Acetal Formation

Question

Acetal formation from aldehyde and alcohol is catalyzed by:

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1. **Requirement**: To form an acetal ($R-CH(OR')_2$), two molecules of alcohol must add to one molecule of aldehyde.
2. **Mechanism Step 1**: The Carbonyl Oxygen is protonated by an **acid catalyst** ($e.g.,$ dry $HCl$ gas or $p-TsOH$).
3. **Significance**: This makes the carbonyl Carbon much more electrophilic, allow the weak nucleophile (alcohol) to attack.
4. **Hydration Barrier**: After forming the hemiacetal, acid is again required to protonate the $-OH$ group, making it a good leaving group ($H_2O$) for the second alcohol attack.
5. **Base contrast**: In base, hemiacetals can form but full acetals generally do not because the $-OH$ group cannot be displaced as $OH^-$ by another $OR^-$ nucleophile.

Acetal Formation

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