Which of the following compounds has the lowest boiling point?
The correct option is D For alkanes, the van der Waals' forces of attraction are weak and hence the boiling point is low.
More the number of carbons in the chain, more is the surface area and hence more is the boiling point. That is why branching decreases the boiling point as the surface area of a branched molecule is less.
As far as the boiling point are concerned, alkenes have more boiling point then alkanes with the same number of 'C' atoms. Due to the presence of π− bond, alkenes are slightly polar. More is the number of π bonds, more is the polarity and higher is the boiling point. Thus, decreasing order of boiling point is : c>b>a>d