4-Methylnonanoic acid (CAS 45019-28-1) — Musky Base Note Fragrance Ingredient
4-Methylnonanoic acid
CAS 45019-28-1
What Is 4-Methylnonanoic acid?
4-Methylnonanoic acid is a synthetic fatty acid used in perfumery to add animalic, musky undertones. It’s found in trace amounts in luxury fragrances and body care products. This ingredient matters because it provides a cost-effective alternative to natural animalic notes, allowing perfumers to create rich, complex scents without using animal-derived materials.
Safety Profile
USE WITH AWARENESSWhat Does 4-Methylnonanoic acid Smell Like?
4-Methylnonanoic acid delivers a pungent, cheesy-fatty opening that evolves into a warm, animalic musk reminiscent of unwashed human skin. The top note carries a sweaty, almost goat-like intensity that quickly mellows into a leathery heart. In drydown, it becomes surprisingly smooth – like expensive suede gloves left in a cedar chest. At dilution, it adds a primal depth that enhances floral absolutes and woody bases, behaving like a molecular bridge between clean and dirty accords.
In Famous Fragrances
Fragrance associations may not reflect actual formulations.
Used here to amplify the animalic musk core, providing the controversial ‘dirty skin’ effect that makes this fragrance legendary. The methyl branching adds diffusion compared to linear fatty acids.
Employed in trace amounts to ground the opulent frankincense with human warmth. The methyl group prevents excessive waxiness in the base.
Chemistry, Properties & Perfumer Guide
The Chemistry
4-Methylnonanoic acid belongs to the branched-chain fatty acid family. Unlike its straight-chain counterparts, the methyl group at the 4-position creates steric hindrance that affects odor perception and volatility. Synthesized via hydroformylation of octene followed by oxidation, or through Grignard reactions with subsequent carbon chain elongation. The branching makes it more soluble in ethanol than linear C10 acids, though still requiring careful dilution due to potency.
Physical & Chemical Properties
| Boiling Point | ~250 °C (estimated) |
|---|---|
| Density | ~0.89 g/cm³ (estimated) |
Perfumer Guide
| Application | Typical % | Range | Notes |
|---|---|---|---|
| Fine Fragrance | 0.01-0.1% | Up to 0.3% | Extremely potent – use sparingly |
| Functional Fragrance | 0.001-0.01% | Up to 0.05% | Masking agent for soap bases |
Classic Accords
Tip: Pre-dilute to 1% in DPG before incorporating into bases to prevent overwhelming blends.
Alternatives & Comparisons
Linear C10 fatty acid with cleaner, more waxy character. Use when animalic effects need tempering.
Shorter chain with more pronounced sweat note. Better for top-mid note animalic effects.
Safety, Regulatory & Sustainability
⚠ Regulatory Disclaimer
General reference only. Consult current IFRA Standards Library before formulating.
IFRA Status
No specific restrictions under current IFRA standards (as of 49th Amendment).
GHS Classification
RIFM Assessment
Not currently evaluated by RIFM. Considered safe at industry-standard usage levels.
Sustainability
As a synthetic material, 4-methylnonanoic acid avoids animal welfare concerns associated with natural animalic materials. Production typically uses petrochemical feedstocks, though bio-based routes from fermented sugars are being explored. Energy-intensive synthesis requires careful waste management.
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References
- Brenna E. et al. (2002). Branched-chain fatty acids in perfumery. Flavour and Fragrance Journal.
Data: PubChem (NIH), PubMed, RIFM, IFRA. Last reviewed: Apr 2026.
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