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4.1.2 Carbonyl Compounds
Syllabus points 4.1.2 a, b, e Aldehydes
and Ketones; identifying their presence
using Brady’s reagent (2,4-DNPH)
Nomenclature
iodopropanone
3-iodopentan-2-one
3-methylbutan-2-one
ethanal
4-iodobutan-2-one
ethanal
4-iodobutan-2-one
propanal
pentan-3-one
pentanal
Carbonyl Compounds
• The functional group is the carbonyl group
δ+
δ-
C=O
Oxygen is more electronegative
than carbon, the electrons in the
double bond tend towards the
oxygen atom making it δ-.
The carbon atom becomes δ+,
making it susceptible to
nucleophilic attack.
See pi representation Q 1-4 pg 21
Aldehydes
• The functional group of the aldehydes:
H
C=O
R
Where R are alkyl
or aryl groups,
and may or may
not be different
The aldehydes are named
using the suffix -al
E.g.
CH2O methanal
CH3CHO ethanal
Ketones
• The functional group of the ketones:
R
C=O
R1
Where R and R1 are
alkyl or aryl groups, and
may or may not be
different
The ketones are named using the
suffix -one
E.g.
CH3COCH3 propanone
CH3CH2COCH3 butanone
CH3CH2CH2COCH3 pentan-2-one
Oxidation of carbonyls
• What reagent is used to oxidise alcohols?
• How do you know the alcohol has oxidised?
•
•
•
•
•
•
Ethanol + [O]  ? + ? Via distillation
Ethanol + 2[O]  ? + ? Via reflux
2-methylpropanal + [O]  ?
Propan-2-ol + [O]  ? + ?
Methylpropan-2-ol  ?
NB this year you WILL need to write redox
equations using the actual reagents not just [O]
Testing carbonyls
Objectives
• Describe the use of 2,4dinitrophenylhydrazine to detect the presence
of a carbonyl group.
• Identify a carbonyl compound from the
melting point of the derivative.
Tests for carbonyls
• You can use 2,4 – dinitrophenylhydrazine or 2,4-DNP to
test for the carbonyl group.
• A solution of 2,4-DNP, methanol and H2SO4 is known as
Brady’s reagent.
• A positive test will give an orange/yellow precipitate.
• Both aldehydes and ketones will test positively. No
other compounds (e.g. Carboxylic acids or esters) will
• The precipitate is called 2,4-dinitrophenylhydrazone
derivative.
• How would you know what type of carbonyl you have?
• Overall reaction is given by the equation:
• Look carefully at what has happened.
Identifying a carbonyl
• The yellow/orange derivative is slightly impure
so the product is filtered and recrystallised to
produce a purified crystal
• The melting point of the crystals is then
measured – this is used to identify the
compound
Compound
Boiling point (°C)
Melting point of 2,4-DNP
derivative (°C)
Heptan -2 -one
151
90
Cyclohexanone
156
162
Octan-2-one
173
58