Question

How many of the following compounds will have lower boiling points as compared to $n$-pentane?
$2,2$-Dimethylpropane, $2$-methylpropane,
$2$-methylbutane, $2,2$-dimethylbutane,
$2$-methylpentane, $2$ -methylbut-$2$-ene,
pent-$2$-yne

Answer 1

Answer: 3

Factors that affect boiling point of alkanes are:
i. Number of $C$-atoms: As the no. of $C$-atom increases, the boiling point increases.
ii. Branching: The boiling point decreases with increase in branching in isomeric alkanes.
Thus, $2,2$-dimethylpropane $(282.5K),2$ -methylbutane ( $301K$ ) and $2$-methylpropane$(261.3K)$ will have lower boiling point as compared to $n$-pentane $(309.1K)$.
Also, boiling points of alkenes and alkynes are higher as compared to corresponding alkanes.