The short answer is that propan-2-ol is a more stable molecule...
The longer answer ought to tell why!
The electronegative oxygen of the -OH group creates more of a dipole when it is at the end of the molecule, relatively destabilising it with respect to the 2 isomer.
In the case of 2- propanol the central OH creates a dipole in which the partially positive carbon atom joined to the -OH group is stabilised by the +I (inductive) effect of two neighbouring methyl groups.
Whenever charge density is reduced in a molecule the molecule becomes relatively more stable. As the products are identical in both combustions then the fact that 1-propanol releases more energy tells us that it starts off from higher energy than 2-propanol.
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