Question

Which one of the following has the minimum boiling point?
A. n-butane
B. $1 -$ butyne
C. $1 -$ butane
D. Isobutene

Answer 1

As we know that the boiling point is the temperature at which a substance boils. It is dependent upon several factors like branching of molecules. Boiling point decreases when branching is increased. Also, it increases when the number of carbon atoms are increased.

Complete step by step answer:
All of the given compounds have the same number of carbon atoms. So we determine the boiling point by comparing the branching of the compound or surface area.
Linear compounds have more surface area than the branched compounds. Branched chain compounds have less surface area because of steric hindrance. Since more molecules are attached to one carbon, it makes the surface area less. Thus the intermolecular forces are weak in branched chain compounds. Thus they will have lower boiling point.
Now let’s focus on the given options. N-butane and $1 -$ butane are linear chain compounds. While isobutene is a branched chain compound. N-butane and $1 -$ butane have high boiling point. But $1 -$ butyne has a high boiling point which is an anomalous behavior. Since it has a triple bond, it must have a very less surface area, thereby decreasing its boiling point. But this is not the case with alkynes. Since the number of $\pi$ electrons is more, the electric field is easily destroyed. Thus it will have very strong intermolecular forces.
Thus comparing all these points, isobutene has minimum boiling point.

Hence, the correct option is D.

Note:
The factors that depend upon the boiling point are:
-Pressure
-Intermolecular forces
-Number of carbon atoms
-Molecular weight
-Surface area