Aldehydes And Ketones
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33.8 Aldehydes and Ketones
Carbonyls are compounds with a C=O bond.
Carbonyls: Aldehydes and Ketones
They can be either aldehydes or ketones
H
H
O
H
O
If the C=O is in the middle of
If the C=O is on the end of the
H
C
C
C
H
the chain it is a ketone
.
H
C
C
chain with an H attached it is an
The name will end in -one
aldehyde.
H
H
H
H
The name will end in –al
CH
COCH
propanone
3
3
CH
CHO
ethanal
3
Solubility in water
CH
3
The smaller carbonyls are soluble
Pure carbonyls cannot hydrogen bond, but
in water because they can form
bond instead by permanent dipole bonding.
O
H
O
C
hydrogen bonds with water.
H
CH
3
-
δ
O
Reactions of carbonyls
In comparison to the C=C bond in
The C=O bond is polarised because
alkenes, the C=O is stronger and does
+
δ
O is more electronegative than
not undergo addition reactions easily.
C
carbon. The positive carbon atom
H
C
CH
attracts nucleophiles.
3
3
This is in contrast to the electrophiles
that are attracted to the C=C .
nucleophile
Oxidation Reactions
Potassium dichromate K
Cr
O
is
Key point: Aldehydes
2
2
7
Primary alcohol
aldehydes
carboxylic acid
an oxidising agent that causes
can be oxidised to
Secondary alcohol
ketones
alcohols and aldehydes to
carboxylic acids, but
oxidise.
ketones cannot be
Tertiary alcohols
do not oxidise
oxidised.
Oxidation of Aldehydes
H
H
H
H
O
O
Reaction: aldehyde
carboxylic acid
Reagent: potassium dichromate (VI) solution and
H
C
C
C
H
C
C
C
+ [O]
dilute sulphuric acid.
O
H
H
H
H
H
H
Conditions: heat under reflux
RCHO + [O]
RCO
H
2
Full Equation for oxidation
2-
+
3+
3CH
CHO + Cr
O
+ 8H
3 CH
CO
H + 4H
O + 2Cr
3
2
7
3
2
2
Aldehydes can also be oxidised using Fehling’s
Observation: the orange dichromate ion
solution or Tollen’s Reagent. These are used
2-
3+
(Cr
O
) reduces to the green Cr
ion
as tests for the presence of aldehyde groups
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7
Tollen’s Reagent
Fehling’s solution
Reagent: Tollen’s Reagent formed by mixing
aqueous ammonia and silver nitrate. The
2+
Reagent: Fehling’s Solution containing blue Cu
ions.
active substance is the complex ion of
Conditions: heat gently
+
[Ag(NH
)
]
.
3
2
Reaction: aldehydes only are oxidised by Fehling’s
Conditions: heat gently
Solution into a carboxylic acid and the copper ions
Reaction: aldehydes only are oxidised by
are reduced to copper(I) oxide .
.
Tollen’s reagent into a carboxylic acid and
2+
Observation: Aldehydes :Blue Cu
ions in solution
the silver(I) ions are reduced to silver atoms
change to a red precipitate of Cu
O. Ketones do
2
not react.
Observation: with aldehydes, a silver mirror forms
coating the inside of the test tube. Ketones
2+
+
CH
CHO + 2Cu
+ 2H
O
CH
COOH + Cu
O + 4H
result in no change.
3
2
3
2
+
+
CH
CHO + 2Ag
+ H
O
CH
COOH + 2Ag + 2H
3
2
3
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