Question

I performed an aldol condensation products lab using cyclopentanone and 4-methylcyclohexanone as ketones and what I believe to be 4-methoxybenzaldehyde to form C21H20O2 and C22H24O2,but I don't understand how to account for the effect of aldehyde structure on reactivity in these reactions

Answer 1

Your question is not very clear; however I am having an inkling that you want to know how an aldehyde reacts differently as compared to a ketone. I will answer the question as precisely as I can.

Aldehydes are in general more reactive toward nucleophilic substitution reactions than ketones. An aldehyde can be represented as R-CHO while a ketone is R-CO-R’. The greater reactivity of aldehydes springs from both electronic and steric considerations.

The carbonyl (C=O) group in both aldehydes and ketones is polarized with the C atom being positively polarized and the O atom negatively polarized. In aldehydes, there is only one alkyl (R-) group that can push electron density to nullify the positive polarization of the carbonyl carbon. Ketones contain two alkyl groups and both can push electron density toward C to nullify the electron deficiency of carbon. Hence, the carbon center in aldehydes is more electron-deficient and consequently, more reactive.

Steric effect also plays an important role here. In case of ketones, there are two alkyl groups and the incoming nucleophile is sterically hindered from approaching the carbonyl carbon. The aldehyde group contains H atom which is much smaller than the alkyl groups and hence a nucleophile can easily approach the carbon center, thereby making aldehydes more reactive.