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Advanced Level Organic Chemistry: The uses and aldehydes and 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 of biochemistry


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. both polar and non-polar.


Flavourings and perfumes

The aromatic aldehyde (c) doc bormolecular structure benzaldehyde almond flavour odour flavor odor(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.

molecular structure phenylethanone acetophenone use in fragrance smells like cherry honeysuckle jasmine strawberry aldehydes and ketones nomenclature (c) doc b 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.

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 (2 x C=C, diene) 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) because this alkene group is not part of a ring system..

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

Note 'al' in the name 'citral' indicating an aldehyde.

 

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.

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 (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!

Vanillin glucoside vanilla flavour odour methyl vanillin ethyl vanillin 4-hydroxy-3-methoxybenzaldehyde 3-ethoxy-4-hydroxybenzaldehyde 2-methoxyphenol molecular structure

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.

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Other naturally occurring aldehydes and ketones

octanal cinnamaldehyde molecular structure skeletal formula advanced A level organic chemistry doc brown

Cinnamaldehyde, molecular structure C6H5-CH=CH-CHO (check versus skeletal formula), is an aldehyde found in the oil of cinnamon bark and has been traditionally used in medicines to treat coughs and sore throats.

Cinnamaldehyde can exhibit E/Z isomerism, the natural product is the E isomer (trans, shown above).

It is used as a flavouring in sweets and beverages and a fragrance additive in perfumes.

It has also been used as a safe insecticide against mosquitos and also has antibacterial and antifungal properties.

Octanal, molecular structure CH3(CH2)6CHO, is found in orange juice and contributes to the fragrance of oranges and also to roses and violets.


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

 


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