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Two drums of material can carry the same CAS number, clear the same 99% purity limit and still smell like two different products. The reason is usually that the specification was written for a molecule rather than for a material. Aroma chemistry is full of isomers and enantiomers: sets of compounds that share a formula and a molecular weight, are often sold under the same name, and yet fit different olfactory receptors and behave differently in a formula. Buying an aroma chemical means knowing which of the numbers on the certificate actually decides the smell.
Carvone, C10H14O, molecular weight 150.22, is the cleanest illustration, and the two optical isomers are sold under separate CAS numbers because they are genuinely different materials. L(−)-Carvone, CAS 6485-40-1, is the spearmint ketone; it makes up as much as 70% of natural spearmint oil, boils near 231 °C and is miscible with ethanol. Its mirror image, (S)-(+)-carvone, CAS 2244-16-8, is the caraway and dill note. The formula and the boiling range are the same; the aroma is opposite. An order that says only “carvone” has not stated a specification, and the same logic applies across the terpene family.
L-Menthol, CAS 2216-51-5, C10H20O, melts at 41–43 °C and reads αD20 −49.3°. That rotation is not a formality. Menthol has three chiral centres, so eight stereoisomers exist, and only the (−)-form fits the TRPM8 cold receptor that produces the cooling sensation. Racemic dl-menthol, CAS 89-78-1, gives roughly half the cooling strength for the same weight, and material sold loosely as “menthol crystals” can be racemic or a blend. The optical rotation on the certificate of analysis, together with the melting range, is what separates the perfumery and flavour grade from the cheaper stereoisomer mixtures.
Citral, CAS 5392-40-5, C10H16O, is a mixture of two geometric isomers: geranial, the trans form, and neral, the cis form. Natural citral runs at 70–80% in lemongrass oil, and the two isomers differ in odour and in reactivity even though they share a formula and a flash point. Because their boiling points sit within a few degrees of each other, the assay alone does not describe the material; the E:Z ratio does. The classical split uses bisulfite solubility, which separates the two cleanly. Citral is also one of the strongest fragrance allergens and yellows with time, so the specification needs a stability statement as well as a percentage.
The same argument governs the citrus terpene. d-Limonene, the orange peel monoterpene, C10H16, and its mirror l-limonene read as citrus and pine respectively; the racemic 50:50 mixture is sold as dipentene, CAS 68956-56-9, a common industrial grade. In fragrance the enantiomer decides the note; in any application the oxidation route matters, because limonene takes up oxygen to form carvone and allergenic hydroperoxides. A technical grade that has sat warm in a full drum is not the same material as a fresh perfumery grade, whatever the assay says.
Linalool, CAS 78-70-6, C10H18O, is an open-chain tertiary allylic alcohol with two double bonds, and its structure explains most of what a buyer has to watch. Three optical isomers occur in nature; the l-form dominates at roughly 70–80% of the total and comes from lavender and ho oil, while the d-form is the coriander-oil type. The alcohol content sets the market: 95% and above is the perfumery grade, while about 92.5% is the route to isophytol and vitamin E. The allylic alcohol autoxidises into hydroperoxides that discolour a formula and sensitise skin, so storage at 2–8 °C with minimal headspace, and a peroxide value on the certificate, are practical requirements rather than formalities. Linalool also isomerises in strong acid and stays stable in alkali, which matters when it is blended into an emulsion.
Geraniol, CAS 106-24-1, C10H18O, is the trans rose alcohol, and its cis isomer nerol shares the formula but reads greener and thinner. Citronellol, CAS 106-22-9, C10H20O, is one double bond shorter and reads as a softer rose; the l-form, known as rhodinol, is the finer of the natural forms. 2-Phenylethanol, CAS 60-12-8, carries the same rose theme on an aromatic rather than a terpene skeleton. The buying point is that source and synthesis leave different trace profiles under one name. Geraniol made by hydrogenating citral and geraniol isolated from palmarosa oil can share the name and the CAS yet differ in the minor components that give the note its fullness, which is why the industry has long warned against trusting a single name as evidence of origin.
Esters carry most of a modern formula. Isobornyl acetate, CAS 125-12-2, C12H20O2, supplies the pine and camphor note, and is a crystalline solid rather than an oil. Methyl dihydrojasmonate, CAS 24851-98-7, C13H22O3, is the soft jasmine workhorse prized for its diffusion and stability. At the fruity end sit methyl octanoate, CAS 111-11-5, C9H18O2, and ethyl laurate, CAS 106-33-2, C14H28O2. All of them hydrolyse, and free acid builds up with time and heat, pulling the pH and the odour with it. That is why a usable ester specification carries ester content, acid value and a storage temperature, not a single GC percentage that can hide the beginning of a hydrolysis.
“Musk” is not one substance class. Muscone, CAS 541-91-3, C16H30O, is the macrocyclic ketone, 3-methylcyclopentadecanone, that Ruzicka identified in natural musk in 1926; the natural L-form is the benchmark, while the commercial racemate runs a rotation close to zero. Musk ketone, CAS 81-14-1, C14H18N2O5, is a nitro aromatic from an entirely different family, and its persistence in the environment is the reason the nitro musks were restricted in several markets. Watermelon ketone, CAS 28940-11-6, C10H10O3, belongs to a third group again. They are not interchangeable on price, on odour profile or on compliance, and a quotation that offers “a musk” has offered very little.
Seven lines make aroma quotations comparable. The chemical name with its CAS, and where chirality exists, the enantiomer and the optical rotation. The assay by GC together with the isomer ratio, since one purity figure hides the rest. The FEMA or JECFA number where the material is for food rather than perfume. An odour and colour reference against a retained sample under a stated light. Stability data for the reaction that actually applies, autoxidation for the allylic alcohols and hydrolysis for the esters. Allergen and sensitizer status for the regulated molecules such as citral and linalool. And storage, packaging and headspace requirements. The name and the CAS fix the molecule; the isomer ratio and the stability data decide whether it still smells the same six months later.
