Which major product is obtained by the addition of $1 \ mol$ of $HCl$ to $1,3$-butadiene at low temperature ($0^{\circ}C$ or below)?

Mustard gas is obtained by

An alkene gives two moles of $HCHO$,one mole of $CO_2$,and one mole of $CH_3COCHO$ on ozonolysis. What is its structure?

Consider compounds $A$, $B$ and $C$ with following structural formulae: $A = CH_{3} - CH_{2} - CH_{2} - CH_{2} - CH_{2} - OH$, $B = CH_{2} = CH - CH_{2} - CH_{2} - CH_{3}$, $C = HO - CH_{2} - CH_{2} - CH(OH) - CH_{3}$. For the conversion of $B$ from $A$, reagent $(D)$ required is . . . . . . and structural formula of product $(E)$ obtained when $C$ undergoes same reaction using excess reagent $(D)$ is . . . . . . .

The reaction of propene with $HOCl$ proceeds via the addition of

What is $X$ in this reaction?