Chapter 7, Problem 7.43P

Interpretation Introduction

Interpretation:

It is to be explained why most ketones and aldehydes exist primarily in their keto forms but $\text{Propanedial}$ exists primarily $\text{(>99\%)}$ in its enol form.

Concept introduction:

As a ketone or aldehyde, the species is called the keto form. In the enol form, the species has a carbon atom that is simultaneously a part of a $\text{C = C}$ bond, characteristic of an alkene, and is bonded to OH, characteristic of an alcohol. In the keto form, there is a hydrogen atom on the carbon atom that is adjacent to the $\text{C = O}$ group, so called alpha carbon, whereas in the enol form, the hydrogen appears on the oxygen atom instead. In a keto-enol tautomerization, the keto form is in equilibrium with its enol form via proton transfer steps. In general, keto form is more stable than enol because the total bond energy for the keto form is roughly greater than that of the enol form.