How does a hemiacetal differ from an acetal?

• A hemiacetal holds two OR groups and no OH on the carbon
• They are identical names for one compound
• A hemiacetal holds one OR and one OH on the same carbon

What partners and catalyst build an acetal?

• Carbonyl plus two alcohols under acid catalysis
• Carbonyl plus water under base catalysis
• Two carbonyls plus one alcohol with no catalyst

Removing water pushes acetal formation forward.

Circle one:   True   False

One carbonyl meets a primary amine; another carbonyl with alpha hydrogens meets a secondary amine. What forms?

• Both give an imine with C=N
• The first gives an imine, the second enamine
• Both give an enamine with C=C-N as final product

How do you unmask an acetal back to the carbonyl?

• Add aqueous acid to hydrolyse it
• Add strong base and heat hard to force it
• Add more dry alcohol

Why is an acetal a good mask during a Grignard step?

• It survives base and nucleophiles but falls to aqueous acid
• It reacts with Grignard reagents before the target does
• It cannot be removed once it has formed

A student wonders why acid is needed twice in acetal formation. How do you answer?

• Acid protonates the carbonyl for attack, then the hydroxyl group for water loss
• Acid is only needed to colour the mixture
• One dose of acid always survives the whole reaction untouched

You must reduce an ester in a molecule that also carries a ketone. What is the plan?

• Reduce directly, since ketones ignore hydrides
• Protect the ketone as an acetal, reduce, then unmask with aqueous acid
• Protect the ester instead and reduce the ketone