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Advanced Level Organic Chemistry: The uses and aldehydes & ketones

Part 5. The chemistry of ALDEHYDES and KETONES

Doc Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US K12 grade 11 grade 12 organic chemistry

Part 5.8 The uses of aldehydes and ketones and their occurrence in nature - natural products


INDEX of ALDEHYDES and KETONES revision notes

All Advanced A Level Organic Chemistry Notes

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Use as solvents themselves

Aldehydes are not used as solvents - unpleasant and can be harmful if inhaled.

The lower ketones are like propanone and butanone are very useful solvents because they dissolve a wide variety of organic compounds.


Flavourings and perfumes

The aromatic aldehyde (c) doc bor(c) doc bbenzaldehyde, C7H6O, has the smell of almonds and is synthesised for use in flavouring food - I love thick layer of marzipan between a thin slice of sugar icing and a thick layer of rich Christmas cake!

Many more complex molecules containing an aldehyde group or ketone group occur in essential oils from plants, often giving, or contributing to, their characteristic odour e.g. citral and carvone whose skeletal formulae are shown below.

 

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.

skeletal formula of citral aldehyde carvone ketone advanced A level organic chemistry revision notes doc brown

A Citral (lemonal), C10H16O, is an aldehyde with all the usual characteristic reactions of an R-CHO molecule.

Citral also has two alkene groups (C=C) as well as the aldehyde group.

It is found in several species of lemon plants (e.g. lemon grass oil) and contributes to the strong citrus 'lemon-like' odour of the fruit.

Citral can exhibit E/Z stereoisomerism via the top C=C bond (geometric isomers).

Citral cannot exhibit R/S stereoisomerism - no chiral carbon.

Note 'al' in the name 'citral'.

 

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 'diene'.

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

skeletal formula of carvone R/S optical isomers stereoisomers has chiral carbon doc brown's advanced organic chemistry revision notesThey 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 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'.


Polymers

The plastic Bakelite is made by a reaction between methanal (formaldehyde) and phenol.


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

 


Doc Brown's Advanced Level Chemistry Revision Notes

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INDEX of ALDEHYDE and KETONE revision notes

 All Advanced Organic Chemistry Notes

 
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