Acetal formation (protecting group): practice and mechanism
With the diol a five-membered ring closes: the carbonyl carbon then carries two ring oxygens (cyclic acetal) and water is lost.
Example
With a diol and acid, a cyclic acetal forms via the hemiacetal and water is released. Acetals are stable to bases and nucleophiles and serve as protecting groups for carbonyls.
Mechanism step by step
- 1
The acid protonates the carbonyl oxygen and one OH of the diol attacks: hemiacetal.
- 2
The hemiacetal OH is protonated and leaves as water (oxocarbenium ion).
- 3
The second OH of the diol attacks and closes the five-membered ring: cyclic acetal.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That is the hemiacetal, only the intermediate. Under acid catalysis the second OH group also reacts and the ring closes.
Often confused with: Imine formation · Reduction with sodium borohydride
Frequently asked questions
How do you remove the protecting group?
With aqueous acid; the reaction then runs backwards and releases the carbonyl compound.
Why protect at all?
To reduce an ester with LiAlH₄ or use a Grignard, for example, without the ketone in the same molecule reacting too.