Enols

organic compounds with a hydroxyl group OH at a carbon-carbon double bond. The simplest enol is vinyl alcohol CH₂=CHOH, which like most enols is unstable in the free state and changes into its isomer acetaldehyde:

The instability of enols is due to their being in tautomeric equilibrium with the corresponding carbonyl compounds:

Since the carbonyl form is energetically favored over the enol one by 54–71 kilojoules/mole (^13–17 kcal/mole), equilibrium is usually displaced in the direction of formation of aldehydes and ketones. Thus acetone contains 2.5 x 10⁻⁴ percent enol and cyclohexanone 2.5 x 10⁻² percent enol. However, in some cases the enol form is stable (for example, with β-diketones, β-ketoaldehydes, and cyclic α-diketones), because the energy difference is offset by energy gain owing to formation of a system of conjugated double bonds, as well as of a hydrogen bond owing to hydroxyl group formation. The enol content of an equilibrium mixture also depends on the solvent, temperature, and other factors.

单词	enols
释义	Enols Enols organic compounds with a hydroxyl group OH at a carbon-carbon double bond. The simplest enol is vinyl alcohol CH₂=CHOH, which like most enols is unstable in the free state and changes into its isomer acetaldehyde: The instability of enols is due to their being in tautomeric equilibrium with the corresponding carbonyl compounds: Since the carbonyl form is energetically favored over the enol one by 54–71 kilojoules/mole (^13–17 kcal/mole), equilibrium is usually displaced in the direction of formation of aldehydes and ketones. Thus acetone contains 2.5 x 10⁻⁴ percent enol and cyclohexanone 2.5 x 10⁻² percent enol. However, in some cases the enol form is stable (for example, with β-diketones, β-ketoaldehydes, and cyclic α-diketones), because the energy difference is offset by energy gain owing to formation of a system of conjugated double bonds, as well as of a hydrogen bond owing to hydroxyl group formation. The enol content of an equilibrium mixture also depends on the solvent, temperature, and other factors.
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