Question

Reactant A is:

(a)

(b)

(c)

(d)

Answer 1

Hint: Aldol condensation is a type of condensation reaction in organic chemistry in which an enolate ion reacts with a carbonyl compound to form a β-hydroxy aldehyde or β-hydroxy ketone (by an aldol reaction) followed by dehydration to form a conjugated enone.

Complete step by step answer:
As we can see from the question that the conditions of aldol condensation if fulfilled therefore it is an aldol condensation. But you might be thinking about how this ring was formed, and which alpha hydrogen was removed. So, to make things clear, let's have a brief discussion about alcohol condensation and its types.
- Aldol condensation: Aldol condensation occurs in aldehydes having α-hydrogen. They are reacted with a dilute base to give β-hydroxy aldehydes called aldols. This reaction is most known as aldol condensation. If this condensation reaction occurs between two different carbonyl compounds, then it is called crossed aldol condensation.
- Crossed aldol condensation: When condensation is between two different carbonyl compounds then it is called as crossed aldol condensation. This includes condensation reaction between two different molecules of an aldehyde or ketone in a polar protic solvent such as water or alcohol constitutes the crossed aldol condensation reaction. When both aldehydes have alpha hydrogens with them, both can form carbanions and can also act as carbanion acceptors. Hence a mixture of four products is formed which has a very little synthetic value.
If one of the aldehydes has no alpha hydrogen with them then it can act only as a carbanion acceptor. In such a case, only two products were formed. A common substrate for the crossed aldol reaction is an aromatic aldehyde, which has no alpha hydrogen.
- Intramolecular Aldol Condensation: Intramolecular Aldol condensations happen when a single compound contains 2 reaction aldehyde/ketone groups. When the alpha carbon of one group attacks the other one, the molecule attacks itself which leads to forming a ring structure. Molecules which contain two carbonyl functional groups have the possibility of forming a ring through an intramolecular aldol condensation reaction. In most of the cases, two sets of α- hydrogens need to be considered. As with most ring-forming reactions, five and six-membered rings are preferred because they have less ring strain.
With reference to the above discussion, let's find out how the mechanism of the reaction in the above question proceeds:
Step 1: NaOH gets dissociate into $N{{a}^{+}}$ and $O{{H}^{-}}$ and this $O{{H}^{-}}$ will take the acidic hydrogen from the compound as shown in the below figure.
\[NaOH\rightleftharpoons N{{a}^{+}}+O{{H}^{-}}\]

Step 2: The negative charge on the carbon atom will attack the carbon of hydrogen because of the presence of a slight positive charge due to the electronegativity difference in oxygen and carbon. This will lead to the formation of a five-membered ring.

Step 3: Now, due to the heat, the compound will lose water molecules and finally result in the desired compound.

Therefore, the correct answer to the given question is an option (C).

Note: You should remember that the selection of reactants is based on the acidity of alpha hydrogen. More the acidic alpha hydrogen is, more will be the rate of reaction and more percentage of product will form from that reactant.