What effect does branching of an alkane have on its melting point?
The melting point of alkanes with even number of carbon atoms is higher than that of the odd numbered from the immediate next lower alkanes.
For example, $$ C_{3}H_{8} $$ is 85.9 K while that of $$ C_{6}H_{14} $$ is 179 K. Since the packing is symmetrical in even numbered alkanes, they have higher melting point.
Therefore, neopentane with a spherical molecule has the highest melting point out of the three isomers due to more compact packing.
The melting point order is
(i) > (ii) > (iii)
(256.4 K > 143.3 K > 113.1 K)
The boiling point order is
(ii) > (iii) > (i)
(309 K > 300.9 K > 282.5 K)
Hence , increases the melting point .