Which one of the following halogen compounds is not useful as a substrate in an $S_N2$ reaction?

Which of the following is not true for $S_{N}1$ reaction?

$A$ bromoalkane $X$ reacts with magnesium in dry ether to form compound $Y$. The reaction of $Y$ with methanal followed by hydrolysis yields an alcohol having molecular formula $C_{4}H_{10}O$. The compound $X$ is

Which of the following alkyl halides gives the most complex mixture of alkenes in an $E_2$ reaction?

Consider the following reaction:
$CH_3-CH_2-CH(Br)-CH_3 \xrightarrow{alc. KOH, \Delta} CH_3-CH_2-CH=CH_2 (I) + CH_3-CH=CH-CH_3 (II)$
Which of the following statements are correct?
$a.$ $I$ is the major product of the reaction
$b.$ $II$ is the major product of the reaction
$c.$ Formation of $I$ is in accordance with Saytzeff's rule
$d.$ $II$ is more stable because it is more substituted

Identify $A, B, C, D, E, R$ and $R^1$ in the following reactions:
$Cyclohexyl-Br + Mg$ $\xrightarrow{dry\,ether} A$ $\xrightarrow{H_2O} B$
$R-Br + Mg$ $\xrightarrow{dry\,ether} C$ $\xrightarrow{D_2O} CH_3-CH(D)-CH_3$
$CH_3-C(CH_3)_2-C(CH_3)_2-CH_3 \xleftarrow{Na/ether} R^1-X$ $\xrightarrow{Mg} D$ $\xrightarrow{H_2O} E$