Why isn't Perkin reaction possible with aliphatic aldehydes? Since the carbon atom of carbonyl group of aliphatic aldehydes is more electrophilic than that of aromatic ones, the reaction must be more feasible in case of aliphatic compounds. Please explain.
In Perkins reaction, condensation occurs between aromatic aldehydes (which cannot undergo self condensation) with an acid anhydirde in presence of sodium or potassium salt of corresponding acid.
Consider Perkins reaction of benzaldehye with acetic anhydride to form cinnamic acid. The first two steps occur in weakly basic medium (sodium acetate). The last two occur in acidic medium (protonation and subsequent dehydration). Notice that the last step is particularly favoured owing to formation of a conjugated product.
This wouldn't be the case for aliphatic aldehydes (with no alpha Hydrogen , else self condensation may occur). Also, if the anhydride used has only one alpha Hydrogen (here acetic anhydride has 3 alpha Hydrogen), then the reaction barely proceeds to completion.Instead an aldol type product is obtained. This shows the role conjugation plays in making the last step fairly irreversible.
Furthermore, this reaction occurs with vinylogs, hetrocyclic aldehydes and even pthalic anhydrides. Notice that in all cases , final product is conjugated.
It would be worthwhile to mention another alternative mechanism, which has been mentioned in Peter Stykes, Wikipedia as well.
Some Support for this mechanism comes from fact that anhydrides with only 1 alpha hydrogen donot give this reaction. Again, here the base promoted dehydration step would be largely irreversible incase of aromatic aldehydes due to conjugation.
Hope this helps.
Perkin, W. H. J. Chem. Soc. 1868, 21, 181 XXIII.—On the hydride of aceto-salicyl.DOI: 10.1039/js8682100181
Peter Stykes. Organic mechanism. Sixth Edition
- SN Sanyal: Reactions,Rearrangements and Reagents.