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Behaviour of Biomolecules Lecture notes: Chemistry of the carboxyl group
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Behaviour of Biomolecules Lecture notes: Chemistry of the carboxyl group BSc Biomedical Sciences course
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BIOL1021: Behaviour of BiomoleculesDr. Ramon Rios: rrt1f11@soton.ac.uk
Behaviour of Biomolecules
1
Lecture 5: Chemistry of the carboxyl group
,REVISION LAST LECTURE
Consider reaction of an aldehyde/ketone with a secondary amine:
O R' R' R'
H N
+ N H + H2O
R catalyst R
R'
Enamine
via hemiaminal formation:
OH OH H
O H 1 OH 2 3
R R N R' R NR'2
R
H R'
R'2NH hemiaminal
then:
H
OH H 4
OH2
5 H
6
R R R NR'2 R NR'2 R NR'2
NR'2 NR'2
+ H2O
The iminium ion has no N-H bond so
• Identical to imine formation except in loss of H must take place from the
adjacent atom instead. 2
the final step.
, Chemistry of Imines
Mechanism: O R'
H N
+ R' NH2 + H2O
R R catalyst R R
First part (hemiaminal formation):
OH OH H
O H 1 OH 2 3
R R
R R R N R' R NR'
R R
H
H H
R'NH2 hemiaminal
1 Protonation of the C=O group on oxygen.
2 Nucleophilic attack by nitrogen lone-pair electrons from the 1 o amine.
3 Loss of a proton gives a hemiaminal intermediate.
Steps 1 3are slow below pH4 because the amine is protonated…
R'NH3 is not nucleophilic. 3
…and
, Compare hemiaminal formation…
OH OH H
O H OH
R R
R R R N R' R NR'
R R
H
H H
R'NH2 hemiaminal
…with hemiacetal formation:
H
O H OH OH
OH
R R
R R R O R' R
R R OR'
H
R'OH
Differ only in the identity of the nucleophile (amine or alcohol).
4
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