Acetyl Chloride vs Thionyl Chloride: Properties, Hazards and Handling

Acetyl chloride and thionyl chloride at a glance

Acetyl chloride (CH₃COCl) and thionyl chloride (SOCl₂) are both colourless, fuming, chlorine-containing liquids that react violently with water, which is why buyers and chemists often look them up together. They do different jobs. Acetyl chloride is the acid chloride of acetic acid and is used to add an acetyl group to other molecules. Thionyl chloride is a chlorinating reagent, used among other things to make acid chlorides such as acetyl chloride in the first place.

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Physical properties compared

Figures are from NOAA CAMEO Chemicals. The NIST Chemistry WebBook’s averaged boiling point for acetyl chloride, 325 ± 2 K (about 52 °C), agrees. Published boiling points for thionyl chloride run from about 75 to 79 °C, so treat any single figure as approximate. Both liquids are denser than water, and the “W” in both NFPA ratings is the warning that they react dangerously with water.

What each chemical is used for

Acetyl chloride is an acetylating agent. It transfers an acetyl group (CH₃CO–) to alcohols to give esters and to amines to give amides, and with a Lewis-acid catalyst such as aluminium chloride it acetylates aromatic rings (Friedel–Crafts acylation). That makes it a reagent in pharmaceutical, dye and fine-chemical synthesis, and in the laboratory.

Thionyl chloride is a chlorinating agent. It converts carboxylic acids into acid chlorides and alcohols into alkyl chlorides, and because its by-products are gases the product is easy to recover. It is used to make pharmaceutical and agrochemical intermediates, and it is also the liquid cathode in lithium–thionyl chloride (Li-SOCl₂) primary batteries.

How thionyl chloride makes acetyl chloride

Thionyl chloride is a standard laboratory reagent for turning a carboxylic acid into its acid chloride. With acetic acid the reaction is:

CH₃COOH + SOCl₂ → CH₃COCl + SO₂ + HCl

Both by-products, sulfur dioxide and hydrogen chloride, are gases and leave the mixture, which simplifies the work-up. The acetyl chloride still has to be fractionally distilled away from any excess acetic acid or thionyl chloride. That works because the three boil well apart: about 51 °C for acetyl chloride, about 76 °C for thionyl chloride and about 118 °C for acetic acid.

Water reactivity and main hazards

Acetyl chloride reacts violently with water, steam, methanol or ethanol, giving off heat and forming hydrogen chloride and acetic acid. In a closed space, even a small amount of water can cause a violent eruption of gas. It is highly flammable, with a flash point of about 4 °C, and its vapour, 2.7 times as dense as air, forms explosive mixtures with air. When heated to decomposition it gives off hydrogen chloride and phosgene, both extremely poisonous gases. The vapour irritates the mucous membranes, and swallowing the liquid or getting it on the skin or in the eyes causes severe irritation.

Thionyl chloride does not burn, but it reacts violently with water to release hydrochloric acid and sulfur dioxide, and it gives off dense, corrosive fumes in moist air. Containers may explode when heated or contaminated with water, and contact with metals may release flammable hydrogen. It is corrosive and toxic: inhaling, swallowing or touching the vapour or liquid can cause severe injury, burns or death. It also reacts violently with bases such as ammonia, sodium hydroxide and amines, with alkali metals and with some esters.

Storage and handling

For both chemicals the first rule is to keep water out. Store them in tightly closed containers in a cool, dry, well-ventilated place, away from water, steam, alcohols and bases, and open them only in dry conditions under extraction. Keep acetyl chloride away from heat, sparks, flames and oxidising agents. Keep thionyl chloride away from alkali metals, and from dimethyl sulfoxide or dimethylformamide, with which it can react explosively when traces of iron or zinc are present.

Spill clean-up and fire-fighting measures are specific to each chemical, so follow sections 5 to 8 of the current safety data sheet (SDS) for fire-fighting, spills, storage and protective equipment.

Transport class and export controls

NOAA CAMEO Chemicals lists acetyl chloride (UN 1717) with the DOT labels Flammable Liquid and Corrosive, and thionyl chloride (UN 1836) with the DOT label Corrosive. Always confirm the class, packing group and labels on the current SDS and the shipping documents for your consignment.

Thionyl chloride is also listed in Schedule 3 (Part B, precursors) of the Chemical Weapons Convention. It is made in large quantities for legitimate industrial use, but countries that are party to the Convention report their national production, imports and exports of Schedule 3 chemicals, and transfers to states outside the Convention need an end-use certificate. Expect a supplier to ask what the material is for. Acetyl chloride is not a scheduled chemical.

Questions to ask a supplier

• Which grade is offered, and what assay does the Certificate of Analysis report for the batch?
• Can you share the current SDS, with the UN number, hazard class and packing group?
• What packaging is used, and is it rated for water-reactive, corrosive liquids?
• How is moisture kept out during filling, storage and transit?
• For thionyl chloride, what end-use or end-user information do you need for my destination?
• Which Incoterms and destination ports can you ship dangerous goods to?

Enquire with Scapex Exports

Scapex Exports is an exporter and supplier of industrial and fine chemicals from India. Related products in our catalogue include thionyl chloride, glacial acetic acid and oxalyl chloride, another reagent for making acid chlorides. Each product page links to its COA and MSDS.

Acetyl chloride is not in our online catalogue. If you need it, name it in your enquiry and we will confirm whether we can quote. Send the product, grade, quantity, packing and destination port, and we will reply with a quotation.

Frequently asked questions

Sources: NOAA CAMEO Chemicals datasheets for acetyl chloride and thionyl chloride; NIST Chemistry WebBook (CAS 75-36-5 and 7719-09-7); PubChem boiling-point records for thionyl chloride; OPCW, Chemical Weapons Convention, Schedule 3; LibreTexts Chemistry, “Making Acyl Chlorides (Acid Chlorides)”.