Why n-Butane has high B. p than iso-Pentane although both have equal no. of carbon atoms along the chain length?

Question

n-Butane =B. p =-1 oC iso pentane =b. p= 28 oC

Answer 1

Pentanes are heavier than butanes, whatever their geometry. So the boiling point of pentanes are higher. This is a general tendency in the series of hydrocarbons. Pentanes and butanes are good examples to show that the chain length has not the most important effect on the boiling point.

n-Pentane boils at $\pu{36°C}$, 2,2-dimethylpropane at $\pu{9.5°C}$. So all pentanes boil between $\pu{9.5°C}$ and $\pu{36°C}$.

In contrast, all butanes boil under $\pu{0°C}$. n-Butane boils at $\pu{-1°C}$ and isobutane at $\pu{-10°C}$.