Citronellal is a key acyclic monoterpenoid aldehyde predominantly found in citronella oil. It is widely recognized for its potent insect-repellent properties, its characteristic lemony-green aroma, and its broad spectrum of biological activities, including antimicrobial, anti-inflammatory, and neuroprotective effects. These activities are primarily mediated through mechanisms such as membrane perturbation and modulation of reactive oxygen species (ROS).

Chemical Properties

Citronellal (3,7-dimethyloct-6-enal, C₁₀H₁₈O; molecular weight 154.25 g/mol) is a colorless to pale yellow liquid with a boiling point of 206–208°C, density of 0.848 g/cm³, and refractive index (nD20) of 1.446. It exists as chiral (R) and (S) enantiomers, each contributing distinct citrus-woody olfactory profiles.

Its aldehyde functional group enables multiple chemical transformations, including reduction to citronellol, Prins cyclization to isopulegol, and catalytic hydrogenation to dihydrocitronellal. With a logP value of 3.23, citronellal exhibits low water solubility (~1.5 g/L). It is acid-sensitive and prone to aldol self-condensation under certain conditions.

Biosynthetic Pathway

Citronellal is biosynthesized via the methylerythritol phosphate (MEP) pathway, where geranyl diphosphate is converted into citronellal through NADPH-dependent geraniol 10-hydroxylase activity. This process occurs notably in Cymbopogon nardus (citronella grass), where citronellal can represent 65–80% of the essential oil composition, as well as in Eucalyptus citriodora.

Biologically, citronellal emission rates can reach 20–100 μg·g⁻¹·h⁻¹ (dry weight) following herbivory stress. In biotechnological systems, engineered Yarrowia lipolytica strains expressing heterologous prenyltransferase and aldehyde synthase pathways have achieved production titers up to 1.2 g/L, highlighting its potential for industrial biosynthesis.