Peracetylation: practice and mechanism
With excess acetic anhydride every OH group becomes an acetate ester, including the anomeric and the primary one. Glucose becomes a pentaacetate.
Example
With excess acetic anhydride every OH group is esterified, including the one on the anomeric carbon. Glucose gives the pentaacetate. The acetyl groups act as protecting groups and make sugars soluble in organic solvents.
Mechanism step by step
- 1
Pyridine activates the acetic anhydride (acylpyridinium ion) and captures the acetic acid released.
- 2
Each OH group attacks an acylating agent at the C=O group; acetate or pyridine leaves.
- 3
With an excess every OH group is converted; glucose gives the pentaacetate.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which reagent achieves this transformation?
Typical mistakes
- With an excess it does not stop at the anomeric OH. Acetic anhydride with pyridine acylates every OH group, not just the most reactive one.
- The primary OH group reacts fastest, but with an excess all secondary OH groups are esterified as well.
Often confused with: Permethylation · Esterification with acetic anhydride
Frequently asked questions
Why peracetylate sugars?
As protecting groups and to make them soluble in organic solvents and crystalline. The acetates can later be removed with NaOCH₃ in methanol (Zemplén).
Is the α/β configuration retained?
In pyridine at low temperature largely yes, because the anomer is acetylated faster than it mutarotates. With NaOAc at high temperature the β-pentaacetate is favoured.