Part 6: MECHANISMS
Note that no other reagents are needed in order to complete any of these sequences, you should only be using what is there.

A
Reaction of a ketone with an alcohol to form a hemi-ketal in an intramolecular sense.  Protonate the ketone carbonyl to make it more reactive, then make the alcohol attack as a nucleophile.

formation of a cyclic hemiketal
In this mechanism, B: could be C=O groups, ROH or the conjugate base of the acid catalyst.
B
Decarboxylation of a b-ketoacid under acidic conditions.  The loss of the CO2 gives the enol which then undergoes acid catalysed tautomerisation to the ketone.

decarboxylation of a b-ketoacid
In this mechanism, B: could be C=O groups, -OH groups or the conjugate base of the acid catalyst.

C
Hydration of an alkyne gives the Markovnikov product, which is the more highly substituted enol via the more stable resonance stabilised secondary vinyl cation which then undergoes acid catalysed tautomerism to the ketone. Notice that the proton donor is the acid catalyst and the nucleophile is water (no hydroxide under acidic conditions).
alkyne hydration
In this mechanism, B: could be C=O groups, -OH groups C=C groups, or the conjugate base of the acid catalyst.
D
A Friedel-Crafts type alkylaion reaction.  The acid protonates the alcohol making a better leaving group. This leaves giving the carbocation, which is the electrophile that adds to the aromatic system, via the arenium ion which then loses a proton so restoring the important aromatic character.  In the final step the base, :B could be the conjugate base of the acid, a water molecule or the original alcohol.

Friedel-Crafts alkylation of benzene
In this mechanism, B: could be ROH, H2O or the conjugate base of the acid catalyst.

[Chem 353 Home]Return to Homepage