Hydroxycitronellal

A conventional Chinese route first reacts citronellal with sodium bisulfite to form an aldehyde adduct, reducing the risk of aldehyde condensation, cyclisation, and polymerisation during subsequent acid-catalysed hydration. The terminal double bond is then hydrated in an acidic aqueous phase, followed by neutralisation, deprotection, extraction, and vacuum distillation to obtain hydroxycitronellal; acidity, reaction temperature, residence time, interphase mass transfer, and citronellal feed purity are the main controls for conversion, selectivity, and by-products such as isopulegol and dimers (Chen, 1996, Journal of Hainan University Natural Science). This route uses accessible reagents and conventional equipment, but it generates sulfite- and acid-containing wastewater and requires control of sulfur-dioxide release and corrosion, so historical yields of approximately 64–65% should not be presented as a universal plant yield. Chinese patent literature also describes direct catalytic hydration of citronellal with water over Pd@ZSM-type porous catalysts, with a reported 60–90°C operating window and high selectivity; this is a lower-waste process-development option rather than an established industry-standard route, and catalyst lifetime, metal residues, heat transfer on scale-up, and long-term economics require plant-specific validation (CN111825543B, 2022).

Hydroxycitronellal primarily contributes deodorising performance by masking surfactant, fatty-acid, and other base odours with a fresh muguet-floral profile, creating a clean-floral impression in soaps, shampoos, body care, fabric care, and air-care fragrances; it should not be presented as a proven chemical odour-neutralising active. As a free aldehyde, it may lose odour quality or undergo side reactions in high-pH soap or detergent bases, so stability should be verified in the actual base, storage conditions, and intended shelf life; hydroxycitronellal dimethyl acetal (CAS 141-92-4) can be evaluated where improved alkaline stability is required. Hydroxycitronellal is IFRA-restricted: under the 51st Amendment, maximum finished-product levels are 0.11% for Category 2, 4.1% for Category 9, 0.78% for Category 10A, and 7.8% for Category 10B, calculated for the actual final-product exposure category (IFRA, 2023, 51st Amendment Standards). It is also an EU fragrance allergen requiring individual labelling in new cosmetics first placed on the EU market after 31 July 2026 when present above 0.001% in leave-on products or 0.01% in rinse-off products (European Commission, 2023, Regulation (EU) 2023/1545). The dimethyl acetal has shown no skin-sensitisation concern at its current declared levels of use, but its compliance status and permitted concentration must still be verified for the final formula using the current supplier IFRA certificate rather than by applying the parent hydroxycitronellal limits or safety conclusions directly (Api et al., 2025, Food and Chemical Toxicology).

Hydroxycitronellal (CAS 107-75-5) is restricted under IFRA Standard 043 on the basis of dermal sensitisation and systemic toxicity; under the 51st Amendment, maximum finished-product concentrations include 0.11% for Category 2 axillary deodorants and antiperspirants, 2.1% for Category 4 fine fragrance, 4.1% for Category 9 rinse-off toiletries and soaps, 0.78% for Category 10A non-aerosol household products, and 7.8% for Category 10B aerosol household products, while the applicable category should be confirmed using the current IFRA Certificate of Conformity (IFRA, 2023, 51st Amendment Standards). For restriction standards, the 51st Amendment applied from 30 March 2024 to new fragrance creations and from 30 October 2025 to existing creations; these dates concern the placing of fragrance mixtures on the market, not directly the launch date of the finished consumer product. Under the EU Cosmetics Regulation, hydroxycitronellal is an individually labelled fragrance allergen: cosmetics first placed on the EU market from 31 July 2026 must declare it above 0.001% in leave-on products and 0.01% in rinse-off products (European Commission, 2023, Regulation (EU) 2023/1545). IFRA limits control the maximum safe use level, whereas EU allergen thresholds trigger consumer-labelling duties; both requirements apply simultaneously, so an IFRA-compliant product may still require hydroxycitronellal to be listed on the ingredient label (Api et al., 2022, Food and Chemical Toxicology).

Hydroxycitronellal is a core material in classical muguet accords, providing a fresh, radiant, slightly sweet floral note with watery-green facets and forming the heart of jasmine, hyacinth, narcissus, and other floral compositions (BASF, 2024, Hydroxycitronellal Technical Information). It can be blended with methyl dihydrojasmonate, cyclamen aldehyde, phenethyl alcohol, and other muguet materials to adjust diffusion, green freshness, petal character, and softness; the working level should be set against the target odour, product base, and remaining regulatory allowance rather than a fixed “typical” dosage. In fine fragrance, lotions, and body-care products, the permitted concentration in the fragrance compound must be back-calculated from the 2.1% Category 4 IFRA limit in the finished product; in high-pH soap and detergent bases, accelerated stability and sensory testing are needed because the free aldehyde may affect long-term odour quality (IFRA, 2023, 51st Amendment Standards). Hydroxycitronellal dimethyl acetal can be evaluated as a more alkali-stable muguet alternative, but its odour strength and overall profile are not identical to the parent material, and its permitted use must be confirmed through its own current IFRA Certificate of Conformity and safety assessment rather than assumed to be unrestricted (Api et al., 2025, Food and Chemical Toxicology).