Acidic Conditions:  Enol reaction introduction:

Enols are formed from carbonyl compounds under acidic conditions.  In step 1, the carbonyl is protonated by an acid.  This may be accomplished using a variety of acids although p-toluene sulfonic acid, used  above, is popular.  In step 2, an alpha- proton is removed to form the enol.  Both steps in this reaction are reversible.  Remember that keto- forms of carbonyls are generally more stable than enol forms of the carbonyl.  The enol may be formed under acidic conditions, but it is not extremely stable and will either revert back to the keto- form, or react with an electrophile.

An enol is not reactive enough to attack neutral alkyl halides or alkyl tosylates.  In fact, it is generally not reactive enough to attack neutral carbonyl functional groups.  However, under acidic conditions, a carbonyl compound may be protonated at the oxygen site. A protonated carbonyl group is reactive enough as an electrophile to interact with the enol, forming a new C-C bond as shown above.  The new product is known as a beta-hydroxy carbonyl.  Under acidic conditions, the alcohol functional group will be eliminated (E1-CB) to give an alpha,beta-unsaturated carbonyl as the final product.   The final elimination step (E1-CB) is shown again below:  (For a review of E1-CB reactions, please click HERE)

As seen above, the carbonyl oxygen, being more basic due to resonance, will protonate before the alcohol.  Hydrogen bonding allows for a very stable 6-membered ring to form, spreading positive charge between both oxygen atoms and making the alcohol a very good leaving group.  The hydrogen bonding that occurs after protonation drives the elimination reaction to completion.

       Anionic Conditions:  Enolate reaction introduction:

As stated earlier, hydrogens alpha- to a carbonyl group are acidic due to resonance and hyperconjugation.  In the presence of a base, these hydrogens may be removed to form an enolate.  The system below is showing enolate formation under what are called reversible conditions.  This means that the proton may be taken off and placed back on the alpha- carbon innumerable times.  There will be consequences to forming an enolate under either reversible or irreversible conditions, and we will be studying this aspect next.

As seen above, an enolate may react with several types of electrophiles.  Enolates carry a negative charge, making them more reactive than enols.  As such, they are more versatile when used as a nucleophile, but the extra reactivity also means that extra care must be taken as well.  Let us begin to delve into controlling enol and enolate formation and how best to use them as nucleophilic reagents.