Question

Which of the compounds has the lowest boiling point?
A.n-butyl alcohol
B.isobutyl alcohol
C.tert-butyl alcohol
D.sec-butyl alcohol

Answer 1

Hint: Boiling point of alkanes increase as the hydrocarbon chain lengthens or molecular weight increases or branching decreases.

Complete step by step answer:
We will write the molecular formula of all the compounds given in the question.
n-butyl alcohol:
$C{H_3} - C{H_2} - C{H_2} - C{H_2} - OH$
It has a molecular formula ${C_4}{H_{10}}O$.
isobutyl alcohol:
${(C{H_3})_2} - CH - C{H_2} - OH$
It has a molecular formula ${C_4}{H_{10}}O$.
tert-butyl alcohol:
${(C{H_3})_3} - C - OH$
It has a molecular formula ${C_4}{H_{10}}O$.
sec-butyl alcohol:
$C{H_3} - C{H_2} - CH(C{H_3}) - OH$
It has a molecular formula ${C_4}{H_{10}}O$.
Since, all the compounds are isomers and have the same molecular formula, therefore, the one with the highest branching will have the lowest boiling point.
tert-butyl alcohol has the most branched structure, therefore, it has the lowest boiling point.
With an increase in branching, the surface area of the molecule decreases; due to which the attractive forces between the molecules (Van der Waals forces) decreases. This decrease in attraction among the molecules, produces weaker Van der Waals forces which can be overcome at relatively low temperature. Hence, the boiling point of the compound decreases.

Hence option C is correct.

Note:
On increasing the molecular mass of the compound or lengthening the hydrocarbon chain, the amount of attractive forces within the compound increases (as the number of bonds increases), this requires more energy to overcome these forces, thereby increasing the boiling point of the compound.