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Advanced Level Organic Chemistry: The uses and aldehydes & ketones
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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
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 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.
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.
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
INDEX of ALDEHYDE
and KETONE revision notes
All Advanced Organic
Chemistry Notes
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or
benzaldehyde, 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!
They are also both a good
test in reading skeletal formulae - match them up with the molecular
formulae!
