Fragrance Chemistry and Ingredient Properties
Functional groups and behaviour
Fragrance ingredients are organic molecules whose odour and reactivity depend on functional groups: alcohols, aldehydes, ketones, esters, lactones, ethers, and hydrocarbons such as terpenes. Candidates should be able to predict basic behaviour: aldehydes oxidise readily to acids, which changes odour and can cause irritation; esters can hydrolyse in aqueous or alkaline conditions; terpenes are prone to oxidation and polymerisation, producing colour and off-notes. These properties drive the need for antioxidants, stabilisers and appropriate packaging.
Natural versus synthetic
A natural essential oil is a complex mixture whose composition varies with origin, harvest and processing; a synthetic is a defined molecule. The trap is assuming "natural" means safer - many potent sensitisers and phototoxicants are natural. Conversely, a synthetic identical to a natural molecule has the same toxicological profile. Questions often test whether a restriction applies to a natural oil because of a constituent, not because of the oil's name.
Purity and specifications
Specifications define identity, purity and limits on impurities. A material may meet its specification yet still contain a restricted constituent above the permitted level, so the specification and the Standard must both be checked. Isomers matter: the same name can cover different isomers with different odour and toxicology.
Worked rule of thumb
When evaluating a natural oil, list its known constituents and check each against the Standards. If the oil contains 2% of a constituent restricted to 0.01% in the finished product, the oil itself can be used at most 0.5% of the finished product, before considering the compound's own dilution.
Traps
Do not confuse odour threshold with safety threshold. Do not assume a material is stable because it is pure; oxidation and hydrolysis depend on conditions. Do not assume a name identifies a single isomer.