$C_2H_5I$ $\xrightarrow{\text{alc. } KOH} X$ $\xrightarrow{Br_2} Y$ $\xrightarrow{KCN} Z$; $Z = \dots$

The rate-limiting step of an $S_N1$ reaction follows the sequence shown below. Based on the assumption that $R$ contains $3$ different groups,which statement is true?
$\mathop R\limits^{\delta \oplus } \,\mathop {Br}\limits^{\delta \Theta } \, \rightleftharpoons \,\mathop {\boxed{{R^ \oplus }Br^\Theta }}\limits_{(a)} \,\,\, \rightleftharpoons \,\mathop {\boxed{{R^ \oplus }}\,\boxed{Br^\Theta }\,}\limits_{(b)} \,\, \rightleftharpoons $ ${\boxed{{R^ \oplus }}}$ $ + $ $\mathop {\,\boxed{Br^\Theta }\,}\limits_{(c)} \,$

Which one is a nucleophilic substitution reaction among the following?

The formation of butane nitrile is possible in which of the following reactions?

Ethyl orthoformate is formed by heating which of the following compounds with sodium ethoxide?

Which is the main organic product obtained by the reaction of $2,2,2-$trichloroethanol with calcium hydroxide?