10.6.2
Nucleophilic addition reaction of hydrogen cyanide to aldehydes and ketones
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Summary of organic reaction mechanisms - A mechanistic introduction to organic chemistry and
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10.6 Aldehydes–Ketones
10.6.2
Nucleophilic addition reaction with hydrogen cyanide
Examples are explained of the organic
chemistry mechanisms for aldehydes and ketones undergoing nucleophilic substitution, nucleophilic addition
reactions are described with diagrams and full
explanation revision notes.
Carbonyl compounds – ALDEHYDES and KETONES –
introduction, Nucleophilic addition of hydrogen cyanide to form a
hydroxy–nitrile.
The revision
notes
include full diagrams and explanation of the mechanisms and the 'molecular' equation and reaction conditions
and other con–current reaction pathways for these reactions of aldehydes
and ketones and products are also explained.
10.6 Carbonyl compounds –
ALDEHYDES
and KETONES
10.6.1 Introduction to aldehyde and ketone reactivity
Aldehydes and ketones
readily undergo nucleophilic attack because of the highly
polar carbonyl bond >Cδ+=Oδ–
caused by the big difference in the electronegativity between carbon (2.5) and
oxygen (3.5).
An electron pair donating nucleophile (Nuc:), will
therefore attack the 'positive carbon' (Cδ+)
to form a C–Nuc bond. A comparison of
electrophilic addition to alkenes with nucleophilic addition to
aldehydes/ketones is included in these notes.
10.6.2
Nucleophilic addition of hydrogen cyanide to aldehydes or ketones to give
hydroxy–nitriles
The organic synthesis of hydroxynitriles from the reaction of cyanide
with aldehydes and ketones
-
Examples of
nucleophilic addition of hydrogen cyanide to aldehydes and ketones
to give hydroxynitriles
-
What is the mechanism
for the addition of hydrogen cyanide to the carbonyl group of an
aldehyde or ketone?
-
e.g.
RR'C=O
+ HCN ==> RR'C(OH)CN
[see mechanism 7 below]
-
The reaction involves
mixing an aldehyde (R = H, R' = H or alkyl) or ketone (R and R' are either alkyl or
aryl, but NOT H) with buffered potassium cyanide solution to provide a
source of negative cyanide ions, the nucleophile.
-
The
product of the nucleophilic addition of hydrogen cyanide is a hydroxynitrile (a
cyanohydrin).
mechanism 7 –
nucleophilic addition of cyanide ion to an aldehyde or ketone R = H,
alkyl or aryl
-
In this nucleophilic
addition reaction, at the functional group centre of the reaction
(>C=O), you change from an unsaturated trigonal planar situation to a
saturated tetrahedral bond network about the carbon atom. This carbon
atom is, in most cases a chiral carbon and the product therefore can
exhibit optical isomerism
(R/S isomerism). However the product is usually a 50:50 mixture of the
enantiomers (non–superimposable mirror–image forms) i.e. a racemic mixture.
-
This discussion only applies if the
nucleophilic addition product has a
chiral carbon i.e.
has R/S isomers.
-
Why is the product an
optically inactive racemate even if the product is an asymmetric
molecule with a chiral carbon and hence exhibits R/S isomerism.
-
The reason can be clearly
argued by considering the mechanism 7
above. The nucleophile attacks the carbon of the polarised carbonyl
group (R2Cδ+=Oδ–)
in a trigonal planar bonding situation which changes to a tetrahedral on
formation of the C–Nucleophile bond. Quite simply, there is a 50:50
chance of which side of the carbonyl group the nucleophile attacks and
therefore a 50:50 chance of which optical isomer is formed as the
configuration about the carbon atom changes.
-
Apart from explaining the
formation of a racemic mixture, you can also argue, in turn, that the
lack of optical activity in the product is itself evidence for an
initial attack of the nucleophile at the carbon of the carbonyl group
and you might reasonably expect a 2nd order rate expression.
Examples where you do or do not get a racemic
mixture of R/S isomers
Nucleophilic addition of hydrogen cyanide to
propanal: DO
CH3CH2CHO
+ HCN ===> CH3CH2CH(OH)CN
The product is 2-hydroxybutanenitrile and
the chiral carbon atom is
highlighted.
Nucleophilic addition of hydrogen cyanide to
propanone: DO NOT
CH3COCH3
+ HCN ===> CH3C(OH)(CN)CH3
The product is
2-hydroxy-2-methylpropanenitrile and has NO chiral carbon
atom because propane is a symmetrical ketone (R = R' in
terms of diagrams above).
APPENDIX
COMPLETE MECHANISM
and Organic Synthesis INDEX
(so far!)
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