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Carbonyl compounds (Topic 17B)
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Aldehydes and ketones are functional group structural isomers. Both have the carbonyl functional group, but differ in the location of this group.
Aldehydes have the group carbon on the terminal carbon, but in ketone, the group carbon is within the carbon chain.
Pure samples of aldehydes or ketones do not exhibit hydrogen bonding – they do not possess . Their volatility is linked to the strength of their permanent dipole forces and London (induced dipole) forces, which increase with chain length.
Short chain aldehydes and ketones are more volatile.
Aldehydes and ketones can, however, form hydrogen bonds through the lone pairs on the carbonyl oxygen, with hydrogens from solvents, such as water.
This makes the shorter chain carbonyl compounds water soluble.
Aldehydes are oxidised to carboxylic acids.
The oxidatising reagent is an acidified dichromate (VI) solution.
The mixture is boiled under reflux, using the apparatus below:
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The volatile aldehyde is prevented from escaping the oxidation reaction as it is condensed and returned to the reaction flask. The product is a carboxylic acid.
The reaction is associated with the colour change from orange to green. This indicates oxidation has occurred, as the chromium species has been reduced from orange to green .
Using to represent the oxidising agent, the oxidation of ethanal to ethanoic acid would be:
The oxidation number of the carbon increases by 1 therefore only 1 mole of is required per mole of ethanal.
or can both be used to reduce aldehydes and ketones.
The carbonyl group is reduced by two ions, forming the primary alcohol from the aldehyde, and the secondary alcohol from the ketone group.
It is common to use the symbol in equations to represent the hydrogen from the reducing agent in a reduction reaction.
Reduction of aldehydes and ketones is written as follows:
For an aldehyde:
For a ketone.
There is a 1:2 mole ratio of carbonyl : .
One hydrogen is from the nucleophilic attack of and the second is from a protic solvent.
Sodium borohydride, , in aqueous ethanolic solution, is a good reducing agent, and can reduce both aldehydes and ketones, but cannot reduce carboxylic acids.
Lithium aluminium hydride () is a very powerful reducing agent and, as well as reducing aldehydes and ketones to alcohols, it can reduce carboxylic acids to primary alcohols.
reacts violently with water, so it is used in a solution of dry ether (ethoxyethane). The reduction is a two stage process after reaction with the salt form of the product is hydrolysed to the alcohol by the addition of water or dilute acid.
When heated under reflux with acidified solution, carbonyl compounds undergo nucleophilic addition reactions. This reaction is very useful in extending a carbon chain in organic synthesis reactions.
The lone pair of electrons on the carbon atom of the nitrile group, forms a new carbon-carbon bond with the carbonyl carbon. An ion then binds to the carbonyl oxygen forming an alcohol group. The reaction saturates the carbonyl bond. The product is a hydroxynitrile.
The product is 2-hydroxypropanenitrile.
The product is 2-hydroxy, 2-methylpropanenitrile.
(Note the nitrile group is a higher priority than the alcohol group, so is named in the suffix.)
The mechanism for nucleophilic addition of to a carbonyl is drawn in two key stages:
1. Nucleophilic attack by the lone pair nitrile ion, , forming a new bond to the of the carbonyl group, resulting in movement of pi bond electrons to the carbonyl oxygen.
2. The of the intermediate ion gaining a proton from the solvent water, or from the .
The organic products are hydroxynitriles which have both the group and the group now bonded to the same carbon.
The reaction of the nucleophile with an aldehyde, or an unsymmetrical ketone, will always produce a product with four different groups bonded to the same carbon atom; a chiral product.
The trigonal planar functional group makes attack equally likely from above or below the group.
Where a chiral hydroxynitrile product is formed, it will always exist as a racemic mixture of two non-superimposable mirror images, known as optical isomers, or enantiomers.
Aldehydes and ketones will react with a solution of 2,4-dinitrophenylhydrazine () in methanol and concentrated sulfuric acid, to give a yellow or orange precipitate.
Alcohols and carboxylic acids DO NOT give a positive result with 2,4-DNPH making the reagent useful in distinguishing compounds containing carbon, hydrogen, and oxygen.
Following the identification of an aldehyde or ketone, the 2,4-DNPH test can also be used to identify a specific aldehyde or ketone.
The solid product is purified and its melting point is determined.
The melting point will be a sharp specific value which is unique to each carbonyl compound. This enables positive identification of the carbonyl compound by comparing the melting point of the derivative to a database of values.
Where an acetyl group, , exists in a compound, reaction with a solution of iodine in sodium hydroxide forms a pale yellow precipitate, . This is triiodomethane (or iodoform) and has an antiseptic odour.
The reaction is known as the iodoform reaction.
The reagent used is an oxidising agent, so alcohols that can be oxidised to a product containing an acetyl group will also give a positive test result.
The Tollen’s reagent test can be used to distinguish aldehydes from ketones.
The reagent is prepared on the day of use, as it is unstable. It consists of a colourless solution of ammoniacal silver nitrate, which is made from an alkaline silver nitrate reacted with concentrated ammonia solution.
The observation for aldehydes is the formation of a ‘silver mirror’. Ketones DO NOT give a silver mirror – the solution remains colourless.
The silver mirror is formed as a redox reaction occurs with the silver ions reduced to silver metal, as the aldehyde is oxidised to a carboxylic acid salt.
Ketones cannot be further oxidised so do not support the reduction of Tollen’s reagent.