🧬 Derivatives Comparison
The C4 Acyl Family: Butyric Acid vs Butyric Anhydride vs Butyryl Chloride
Same butyryl backbone, three very different reactivities. Here is how to choose the right acylating agent - and handle each one safely.
To build a butyrate ester or amide, a chemist has a choice: use butyric acid directly, or use one of its more reactive derivatives. Together, butyric acid, butyric anhydride and butyryl chloride form the C4 acyl family - all carrying the same four-carbon "butyryl" group, but climbing a ladder of reactivity from mild to aggressive. 🧬 The trade-off is simple in principle: more reactivity usually means faster, cleaner acylation, but greater hazard and harder handling.
This guide compares the three so you can match the reagent to the job. It is part of our butyric acid series - for the parent acid's full profile, see the pillar guide Butyric Acid Uses and Applications, and for the isomer angle, Butyric Acid vs Isobutyric Acid.
⚗️ The Reactivity Ladder
In acylation chemistry, the "leaving group" determines how reactive an acyl compound is. Rank them from mild to aggressive: 🔬
🟢 Butyric acid (–OH leaving group) - least reactive; usually needs a catalyst and heat.
🟡 Butyric anhydride (–OOCR leaving group) - moderately reactive; a common, manageable acylating agent.
🔴 Butyryl chloride (–Cl leaving group) - highly reactive; fast acylation, but hazardous and moisture-sensitive.
🟢 Butyric Acid - The Mild Parent
Formula: CH₃CH₂CH₂COOH · CAS 107-92-6
The starting point of the family. As a carboxylic acid it is the least reactive acylating agent - direct esterification (Fischer esterification) typically needs an acid catalyst and heat, and produces water as a by-product. It is the go-to where cost and simplicity matter and reaction speed is not critical, and it is the raw material from which the anhydride and acyl chloride are made. It is corrosive but far easier to handle than its derivatives.
🟡 Butyric Anhydride - The Practical Middle Ground
Formula: (CH₃CH₂CH₂CO)₂O · CAS 106-31-0 · MW ~158 · bp ~198 °C
Formed from two butyric acid units joined through an oxygen. The anhydride is more reactive than the acid and acylates alcohols and amines readily, releasing butyric acid (rather than water) as the by-product. It offers a good balance: brisk reactivity without the extreme hazards of an acyl chloride. 🧪
It is widely used to introduce butyryl groups - for example in making cellulose acetate butyrate (CAB) and other butyrate esters. See the CAB story in Cellulose Acetate Butyrate (CAB) in Coatings and Inks. Butyric anhydride is still corrosive and reacts with water, so it requires careful, dry handling.
🔴 Butyryl Chloride - The Aggressive Specialist
Formula: CH₃CH₂CH₂COCl · CAS 141-75-3 · MW ~106.6 · bp ~102 °C
The most reactive member. As an acyl chloride, butyryl chloride acylates alcohols and amines rapidly and almost quantitatively - releasing hydrogen chloride (HCl) as it does. That speed makes it invaluable in pharmaceutical, agrochemical and specialty synthesis (including as an acylation reagent and peroxide precursor), but it comes with serious hazards. ⚠️
Butyryl chloride reacts violently with water, hydrolyzing to butyric acid and HCl. It is chosen when maximum acylating power is needed and moisture can be rigorously excluded - otherwise the anhydride or acid is safer.
🚨 Butyryl Chloride - GHS Safety Disclosure
Acyl chlorides demand extra caution. Butyryl chloride carries the GHS signal word Danger, with hazard statements including H225 (highly flammable liquid and vapour) and H314 (causes severe skin burns and eye damage). It ships as UN 2353, hazard class 3 (8), Packing Group II - both flammable and corrosive.
🔥 On contact with water it releases HCl; heating or combustion can generate hydrogen chloride and other toxic gases. Handle only in dry conditions with full PPE, splash protection and respiratory protection, and keep it away from water, alcohols, strong bases and oxidizers.
Always follow the product SDS. Reference data: ECHA (InfoCard 100.004.999) and PubChem (CID 8855). For the parent acid's handling, see Butyric Acid Safety, Handling and Storage.
📊 The C4 Acyl Family Compared
| Property | Butyric Acid | Butyric Anhydride | Butyryl Chloride |
|---|---|---|---|
| CAS | 107-92-6 | 106-31-0 | 141-75-3 |
| Reactivity | Low | Moderate | High |
| By-product of acylation | Water | Butyric acid | HCl |
| Water reaction | Miscible/stable | Slow hydrolysis | Violent hydrolysis |
| Key hazard | Corrosive | Corrosive | Flammable + corrosive |
| Typical choice when… | Cost/simplicity | Balanced route | Max acylating power |
💡 Values approximate, for orientation. Confirm against supplier specifications and each product's SDS.
🧭 Which One Should You Use?
🔹 Simple esterification, cost-sensitive, speed not critical? → butyric acid.
🔹 Reliable acylation with manageable hazard? → butyric anhydride.
🔹 Fast, near-quantitative acylation, moisture excluded, hazard accepted? → butyryl chloride.
🔗 The same acid–anhydride–chloride pattern repeats one carbon down, in the C3 (propionyl) family. If you work across chain lengths, explore our propionic anhydride and propionyl chloride product pages, backed by the Sinolook group.
💬 Frequently Asked Questions
🔹 What is the difference between butyric acid, butyric anhydride and butyryl chloride?
They share the same butyryl (C4) group but differ in the leaving group and therefore reactivity. The acid is mildest, the anhydride moderate, and the acyl chloride most reactive. They also differ in by-products (water, butyric acid, HCl respectively) and hazard level.
🔹 Why use an anhydride or acyl chloride instead of the acid?
Because they acylate faster and more completely. Direct esterification with the acid is slow and reversible; the anhydride and acyl chloride drive the reaction forward more efficiently - useful when yield, speed or difficult substrates are involved.
🔹 Why is butyryl chloride so hazardous?
It is both highly flammable (H225) and corrosive (H314), and it reacts violently with water to release hydrogen chloride. Heating or combustion can produce toxic gases. It must be handled dry, with full PPE, and kept away from water, alcohols, bases and oxidizers - always per its SDS.
🔹 How is butyryl chloride made?
It is typically produced by chlorinating butyric acid (for example with reagents such as thionyl chloride), replacing the acid's –OH with –Cl to form the reactive acyl chloride.
🔹 Which is best for making cellulose acetate butyrate (CAB)?
Butyric anhydride is commonly used to introduce butyryl groups onto cellulose in CAB manufacture, offering a good balance of reactivity and handling. See our dedicated CAB article for how the resin is built and used.
📚 Related Reading
🔬 Isomers
Butyric Acid vs Isobutyric Acid →From derivatives to isomers - the other way the C4 acid family branches.
🎨 Application
Cellulose Acetate Butyrate (CAB) in Coatings and Inks →Where butyric anhydride's acylating power builds a high-performance resin.
⚠️ Safety
Butyric Acid Safety, Handling and Storage →Handling the parent acid - the baseline before you step up the reactivity ladder.
Sourcing Butyric Acid or Its Derivatives? 📩
We supply butyric acid (CAS 107-92-6) and can advise across the wider acyl family for your synthesis needs, with full specifications, CoA and SDS. Tell us your reagent, grade and volume and our team will match documentation and packaging. Sinolook Chemical, backed by the Sinolook group, serves customers in 50+ countries.
