🔬 Isomer Comparison
Butyric Acid vs Isobutyric Acid: Structure, Odor and Applications
Same formula, different backbone. Here is how the straight-chain and branched C4 acids diverge - and why the difference matters when you specify.
On paper, butyric acid and isobutyric acid look almost identical: both are C₄H₈O₂ with a molecular weight of about 88 g/mol. But they are structural isomers - same atoms, arranged differently - and that one difference is enough to change how they smell, how they behave and where they are used. 🔬 Confusing the two, or assuming they are interchangeable, is a genuine sourcing error.
This article puts n-butyric acid (CAS 107-92-6) and isobutyric acid (2-methylpropanoic acid, CAS 79-31-2) side by side. It is part of our butyric acid series - for the full application map, see the pillar guide Butyric Acid Uses and Applications.
⚗️ The Core Difference: Straight vs Branched
n-Butyric acid is a straight (normal) chain - four carbons in a row, with the acid group at the end:
CH₃–CH₂–CH₂–COOH (n-butyric acid)
Isobutyric acid is branched - a methyl group sits on the second carbon:
(CH₃)₂CH–COOH (isobutyric acid / 2-methylpropanoic acid)
📊 Property Comparison
| Property | n-Butyric Acid | Isobutyric Acid |
|---|---|---|
| CAS number | 107-92-6 | 79-31-2 |
| Structure | Straight chain | Branched |
| Formula / MW | C₄H₈O₂ / ~88.1 | C₄H₈O₂ / ~88.1 |
| Boiling point | Higher (~163 °C) | Lower (~155 °C) |
| Odor | Rancid butter | Pungent, cheesy/sweaty |
| Hazard | Corrosive | Corrosive |
💡 Approximate values for orientation. Verify exact data against supplier specifications and PubChem.
🧩 Why Branching Changes Behavior
Branching makes the isobutyric molecule more compact. A more compact shape generally lowers the boiling point (the straight chain packs and interacts a little more effectively), and it subtly shifts the odor character. These are small structural changes with real, practical consequences - which is exactly why the two isomers are specified separately and are not drop-in substitutes. ✅
👃 Odor: Both Unpleasant, Differently So
Neither isomer is pleasant in free-acid form. n-Butyric acid is the classic "rancid butter" smell. Isobutyric acid is often described as pungent and cheesy or sweaty. Interestingly, like n-butyric acid, isobutyric acid also forms fruity esters used in flavors and fragrances - the same acid-to-ester transformation covered for the straight-chain isomer in Butyric Acid Esters in Flavors & Fragrances, and the odor chemistry in Why Does Butyric Acid Smell So Bad? 👃
🎯 Applications: Where Each Is Used
n-Butyric acid dominates in animal feed (as butyrate), flavor esters, cellulose acetate butyrate (CAB) coatings and pharmaceutical intermediates - the full range mapped in our pillar guide.
Isobutyric acid is used in its own set of esters and derivatives, as an intermediate in specialty synthesis, and in some flavor and fragrance applications where its particular character is wanted. When a specification calls for one isomer, substituting the other can change product performance, odor or regulatory status - so specify deliberately. 🎯
🔗 Part of the Short-Chain Acid Family
Butyric acid also sits in a family of neighboring short-chain carboxylic acids - propionic (C3) and valeric/pentanoic (C5). If you are comparing across chain lengths rather than isomers, see our related coverage of N-Valeric Acid and the wider short-chain carboxylic acids comparison. 🧪
And for butyric acid's own reactive relatives - anhydride and acyl chloride - see Butyric Acid vs Butyric Anhydride vs Butyryl Chloride.
💬 Frequently Asked Questions
🔹 Are butyric acid and isobutyric acid the same?
No. They share the formula C₄H₈O₂ but are structural isomers: n-butyric acid is a straight chain, isobutyric acid is branched (2-methylpropanoic acid). That difference changes boiling point, odor and applications, so they are specified separately.
🔹 What is the CAS number for isobutyric acid?
Isobutyric acid (2-methylpropanoic acid) is CAS 79-31-2, while n-butyric acid is CAS 107-92-6. Always confirm the CAS number when ordering to be sure you are getting the correct isomer.
🔹 Which has the higher boiling point?
n-Butyric acid boils slightly higher (around 163 °C) than isobutyric acid (around 155 °C). The branched isobutyric molecule is more compact, which generally lowers its boiling point relative to the straight chain.
🔹 Do they smell the same?
Both are unpleasant as free acids but distinct: n-butyric acid smells of rancid butter, while isobutyric acid is often described as cheesy or sweaty. Both form fruity esters used in flavors and fragrances.
🔹 Can I substitute one for the other?
Generally no. If a process or product specifies a particular isomer, substituting the other can alter performance, odor or regulatory status. Specify the exact isomer (and confirm its CAS number) for your application.
📚 Related Reading
🧬 Derivatives
Butyric Acid vs Butyric Anhydride vs Butyryl Chloride →From isomers to derivatives - the other way the C4 acid family branches.
🧪 Neighbor Acid
N-Valeric Acid (Pentanoic Acid) →The next acid up the chain - compare C4 with its C5 relative.
🗺️ Pillar Guide
Butyric Acid Uses and Applications →The complete map of where n-butyric acid is used across industry.
Need the Right C4 Acid Isomer? 📩
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