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What is a carbonyl compound?
an organic molecule that contains a C=O group
Carbonyl compounds include
aldehydes, ketones, carboxylic acids, esters, amides, acyl chloride and acid anhydrides
Primary alcohols can be oxidised to form
aldehydes then further oxidation to carboxylic acids
The oxidising agents used are acidified potassium or sodium dichromate(VI) (K2Cr2O7 or Na2Cr2O7) which will change colour
from orange to green
Alternatively, acidified potassium manganate(VII) (KMnO4) can be used, the colour change is from
purple to colourless
In order to form the aldehyde, distillation apparatus should be used otherwise
the aldehyde will undergo further oxidation
The aldehyde product has a lower boiling point than the alcohol reactant so
it can be distilled off as soon as it forms
To form a ketone, which can not undergo further oxidation,
reflux apparatus can be used
Compounds containing -C=O groups will undergo a condensation reaction with
2,4-dinitrophenylhydrazine
A condensation reaction is a reaction in which
two molecules join together and a small molecule (such as water or hydrogen chloride is eliminated)
The positive result for this reaction test is
a deep orange precipitate being formed
The pos result precipitate can be purified by
recrystallisation
The melting point of the formed precipitate can then be measured and compared to literature values to find out
which specific aldehyde or ketone had reacted with 2,4-DNPH
The 2,4-DNPH test is especially useful as
other carbonyl compounds such as carboxylic acids and esters do not give a positive result
Aldehydes and ketones have different chemical properties which means
they can be identified
The carbonyl group in an aldehyde is always situated
at the end of the chain
The simplest aldehyde is
methanal, HCHO
The carbonyl group in a ketone is always situated
in the middle of the chain
The simplest ketone is
is propan-2-one, CH3COCH3, as you need an alkyl group either side of the carbonyl carbon in a ketone
In order to determine whether a compound is an aldehyde or a ketone two chemical tests can be performed:
Fehling’s solution
Tollens’ reagent
Fehling’s solution is
a blue alkaline solution which contains copper(II) ions, which act as the oxidising agent
Note
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