Alcohol Acetylation Ac2o H2so4

Alcohol Acetylation with Acetic Anhydride (Ac2O / H2SO4)

Acetic anhydride under acid catalysis converts an alcohol (ROH) into an acetate ester (ROAc). Mechanistically, this is nucleophilic acyl substitution at a carbonyl (addition–elimination), not SN1/SN2 at carbon.



Quick Summary

  • Reagents/conditions: Acetic anhydride (Ac₂O) + catalytic sulfuric acid (H₂SO₄).
  • Outcome: ROH → ROAc (acetate ester); carbon skeleton unchanged.
  • Byproduct: acetic acid (AcOH).
  • Mechanism: carbonyl activation → ROH attack → collapse (acetate leaves) → deprotonation.
  • Common pitfall: drawing SN1/SN2 at carbon instead of acyl substitution at the carbonyl.


Mechanism — Four Steps (Activation, Attack, Collapse, Product)

Step 1: Protonation activates an acetic anhydride carbonyl for alcohol acetylation.
Step 1 — Protonation activates an anhydride carbonyl, increasing electrophilicity at the acyl carbon.
Step 2: Alcohol oxygen attacks the activated acyl carbon of acetic anhydride to form a tetrahedral intermediate.
Step 2 — The alcohol oxygen attacks the activated acyl carbon (addition to the carbonyl), giving a tetrahedral intermediate.
Step 3: Tetrahedral intermediate collapses to re-form the carbonyl and expel acetate leaving group.
Step 3 — Collapse reforms the carbonyl and expels acetate as the leaving group (addition–elimination at the acyl carbon).
Step 4: Deprotonation gives the neutral acetate ester and acetic acid byproduct.
Step 4 — Deprotonation gives the neutral acetate ester (ROAc). The leaving group becomes acetic acid (AcOH), and the acid catalyst is regenerated.


Worked Examples

Each example shows substrate → reagent → product as an acetate ester (ROAc).

Example A — Primary alcohol → acetate ester

Primary alcohol substrate for acetylation Ac2O with catalytic H2SO4 reagent card Primary acetate ester product

Exam cue: the O atom stays in the product (R–O–C(=O)CH₃), so this is not substitution at carbon.

Example B — Secondary alcohol → acetate ester

Secondary alcohol substrate for acetylation Ac2O with catalytic H2SO4 reagent card Secondary acetate ester product

Key idea: the carbonyl is the electrophile (addition–elimination), not the alcohol carbon.

Example C — Tertiary alcohol (fringe)

Tertiary alcohol substrate for acetylation (fringe case) Ac2O with catalytic H2SO4 reagent card Tertiary acetate ester product (potential outcome)

Exam trap: under strongly acidic, heated conditions, tertiary alcohols can dehydrate (E1). When Ac₂O is explicitly present, many courses still treat this as acylation to ROAc.



Scope & Limitations

  • Alcohols: Primary and secondary alcohols typically acetylate cleanly; bulky substrates can be slower.
  • Multiple OH groups: Polyols can acetylate more than once unless the problem specifies limited equivalents or selectivity.
  • Competing nucleophiles: Amines can compete strongly (N-acetylation) if present.
  • Acid sensitivity: Strongly acid-labile groups may not survive H₂SO₄ conditions.


Practical Tips & Pitfalls

  • Think “carbonyl substitution” whenever you see Ac₂O (an acid derivative).
  • The leaving group is acetate, and the common byproduct is acetic acid after proton transfer.
  • Don’t confuse this with Fischer esterification: here the acyl donor is Ac₂O (activated), not a carboxylic acid.


Exam-Style Summary

ROH + (CH₃CO)₂O → RO–C(=O)CH₃ + CH₃CO₂H
Mechanism = activation of the anhydride carbonyl + nucleophilic addition of ROH + collapse + deprotonation.



FAQ

Is this SN1 or SN2?
Neither — it’s nucleophilic acyl substitution at a carbonyl (addition–elimination).

Where does the “acetate” piece go?
The alcohol oxygen becomes part of the ester, and the other half of the anhydride ends up as acetic acid (AcOH).



Interactive Toolbox

  • Mechanism Solver — choose the Ac₂O / H₂SO₄ reagent and replay each RDKit-rendered step with curved arrows.
  • Reaction Solver — test new alcohol substrates and compare against competing acid-driven transformations.
  • IUPAC Namer — name the substrates and acetate ester products from the worked examples.


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