Aldehydes and ketones
Part-4
Mechanism of Cannizaro's of benzaldehyde
(b) Cross Cannizaro's Reaction
Reaction of carbonyl compounds (does not having
molecules of acid and alcolol.
) with alkali forms each
Whenever HCHO is present, there are only two main products otherwise there will be four
products in all other cases. Because HCHO contains maximum reducing hydrogen atom so easily
undergo oxidation process.
One of the most important applications is the crossed Cannizzaro reaction between
formaldehyde and other aldehydes containing
(iv) Bimolecular Reduction or Pinacol Reaction
Two molecules of acetone undergo reduction in the presence of Mg/Hg to form Pinacol. Upon
treatment with mineral acids, 2,3-dimethyl 2,3- butane diol (pinacol) is converted into methyl
ter-butyl ketone (pinacolone). The 1,2-diol undergo dehydration in such a way that
rearrangement of the carbon skeleton occurs. Other 1,2 diols undergo analogous reactions,
which are known as pinacol pinacolone type rearrangement.
Mechanism
Pinacol–Pinacolone type Rearrangement
As the migrating group migrates with its electron pair, the more nucleophilic group might be
expected to migrate. Thus, the order of migratory attitude amongst the aryl groups is p-anisyl
> p-tolyl > phenyl > p-chlorophenyl, etc.
Remember, electron-attracting groups will retard the migration. The migratory aptitude
amongst the alkyl groups is Me3C > Me2CH > Me. However, the stability of the initially formed
carbocation may offset the migratory attitude order. Thus, in the compound 1, 1-dimethyl-2, 2diphenyl glycol, the resonance-stabilized carbocation (I) is formed instead of (II) and so it is the
methyl group and not the phenyl group which migrates, contrary to the above sequence.
Steric hinderance may affect the rate of migration–p-anisyl group migrates 1000 times faster
than o-anisyl group.
Migrating group attacks from the trans side or back side of to the leaving group. This has
significant aspect in cyclic systems. Thus, the two isomers of 1, 2-dimethyl-cyclohexane-1, 2diol give different products due to different orientations of the methyl and hydroxyl groups.
The one (III) in which the Me and OH groups are trans to each other gives 2, 2dimethylcyclohexanone by methyl shift. The other (IV) in which the Me and OH groups are cis
to each other undergoes ring methylene group shift instead of Me-shift with consequent ring
contraction to give 1-acetyl-1-methylcyclopentane (V).
Applications
Ketones from cyclic diols Pinacol rearrangement has been also applied to prepare ketones
which are very difficult to prepare by other method.
(v) Reaction with NH3
When carbonyl compounds reacts with ammonia, nucleophilic addition reaction occurs but
products are formed according to praportions between ammonia and carbonyl compound. For
example
(vi) Reaction with Ammonia Derivatives NH2 – Z is an example of nucleophilic addition
elemination reaction where – Z groups are following in nature. First attack from nucleophilic
site of nitrogen atom to electrophilic carbon atom then elimination of water occurs
simultaneously.
These reactions are usually carried out in weakly acidic medium because weak acid catalyses
the reaction by protonating carbonyl oxygen but in presence of excess of acid nucleophile is
also protonated which reduces its reactivity, so optimum pH is necessary. When zine
compounds reacts with carbonyl compound they form crystalline solid as zone derivatives.
The reaction with 2,4–DNPH is used for sepration of carbonyl compound with noncarbonyl
compound.
Always remember, presence of traces of
creates better polarity in the carbonyl
compound and more electrophilic nature of carbon atom but on the other hand presence of
excess H+ in the ammonia derivative makes it a bad nucleophile. For example when NH2NH2
(hydrazine) in presence of excess of acid converted into NH2N+H3 cation (hydrazinium) which is
a bad nucleophile. Hence optimum pH is a very important condition for this reaction,only
tracer amount of H+ are required.