into a more stable tertiary carbocation.
$CH_3-\overset{+}{C}H-CH(CH_3)-CH_3 \xrightarrow{1,2\text{-hydride shift}} CH_3-CH_2-\overset{+}{C}(CH_3)-CH_3$ ($3^\circ$ carbocation, more stable)
Step 3: $Br^-$ attacks the tertiary carbocation.
$CH_3-CH_2-\overset{+}{C}(CH_3)_2 + Br^- \rightarrow CH_3-C(Br)(CH_3)-CH_2CH_3$
The major product (X) is 2-bromo-2-methylbutane, $CH_3-C(Br)(CH_3)-CH_2CH_3$.