Question #a306c

Nov 25, 2016

The product depends on which reactant is in excess.

Explanation:

The aldol condensation

An aldol condensation is usually a base-catalyzed reaction in which an aldehyde or ketone with α-hydrogens reacts with a carbonyl compound to form
a β-hydroxyaldehyde or β-hydroxyketone, followed by dehydration to give a conjugated enone.

For example

$\text{RCH=O" + "H-CH"_2"COR" stackrelcolor(blue)("OH"^"-"color(white)(m))(→) "RCH(OH)CH"_2"COR}$

$\text{RCH(OH)CH"_2"COR" → "RCH=CHCOR"+ "H"_2"O}$

You can find the mechanism in this Socratic answer.

In this problem, 4-hydroxybenzaldehyde is the aldehyde and cyclopentanone is the ketone with the α-hydrogen.

If there is excess cyclopentanone

There will be only enough 4-hydroxybenzaldehyde to react at $\text{C-2}$ of the cyclopentanone.

The major product will be 2-(4-hydroxybenzylidene)cyclopentanone.

The product is probably a mixture of ($E$) and ($Z$) isomers.

If there is excess aldehyde

Carbon 5 of the cyclopentanone still has α-hydrogens at $\text{C-5}$, so it can undergo further condensation with the excess aldehyde to form 2,5-bis(4-hydroxybenzylidene)cyclopentanone.

The product is probably a mixture of ($E , Z$), ($E , E$), and ($Z , Z$) isomers.