Aldehydes and Ketones
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Aldehydes and Ketones
- Only methanal is a gas at RTP the rest are liquids then solids.
- Only those with low molecular mass are completely miscible in water, solubility in water decreases as molecular mass increases. They are all soluble in organic solvents.
Nature of the carbonyl group
The carbonyl group is simply a carbon atom with a double bond to an oxygen atom. The oxygen atom has two lone pairs of electrons. The double bond consists of one σ bond and one π bond. As the oxygen is highly electronegative, the electrons in both these bonds are not distributed equally, there being a strong pull towards the oxygen. This is greater in the π bond than the σ bond because the bonds are more mobile. This polarisation is often represented as:
Where the π cloud is distorted, and the σ cloud is slightly distorted.
The three sigma bonds attached to the carbon are in one plane, with bond angles of 120o, the π cloud sits above and below this plane.
This polarisation of the C=O means that they are susceptible to attack by nucleophile on the δ+ carbon and the electrophiles on the δ- oxygen. The two lone pairs on the oxygen make it particularly susceptible to attacks by protons and Lewis acids.
Reducing properties of aldehydes
Aldehydes carry a hydrogen atom next to their carbonyl group. This hydrogen is activated by the carbonyl group and is readily oxidised to OH. Aldehydes are therefore readily oxidised to carboxylic acids.
Ketones are not readily oxidised at all. They have no effect on mild oxidising agents. This is because they do not have an oxidisable hydrogen atom joined to the carbonyl group.
a) When warmed in Fehlings solution, an alkaline solution of Cu2+ ions (a red precipitate of copper (I) oxide) is produced with an carboxylic acid.
This is used to detect the aldehyde group in reducing sugars.
b) Aldehydes are oxidised in Tollens reagent (aqueous Silver nitrate, ammonia and sodium hydroxide) to give a 'silver mirror'.
CH3CHO(l) + 2[Ag(NH3)]2+(aq) + OH-(aq) → CH3COO-(aq) + 2NH4+(aq) + 2Ag(s) + 2NH3(g)
c) Aldehydes will reduce acidified potassium dichromate (VI) and manganate(VII) solutions to green and colourless respectively.
5CH3CHO(l) + 2MnO42-(aq) + 6H+(aq) 5CH3COOH(aq) + 2Mn2+(aq) + 3H2O(l)
Nucleophilic addition reactions
Basic mechanism is:
a) With HCN
The HCN is usually generated in situ by the reaction of dilute H2SO4 on KCN at room temp.
b) With NaBH4 and LiAlH4
The BH4- and the AlH4- ion acts as a source of H- ion. The aldehyde or ketone is reduced to the appropriate alcohol.
3. Addition - Elimination reactions
All involve the lone pair on the NH2 group attacking the +ve charged carbon atom on the C=O group.
a) Hydroxylamine NH2OH
b) Hydrazine NH2-NH2
c) Phenyl hydrazine C6H5NHNH2
d) 2,4 - dinitrophenylhydrazine
4) The Haloform reaction:
When chlorine is passed into ethanal, 2,2,2 -trichloroethanal is formed.
If the product is treated with aqueous Alkali, trichloromethane is produced.
Reaction with Iodine.
If iodine is dissolved in alkali, the OI- ion is produced which reacts in a similar manner to chlorine.