base catalysed. The aldol condensation product 5 eliminates water through the enolate ion 6 to form pseudoionone 7. The reaction proceeds by acid catalysis where the double bond in 7 opens to form the carbocation 8. A rearrangement reaction of the carbocation follows with ring closure to 9. Finally a hydrogen atom can be abstracted from 9 by an acceptor molecule (Y) to form either 10 (extended conjugated system) or 11. Genetic differences in odor perception A single-nucleotide polymorphism in the OR5A1 receptor (rs6591536) causes very significant differences in the odor perception of beta-ionone, both in sensitivity and also in subjective quality. Individuals who contain at least one G allele are sensitive to beta-ionone and perceive a pleasant floral scent, while individuals who are homozygous AA are ~100 times less sensitive and at higher concentrations perceive a pungent sour/vinegar odor instead. See also Irones, a group of related chemical compounds Alpha-isomethyl ionone, a type of ionone References Perfume ingredients Flavors Enones Sesquiterpenes Apocarotenoids Cyclohexenes