Carboxylic Acid → Acid Chloride with Thionyl Chloride (SOCl₂)

Thionyl chloride transforms carboxylic acids (RCO₂H) into acid chlorides (RCOCl) under dry conditions. The textbook path proceeds through a chlorosulfite (mixed anhydride) that collapses with loss of SO₂ and HCl gases—the evolving gases remove themselves from solution, so the reaction is effectively irreversible and high-yielding. A catalytic drop of DMF often accelerates unreactive or deactivated acids by generating a Vilsmeier-type chlorinating agent.

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Quick Summary

Reagents / conditions	SOCl₂ (1.1–2.0 eq), dry CH₂Cl₂ or neat SOCl₂, 0 °C → reflux as needed; optional cat. DMF (0.05–0.2 eq) and/or pyridine/Et₃N to trap HCl
Outcome	RCO₂H → RCOCl + SO₂(g) + HCl(g); gases vent, making the reaction effectively irreversible.
Mechanism	Acyl chlorosulfite formation → Cl⁻ attack → collapse to SO₂/HCl + RCOCl. Optional DMF catalysis accelerates via Vilsmeier-type activation.
Why it works	Gas byproducts (SO₂, HCl) leave the solution, pushing equilibrium to the acid chloride.
Stereochemistry	No rearrangements—the acyl carbon remains sp²; only the OH is replaced by Cl.
Contrasts & pitfalls	Moisture hydrolyzes both SOCl₂ and RCOCl; formic acid cannot furnish HCOCl (decomposes to CO). DMF catalysis speeds sluggish, deactivated acids.

Mechanism — Chlorosulfite Path (Three Core Steps)

All frames are exported directly from the RDKit builder, so the curved arrows, overlays, and seafoam highlights mirror the interactive Mechanism Solver experience. (Cat. DMF accelerates stubborn acids but follows the same net arrow logic.)

Mechanistic Checklist (Exam Focus)

• Draw the acyl chlorosulfite intermediate (RCO–O–SOCl) and explicitly show Cl⁻ attack on the acyl carbon.
• Mention SO₂/HCl gas evolution as the thermodynamic driver; it explains why the reaction is effectively irreversible.
• DMF catalysis doesn’t change the net stoichiometry—just faster activation (Vilsmeier-type).
• Moisture destroys both SOCl₂ and RCOCl; depict a dry setup and note that water would revert to the acid.
• No rearrangements: the carbonyl carbon remains sp² before and after; only the –OH is replaced by –Cl.

Worked Examples

Scope & Limitations

• Substrates: Aliphatic, benzylic, and aromatic carboxylic acids convert smoothly; formic acid is a notable exception (HCOCl cannot be isolated).
• Functional groups: Free amines, thiols, or alcohols may be acylated/chlorinated; protect them or adjust order of operations.
• Moisture sensitivity: Water hydrolyzes both SOCl₂ and RCOCl; ensure dry glassware, inert atmosphere if possible.
• Reagent alternatives: Oxalyl chloride (ClCOCOCl) + cat. DMF or PCl₃/PCl₅ can replace SOCl₂ when gas handling is problematic.
• Gas management: SO₂ and HCl are corrosive/lachrymatory—use scrubbers or trap bottles when scaling up.

Practical Tips

• Vent properly: Equip the setup with a gas trap or bubbler; SO₂/HCl must be vented safely.
• Temperature ramp: Chill to 0 °C before adding SOCl₂, then warm as required—addition is exothermic.
• Catalytic DMF: One or two drops often boosts sluggish substrates; more is unnecessary and may complicate workup.
• HCl scavengers: Pyridine or Et₃N (0.5–1.0 eq) reduce corrosion and keep reactions homogeneous.
• Quench carefully: Destroy excess SOCl₂ by slow addition of ice-cold NaHCO₃ or MeOH under cooling; gases will evolve vigorously.

Exam-Style Summary

Net reaction: RCO₂H + SOCl₂ → RCOCl + SO₂(g) + HCl(g)
Mechanism spine: Carboxyl oxygen adds to SOCl₂ → chlorosulfite → Cl⁻ attack → collapse to SO₂/HCl → acid chloride. Catalytic DMF simply accelerates activation (Vilsmeier-type) without changing products.

Pitfalls to flag: Forgetting gas evolution (driving force), assuming reversibility (it’s not), overlooking moisture sensitivity, or claiming formyl chloride can be bottled.

FAQ

Why add DMF if SOCl₂ already works?
DMF and SOCl₂ form a Vilsmeier-type chloroiminium that’s an even better chlorinating agent, so hindered or electron-poor acids react faster/cleaner.

Can I isolate formyl chloride using SOCl₂?
No. Formyl chloride is unstable and decomposes; formic acid typically produces CO/CO₂ under these conditions.

Do I need a base (pyridine/Et₃N)?
Not strictly, but adding a base suppresses HCl corrosion and helps keep the reaction homogeneous, especially on scale.

Interactive Toolbox

• Mechanism Solver — Replay the chlorosulfite steps (and optional DMF introduction) exactly as shown above.
• Reaction Solver — Chain the freshly formed acid chloride directly into esterification or amide formation logic.
• IUPAC Namer — Confirm acid chloride names (acetyl chloride, benzoyl chloride, pivaloyl chloride, …).

Related Reading

• Acid Chloride → Ester with ROH + Pyridine — Follow-up acylation once the chloride is formed.
• Carboxylic Acid → Alcohol using LiAlH₄ — Alternative fate of the acid group (reduction vs activation).
• Amide Formation from Acid Chlorides + Amines — Typical downstream use of the freshly generated acid chloride.