(a) Write the \( S_{N} 1 \) and \( S_{N} 2 \) mechanism for the following reactions :
\( 21 / 2 \times 2=5 \)
(i) \( \left( CH _{3}\right)_{3} CBr +\stackrel{\ominus}{ O } H \longrightarrow\left( CH _{3}\right)_{3} COH + Br \)
(ii) \( CH _{3} Br +\stackrel{\ominus}{ O } H \longrightarrow CH _{3} OH + Br \)
(b) Complete the following reactions : \( 1 \times 5=5 \)
(i)
(ii) \( CH _{3}- CH _{2}- OH + PCl _{5} \longrightarrow \) ?
(iii) \( CH _{3}- CH = CH _{2}+ HBr \longrightarrow \) ?
