Q36E - Organic Chemistry (Mcmurry)

Question

Aldehydes and ketones undergo acid-catalyzed reaction with alcohols to yield hemiacetals, compounds that have one alcohol-like oxygen and one ether-like oxygen bonded to the same carbon. Further reaction of a hemiacetal with alcohol then yields an acetal, a compound that has two ether-like oxygens bonded to the same carbon.

(a) Show the structures of the hemiacetal and acetal you would obtain by reaction of cyclohexanone with ethanol.

(b) Propose a mechanism for the conversion of a hemiacetal into an acetal.

Step-by-Step Solution

Answer

(a) 

(b) 

Step-by-Step Solution Step 1: Structure of hemiacetal and acetal by the following reaction

(a) Hemiacetal and acetal can be prepared in the following way:

Step 2: Protonation

Protonation is defined as the addition of a proton to an atom, molecule, or ion. It is different from hydrogenation because in protonation, a change in charge of protonated species occurs, while the charge remains unaffected during hydrogenation, for example, the formation of the ammonium group ${{\mathrm{NH}}_4}^{+}$  from ammonia ${\mathrm{NH}}_3$ . Protonation mainly occurs in the reaction of an acid with a base to form a salt.

Step 3: Dehydration

A dehydration reaction can be defined as the chemical reaction in which a water molecule is eliminated from the reactant molecule.

Step 4: Propose a mechanism

(b) 

Mechanism of the conversion of a hemiacetal into an acetal

The reaction mechanism can be done in four steps.

1. Protonation

2. Dehydration or loss of water

3. Addition of ethanol

4. Loss of proton