(a) Why does p-dichlorobenzene have a higher m.p. than its o- and m-isomers? 
(b) Why is (±)-Butan-2-ol optically inactive?

OR

(a) The dipole moment of chlorobenzene is lower than that of cyclohexyl chloride. 
(b) Write the equations for the preparation of 1-bromobutane from : 
(i) 1-butanol (ii) but-l-ene

(a) p-isomers are comparetively more symmetrical and fit closely in the crystal lattice, thus require more heat to break these strong forces of attraction. Therefore higher melting point than o- and m-isomers 

(b) (±)-Butan-2-ol is optically inactive because in racemic mix one type of rotation is cancelled by other.

OR

(A) Chlorobenzene has lower dipole moment than cyclohexyl chloride due to lower magnitude of -ve charge on the Cl atom and shorter C – Cl distance. Due to greater S-character, a sp2-hybrid carbon is more electronegative than a sp3-hybrid carbon. Therefore, the sp2-hybrid carbon of C – Cl bond in chlorobenzene has less tendency to release electrons to Cl than a sp3 hybrid carbon of cyclohexyl chloride.

(B)
(i) Preparation of 1-bromobutane from 1-butanol
$\begin{array}{r}{\mathrm{CH}}_3-{\mathrm{CH}}_2-{\mathrm{CH}}_2-{\mathrm{CH}}_2-\mathrm{OH}+\mathrm{HBr}\stackrel{{\mathrm{H}}_2{\mathrm{SO}}_4}{⟶}\\ {\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{CH}}_2-\mathrm{Br}+{\mathrm{H}}_2\mathrm{O}\\ \end{array}$

(ii) Preparation of 1-bromobutane from but-1-ene