Q40. - Organic Chemistry

Question

Question: Vitamin C is a stable enediol. Draw the structure of the two keto tautomers in equilibrium with the enediol and explain why the enediol is more stable than the other tautomer.

Step-by-Step Solution

Answer

Answer

 

The structure of the two keto tautomers in equilibrium with the enediol are shown below:-

The enediol is more stable than the other tautomers because resonance stabilization is observed in this case which is not observed in the case of the keto form.

Step 1: Definition of tautomers

Tautomers are structural isomers of chemical compounds that readily interconvert in an acidic or a basic medium in the presence of water. The process of conversion of one tautomer into another is known as tautomerization. One such example is keto-enol tautomers.

Step 2: Bonding pattern in case of a keto-enol tautomerization

Enol and keto forms are tautomers that are present in the carbonyl group and that differ in the position of a double bond and a proton. They are constitutional isomers who are found to be in equilibrium with each other. A keto has a C=O and an additional C-H bond while an enol has a C-OH and a C=C bond.

Step 3: Based on stability

Generally, keto is found to be more stable than enol since C=O is a stronger bond than C=C, but in this case, enol is more stable because of the resonance stabilization as shown:-

 

Hence, the stability of the negative charge is observed due to the -I effect of the alcoholic group.

Hence, the stability of the negative charge is observed due to the extended resonance.

 

After the consideration of the valid points, the structure of the two keto tautomers in equilibrium with the enediol are shown below:-

Representation of a keto-enol tautomerization of vitamin C

 

The enediol is more stable than the other tautomers because resonance stabilization is observed in this case which is not observed in the case of the keto form.