[Author © Dr Phil Brown PhD: Doc Brown's advanced level organic chemistry exam revision notes suitable for students of UK A level chemistry courses & US K12 grade 11, grade 12 and AP honors chemistry courses: The chemistry of aldehydes & ketones [page updated April 8th 2026 *]

Phenylethanone (left, acetophenone) is a ketone used in fragrances to smell like cherry, honeysuckle, jasmine and strawberry.

There are many aldehydes and ketones in the natural products of plants including terpenoids - a very diverse group of compounds contributing to the smell and taste of many natural products and perfume formulations.

These are unsaturated compounds (C=C functional group), but also contain an aldehyde of ketone group.

Some of these terpenoids are illustrated below.

B Carvone, C₁₀H₁₄O, is a cyclic ketone found in spearmint and caraway oil.

It has one ketone group and behaves as a ketone, but it also two C=C double bond groups, so is also a 'diene'.

Both citral and carvone behaves as a 'double' alkene, reacting quantitatively with two molecules of bromine.

They are also both a good test in reading skeletal formulae - match them up with the molecular formulae!

Carvone cannot exhibit E/Z stereoisomerism via the top C=C bond (geometric isomers) - one C=C has two identical end groups (H) and the top left C=C is part of the ring.

Citral can exhibit R/S stereoisomerism - it has a chiral carbon - the bottom one of the hexagonal ring - see diagram on right for the non-superimposable mirror image forms.

Note 'one' in the name 'carvone' indicating a ketone.

 

'Vanilla' one of our favourite tastes!

Natural vanillin is extracted from the seed pods of Vanilla planifolia, a vining orchid native to Mexico, but now grown in tropical areas around the globe.   The harvested green seed pods contain vanillin in the form of a glucoside, but the green pods do not have the flavour or odour of vanilla. Vanillin glucoside is shown on the top left of the diagram

1. Vanillin glucoside consists of the basic vanillin molecular structure joined to cyclic hexose sugar molecule).

2. From the natural sourced vanillin glucoside, vanillin (methyl vanillin, 4-hydroxy-3-methoxybenzaldehyde) is extracted.

3. The production of vanillin from natural sources is quite expensive, so several cheaper synthetic routes have been devised including using 2-methoxyphenol which is another synthetic product of the petrochemical industry.

4. 'ethyl vanillin', 3-ethoxy-4-hydroxybenzaldehyde, is a synthetic analogue of vanillin with a much stronger vanilla flavour and odour than natural vanillin.   Using synthetic 'vanilla' molecules is cheaper, but those with sensitive taste buds might be able to tell the difference from naturally sourced vanilla flavouring.

For 4. note the change in order of the substituents from 2. due to the IUPAC alphabetical nomenclature rule.

A nose footnote!

Much of our sensation of taste is actually due to the smell of the food we are eating.

When eating, we smell these flavours (aldehydes, ketones and esters too - think of fruity odors/tastes etc.) and these odours make the food appetising.

The flavour molecules like vanillin, are sufficiently volatile to reach the olfactory receptors in the nose - note that if you have a heavy cold and blocked nose, your appetite seems decreased and the food doesn't taste as good.

Preservatives, herbicides - many aldehydes are harmful and some toxic

Methanal (formaldehyde) solution is used to preserve anatomical specimen - never forgot the smell of preserving the body of a rat I dissected as a student !!!

It can also be used to sterilise surgical equipment - antiseptic/disinfectant properties.

Propenal ('Acrolein') is an unsaturated aldehyde (H₂C=CH-CHO) is used as a contact herbicide