Ethylene reacts with bromine to form

$CH_3-CHCl-CHCl-CH_3$ $\xrightarrow[dry \ ether]{Na} X$ $\xrightarrow[(1) \ BH_3, THF; (2) \ H_2O_2, OH^-_{(aq)}] Y$; $Y$ is:

Rank the following alkenes in order of decreasing heats of hydrogenation (largest first):
$1$. $1-$butene
$2$. $2-$methyl$-2-$butene
$3$. trans$-2-$butene
$4$. cis$-2-$butene

Reagent $(A)$ in the reaction is

The major product of the reaction of $1,3$-cyclohexadiene with $1 \text{ mole}$ of $HBr$ is:

How many products will be formed in this reaction?