Question

What effect does branching of an alkane chain has on its melting point ?

• A. Increases its melting point
• B. Decreases its melting point
• C. doesn't change its melting point
• D. Cannot be determined

Answer 1

The correct option is A Increases 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, C3H8 is 85.9 K while that of C6H14 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)