Question

Why is keto is more stable than enol?

Answer 1

The main reason is that `"C=O"` bond is more stable than a `"C=C"` bond.

Explanation:

The essential bond changes in a keto-enol tautomerism are

`"H-C-C=O ⇌ C=C-O-H"`

The `"C-H"`, `"C-C"`, and `"C=O"` bonds in the keto form become `"O-H"`, `"C-O"`, and `"C=C"` bonds in the enol.

For the keto form, the bond energies in kilojoules per mole are

`"C-H"color(white)(ll) = color(white)(ll)413`
`"C-C"color(white)(ll) =color(white)(ll) 347`
`"C=O" = color(white)(ll)745`
`"Tot".color(white)(l) = stackrel(——)(1505)`

For the enol, the bond energies are

`"O-H" color(white)(l)= color(white)(ll)467`
`"C-O" color(white)(l)= color(white)(ll)358`
`"C=C" = color(white)(ll)614`
`"Tot." color(white)(l)= stackrel(——)(1439)`

The difference in the sums of the bond energies is

`sum_"keto"("BE") – sum_"enol"("BE") = "(1505-1439) kJ/mol" = "66 kJ/mol"`

The keto form is thermodynamically more stable by about 66 kJ/mol.

Hence the position of equilibrium strongly favours the keto form.