Question: 78: The decreasing order of boiling points of the following alkanes is :
(a) Heptane
(b) Butane
(c) 2-methylbutane
(d) 2-methylpropane
(e) Hexane
Choose the correct answer from the options given below :
(1) (a) > (c) > (e) > (d) > (b)
(2) (c) > (d) > (a) > (e) > (b)
(3) (a) > (e) > (b) > (c) > (d)
(4) (a) > (e) > (c) > (b) > (d)
Answer: Option (4)
Explanation:
The boiling point of alkanes depends mainly on two factors: molecular mass and branching.
As the molecular mass increases, the surface area and van der Waals forces increase,
leading to a higher boiling point. Hence, alkanes with more carbon atoms have higher boiling points.
For alkanes having the same molecular formula, increased branching reduces surface area
and weakens intermolecular attractions, resulting in a lower boiling point.
Heptane has the highest molecular mass among the given compounds, so it has the highest boiling point.
Hexane has fewer carbon atoms than heptane but more than the remaining alkanes, so it comes next.
2-methylbutane and butane have fewer carbon atoms than hexane.
Among them, 2-methylbutane has more carbon atoms than butane, so its boiling point is higher.
2-methylpropane is the most highly branched and has the lowest molecular mass among the given compounds, so it has the lowest boiling point.
Therefore, the correct decreasing order of boiling points is:
(a) Heptane > (e) Hexane > (c) 2-methylbutane >
(b) Butane > (d) 2-methylpropane.