Which of the following has the highest boiling point ?
i. 2-Methyl pentane
ii. 2,3-Dimethyl butane
iii. 2,2-Dimethyl butane
b. What effect does branching of an alkane chain has on its melting point ?

Explanation

a. 2-Methyl pentane has the highest boiling point because the boiling point in alkane depends upon the chains off atoms and the molecular mass of the molecules. Straight-chain molecules have higher boiling point than the branched-chain compounds. Also, teh lesser branched chain has low boiling point because in higher branched molecules, the molecular mass increases and boiling point also increases. The boiling point order is as follows:
2-Methyl pentance gt 2,3-Dimethyl butane gt 2,2-Dimethyl butane

The boiling point decreases because branching increases.
b. 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, the melting point of `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 highesty melting point ouot of the three isomers due to more compact packing.

The melting point order is:
(i) gt (ii) gt (iii)
`(256.4K gt 143.3K gt 113.1K)`
The boiling point order is:
(ii) gt (iii) gt (i)
`(309 K gt 300.9 K gt 282.5 K)`