Chapter 25, Problem 101AP

All alkanes give off heat when burned in air. Such combustion of alkanes is exothermic, and the sign of $\Delta H°$ is negative. The general equation for the combustion of the alkanes of molecular formula ${\text{C}}_{\text{5}}{\text{H}}_{\text{12}}$ is

${\text{C}}_{\text{5}}{\text{H}}_{\text{12}}\text{+8}{\text{O}}_{\text{2}}\to \text{5C}{\text{O}}_{\text{2}}\text{+6}{\text{H}}_{\text{2}}\text{O}$

The values $\Delta H°$ for the combustion of the three pentane isomers are

$\text{CH 3 CH 2 CH 3 CH 2 CH 3=-3536 kJ/mol}$ $\text{(CH3) 2 CHCH 2 CH 3=-3529 kJ/mol}$ $\text{(CH3) 4C=-3515 kJ/mol}$

(a)

What do these data tell you about the effect of chain branching on the relative potential energies and stabilities of these isomers?

(b)

Assume this effect is general to predict which one of the $\text{18 }{\text{C}}_{\text{8}}{\text{H}}_{\text{18}}$ isomers should be the most stable.