What will be the product formed when cyclohexanone undergoes Aldol condensation?

The correct answer is option 4.

When cyclohexanone is treated with strong base, two molar equivalents of cyclohexanone may form a new C–C bond. The resulting product, a “\(\beta \)-hydroxy aldehyde”   contains both an aldehyde and an alcohol. In addition to the new C–C bond, a C-H bond has broken, as has a C–O (pi) bond.

When the resulting aldol addition product (the beta-hydroxy aldehyde) is heated with base, it can lose water to give a new C-C pi bond. This is referred to as an aldol condensation reaction since water is formed as a byproduct.

Mechanism:

When considering what a mechanism might look like, the first thing to note is that we are using a strong base, NaOH. This raises a question about the most acidic proton here. [Hint: it’s not the C-H attached to the aldehyde, as that would result in an acyl anion, which is not resonance-stabilized.]

By examining the bonds that form and break, you can observe that we are breaking a C-H bond on the carbon directly adjacent to the aldehyde carbonyl (i.e., the alpha carbon).

The removal of this C-H bond by the strong base results in a resonance-stabilized enolate ion.

Therefore, a necessary requirement for the aldol reaction is that the aldehyde be enolizable (i.e. have a proton on the alpha carbon). No proton → no enolate → no aldol.

The key to the reaction is that the enolate, once formed, is an excellent nucleophile and can rapidly react with any available electrophiles hanging around in solution – such as, for example, aldehydes. The product requires that we form C–C and break C–O (pi). The enolate thus adds to the aldehyde in Step 2 (form C–C, break C–O (pi) ), an example of the most classic mechanistic step of carbonyls, addition.

In this way, all starting aldehyde is eventually converted to the aldol addition product, despite the unfavorable acid-base equilibrium. After the addition reaction, the resulting negative charge on oxygen then undergoes protonation by the solvent (Step 3, form O–H).

If the resulting aldol addition product (the beta-hydroxy aldehyde) is heated with base, it can lose water to give a new C-C pi bond. This is referred to as an aldol condensation reaction since water is formed as a byproduct.

For our purposes the most important distinguishing factor is “HEAT“, which tends to favor elimination reactions. The elimination step works like this and is an example of an \(E_{1_{CB}}\) reaction.