ong>Answer:

Two or more molecules display isomerism when they have the same number of atoms of each TYPE but have these bonded together either in a different sequence (structural isomers) or with a different ARRANGEMENT in space (stereoisomers). There are two kinds of stereoisomerism: geometrical and optical.

Since isomers have different molecular shapes, isomeric fragrance molecules often have quite different odour characteristics, even though their structures are quite similar. One example is the group of isomers with the chemical FORMULA C10H18O.

Ten CARBON atoms

This combination of elements can form alcohols or aldehydes. Roll your mouse over the highlighted names to see the structures in the PICTURE on the left.

Figure 2.4. Isomers of C10H18O:

Geraniol

Linalool Nerol

Citronellal

Alcohols

Geraniol, nerol and linalool are all alcohols. They tend to have floral odours.

Geometric isomers

Geraniol and nerol are geometric (cis-trans) isomers of each other. Geraniol has a waxy, sweet rose odour; nerol also smells of sweet rose with a citrus tang.

Structural (position) isomer

Linalool is a structural (position) isomer of geraniol and nerol. It has two optical isomers. R-linalool has an odour of bergamot and lavender.

Structural (functional group) isomer

Citronellal is a structural (functional group) isomer of Geraniol, nerol, and linalool. It is an aldehyde and it has two optical isomers. It has a citrus odour and is used as a soap perfume and in insect repellants.

Explanation:

Geraniol and nerol are geometric (cis-trans) isomers of each other. Geraniol has a waxy, sweet rose odour; nerol also smells of sweet rose with a citrus tang. Linalool is a structural (position) isomer of geraniol and nerol