# Why is 1,3-pentadiene favored in the reaction of pent-3-en-2-ol with sulfuric acid?

Let's say we have the following reaction, a simple dehydration reaction.

Why do we get 1,3-pentadiene and not 2,3-pentadiene?

The reaction conditions (strong acid and heat) suggest that an equilibrium mixture of possible products will result. Around room temperature, each 1.4 kcal/m difference in energy between two isomers will shift the equilibrium between them by a power of 10 ($\ce{\Delta G=-RTlnK}$). A 15 kcal/m energy difference between your two dienes suggests that the equilibrium will be ~ $\mathrm{10^{10}:1}$ favoring the 1,3-pentadiene. If any of the allene forms, it will rapidly convert to the 1,3-pentadiene.