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Part 5.
The chemistry of
ALDEHYDES and KETONES
Selected condensation reactions
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Brown's Chemistry Advanced Level Pre-University Chemistry Revision Study
Notes for UK KS5 A/AS GCE IB advanced level organic chemistry students US
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notes on the chemistry of aldehydes and ketones
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Part 5.6
Examples of the addition - elimination condensation reactions of aldehydes and ketones
- including reaction product with 2,4-dinitrophenylhydrazine for identification
Sub-index for Part 6.6
5.6.1
A basic addition-elimination condensation reaction of aldehydes or
ketones with compounds containing an amino group
5.6.2
The
reaction between 2,4-dintrophenylhydrazine with aldehydes or ketones
5.6.3
The preparation
and use of the product from reacting aldehydes and ketones with
2,4-dinitrophenylhydrazine
5.6.1
A basic addition-elimination condensation reaction of aldehydes or
ketones with compounds containing an amino group
In this case, molecules with an amino group (-NH2),
can undergo a condensation reaction with aldehydes and ketones.
R2C=O
+ H2N-X ===> R2C=N-X +
H2O
R = H or alkyl for aliphatic
aldehydes and ketones
This is also an example of a addition -
elimination
reaction, where two molecules are joined together by the elimination of a
small molecule - in this case water.
e.g. aldehydes and ketones react with
hydrazine (H2N-NH2), so X = -NH2
R2C=O
+ H2N-NH2 ===> R2C=N-NH2 +
H2O
(CH3)2C=O
+ H2N-NH2 ===>
(CH3)2C=N-NH2 +
H2O
The product here is called a
hydrazone e.g. if R = CH3, the product is called
propanone hydrazone
In these reactions, the lone pair on
the nitrogen of the -NH2 group, enables it to act as a
nucleophile that can attack the delta plus carbon atom of the carbonyl
group, the polar bond (δ+C=Oδ-)
of the aldehydes and ketones functional group.
Unlike the
addition of HCN, there is no addition of a proton (H+)
to the oxygen of the carbonyl group.
Instead the initial product is a combination of the two
molecules, before water is eliminated to give the final product, which has a
C=N double bond in its molecular structure.
Note on E/Z
isomerism - a type of stereoisomerism (old name geometric isomerism).
Around the carbon and nitrogen atoms
of the C=N bond, there three groups of bonding and non-bonding
electrons. This gives a
trigonal planar bond arrangement with bond angles of ~120o.
Therefore you have a similar situation to that in
alkenes (>C=C<) and the possibility of E/Z isomerism.
However, this is only possible if R and R' are
different groups, so here the carbonyl compound must be an aldehyde
(automatically unsymmetrical at the >C=O group) or an unsymmetrical
ketone, where the two alkyl or aryl groups attached to the C=O are
different e.g. all the '2-ones' like butanone or pentan-2-one.
TOP OF PAGE
5.6.2 The
reaction between 2,4-dintrophenylhydrazine with aldehydes or ketones
This is another example of an addition -
elimination condensation reaction.
R2C=O
+ H2N-X ===> R2C=N-X + H2O
BUT, this time X is a bit more complicated - a lot bigger.
The
general equation for the addition-elimination reaction between
2,4-dinitrophenylhydrazine and aldehyde or ketone - also an example of a
condensation reaction.
R, R' = H, alkyl or aryl.
The product is called a
2,4-dinitrophenylhydrazone
or just 'dinitrophenylhydrazone'
(one letter changed).
The equation for the addition-elimination reaction between
2,4-dinitrophenylhydrazine and ethanal.
The product is called ethanal
2,4-dintrophenylhydrazone. Here
there is the possibility E/Z isomers and it is theoretically the same
for all aldehydes.
The equation for the addition-elimination reaction between
2,4-dinitrophenylhydrazine and propanone.
The product is called propanone
2,4-dintrophenylhydrazone. Here
there is no possibility of E/Z isomers,
and it's the same for other
symmetrical ketones like pentan-3-one CH3CH2COCH2CH3
BUT, unsymmetrical ketones like
butanone CH3COCH2CH3 and pentan-2-one
CH3COCH2CH2CH3, would
theoretically form E/Z isomers.
TOP OF PAGE
5.6.3 The preparation and use of the product
from reacting aldehydes and ketones with 2,4-dinitrophenylhydrazine
Brady's reagent is an aqueous
orange solution of 2,4-dinitrophenylhydrazine and is used as
a test for aldehyde and ketone carbonyl compounds - BUT, it
does NOT distinguish between them.
When a few drops of an
aldehyde or ketone is shaken with a few cm3 of
Brady's reagent a yellow-orange precipitate is formed if
the compound is an aldehyde or ketone - a simple test.
The dinitrophenylhydrazone
precipitate is insoluble in water.
However, you can filter of
the precipitate and recrystallise with the minimum volume
of hot ethanol.
Filter off again and dry on a
filter paper or watch glass.
You can then determine the
melting point of the dinitrophenylhydrazone.
Each dinitrophenylhydrazone
has a unique melting point.
So you can match the
melting point of the dinitrophenylhydrazone with a data table and identify the aldehyde or
ketone.
More on the tests for aldehydes
and ketones
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