Be sure you fully understand Mechanism 18.4 by writing equations for steps 1–3. Use curved...

Be sure you fully understand Mechanism 18.4 by writing equations for steps 1–3. Use curved arrows to show electron flow.

Mechanism 18.4

Acetal Formation from Benzaldehyde and Ethanol

THE OVERALL REACTION:

THE MECHANISM:

Steps 1–3: Acid-catalyzed nucleophilic addition of 1 mole of ethanol to the carbonyl group. The details of these three steps are analogous to the three steps of acid-catalyzed hydration in Mechanism 18.2. The product of these three steps is a hemiacetal.

Step 4: Steps 4 and 5 are analogous to the two steps in the formation of carbocations in acid-catalyzed reactions of alcohols. Step 4 is proton transfer to the hydroxyl oxygen of the hemiacetal.

Step 5: Loss of water from the protonated hemiacetal gives an oxygen-stabilized carbocation. Of the resonance structures shown, the more stable contributor satisfies the octet rule for both carbon and oxygen.

Step 6: Nucleophilic addition of ethanol to the oxygen-stabilized carbocation

Step 7: Proton transfer from the conjugate acid of the product to ethanol

Mechanism 18.2

Hydration of an Aldehyde or Ketone in Acid Solution

THE OVERALL REACTION:

THE MECHANISM:

Step 1: Protonation of the carbonyl oxygen

Step 2: Nucleophilic addition to the protonated aldehyde or ketone

Step 3: Proton transfer from the conjugate acid of the geminal diol to a water molecule