Question

Draw the structure of the product formed when the following compound is heated in aqueous base. The formula for the product is C₈H₁₂O.

Answer 1

Concepts and reason

When a compound contains two carbonyl groups and in the presence of base compound undergo an intramolecular aldol condensation reaction. The reaction occurs when product of five or six membered ring is formed.

Fundamentals

The aldol condensation is a type of reaction in which two molecules of α-hydrogen containing aldehyde or ketone reacts with each other in the presence of base like sodium hydroxide/heat and formed α-β unsaturated aldehyde or ketone. In case of intramolecular aldol condensation reaction, the compound contains two carbonyl groups and in the presence of base compound undergo intramolecular aldol condensation reaction and cyclic α-β unsaturated ketone is formed.

The general mechanism of intramolecular aldol condensation reaction is as follows:

In the compound, two types of α-hydrogens are present, that is, ${{\rm{H}}_{\rm{a}}}$ and ${{\rm{H}}_{\rm{b}}}$ . When ${{\rm{H}}_{\rm{a}}}$ hydrogen is abstracts the enolate ion is formed. The formed enolate ion undergo intramolecular reaction and reacts with another carbonyl group which is presents in the same molecule and five-membered ring is formed. But the enolate formed after removal of ${{\rm{H}}_{\rm{b}}}$ hydrogen reacts with another carbonyl group which is presents in the same molecule and three-membered ring is formed. The three-membered ring in highly unstable, therefore, path b does not occur and path a is occurred; only five-membered ring is formed.

In the third step, species (m) reacts with water and in presence heat, the final product, that is, five membered cyclic α-β unsaturated ketone is obtained.

The structure of given compound with marked α-hydrogens is as follows:

In the first step of the reaction, abstract of the α-hydrogen, that is, ${{\rm{H}}_{\rm{a}}}$ takes place and enolate ion is formed. The reaction is as follows:

In the second step of the reaction, the enolate ion which is resonance with carbanion, attacks the second carbonyl group presents in the compound and an intermediate (m) is formed.

The intermediate reacts with water and β hydroxy cyclic ketone is formed. In the presence of heat, the β-hydroxy cyclic ketone is converts into five membered cyclic α-β unsaturated ketone.

Ans:

The product is as follows:

Answer 2

aldol reaction

Answer 3