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Name Reactions part 1 sem iv poc iii
- 3. Step 1: Since the reaction is
carried out in an acidic
medium, the hydroxide group
of the pinacol is protonated by
the acid.
Step 2: Water is now removed
from the compound, leaving
behind a carbocation. This
carbocation is tertiary and
therefore stable.Step 3: The methyl group shifts
to the positively charged carbon
in a rearrangement of the
compound.
Step 4: The oxygen atom which is
doubly bonded to the carbon is
now deprotonated, giving rise to
the required pinacolone.
Mechanism
Reaction
© Dr. Atul R. Bendale
- 5. Hofmann rearrangement, also known as Hofmann degradation [and not to be confused with Hofmann elimination] is the
reaction of a primary amide with a halogen (chlorine or bromine) in strongly basic (sodium or potassium hydroxide) aqueous
medium, which converts the amide to a primary amine, which is one carbon less than the amide. For example
The carbamic acid spontaneously
loses CO2, yielding the amine
product.
Base abstracts an acidic N-H
proton, yielding an anion.
The anion reacts with bromine
in an α-substitution reaction to
give an N-bromoamide.
Base abstraction of the
remaining amide proton
gives a bromoamide anion.
The bromoamide anion rearranges as the R
group attached to the carbonyl carbon
migrates to nitrogen at the same time the
bromide ion leaves, giving an isocyanate.
The isocyanate adds water in a
nucleophilic addition step to yield
a carbamic acid(aka urethane).
Amide
Amine
The reaction actually uses Br₂ + NaOH, but
this is equivalent to using NaOBr, because
the reaction produces NaOBr in situ.
Mechanism
Reaction
© Dr. Atul R. Bendale
- 7. Step 1: An acid/base reaction.
Protonation of the carbonyl activates it
while creating a more reactive
nucleophile, the percarboxylate.
Step 2: Now the nucleophilic O attacks
the carbonyl C with the electrons from
the C=O π bond going to the positive O.
Step 3: Electrons from the O come back
(this reforms the π bond of the C=O) and
we migrate the C-C electrons to form a
new C-O bond displacing the carboxylate
as a leaving group.
Step 4: Finally an acid/base reaction
reveals the C=O and therefore the ester
product
Baeyer-Villiger oxidation is the oxidation of a ketone to a carboxylic acid ester using a peroxyacid as the oxidizing agent. The reaction of the
ketone with the acid results in a tetrahedral intermediate, with an alkyl migration following to release a carboxylic acid.
Ketones, RCOR', are oxidised by peracids (or hydrogen peroxide) to give esters, RCO2R'.
Aldehydes, RCHO, are oxidised under the same conditions to give carboxylic acids, RCO2H.
Mechanism
Reaction
Concept © Dr. Atul R. Bendale
- 9. 0
Benzilic acid rearrangement converts a 1,2-diketone into an α-hydroxycarboxylic acid containing a rearranged carbon skeleton. This
reaction receives its name from the reaction of benzil with potassium hydroxide to form benzilic acid. It can be viewed as
an intramolecular disproportionation reaction, as one carbon center is oxidized while the other is reduced.
The reaction has been shown to work in aromatic, semi-aromatic, aliphatic, and heterocyclic substrates. The reaction works best when
the ketone functional groups have no adjacent enolizable protons, as this allows aldol condensation to compete. It has been found that
aryl groups more readily migrate than alkyl groups, and that aryl groups with electron-withdrawing groups migrate the fastest.
Mechanism
Reaction
Addition of OH – to the
carbonyl carbon as
a nucleophilic addition to
form the alkoxide.
The next step requires a
bond rotation to
conformer which places the
migrating group in position
for attack on the second
carbonyl group
The carboxylic acid in intermediate is less
basic than the alkoxide and therefore
reversible proton transfer takes place
favoring intermediate
it is protonated on acidic
workup to the final α-
hydroxy–carboxylic acidAnimation © Dr. Atul R. Bendale
- 11. 0
The acid-catalyzed conversion of an oxime into an amide is known as Beckmann rearrangement: The reaction begins by protonation of
the alcohol group forming a better leaving group. Its mechanism follows the same pattern as a pinacol reaction – acid converts the oxime
OH into a leaving group, and an alkyl group migrates on to the nitrogen as water departs. The product cation is then trapped by water to
give an amide.
Mechanism
Reaction
(a) Protonation of oxime
(b) rearrangement of imine
(Rate determining step)
(c) Attack of carbocation
(d) Deprotonation
© Dr. Atul R. Bendale
- 13. 0
The Schmidt reaction is an organic reaction in which an azide reacts with a carbonyl derivative, usually a aldehyde, ketone,
or carboxylic acid, under acidic conditions to give an amine or amide, with expulsion of nitrogen
Schmidt Reaction Mechanism for Producing Amines
The carbamate is now deprotonated.
The subsequent removal of CO2 yields
the required amine.
This mechanism begins with the formation
of an acylium ion from the protonation of
the carboxylic acid followed by the removal
of water.
This acylium ion is now reacted with
hydrazoic acid, leading to the formation of
a protonated azido ketone.
Now, the protonated azido ketone and the R group
undergo a rearrangement reaction, resulting in the
migration of the carbon-nitrogen bond and the removal
of dinitrogen leading to the formation of a protonated
isocyanate.
Now, a carbamate is formed when water is introduced
to attack the protonated isocyanate.
Mechanism
Reaction
Acid
Amine
© Dr. Atul R. Bendale
- 14. Schmidt Reaction Mechanism for Producing Amides
The relocation of a proton
belonging to the tautomer of
the amide gives the final
amide product.
Mechanism
Reaction
This Mechanism begins with the protonation of the
ketone, leading to the formation of an O-H bond.
The subsequent nucleophilic addition of
the azide leads to the formation of an
intermediate.
Water is now removed from this intermediate
via an elimination reaction, forming a
temporary imine.
An alkyl group which was a part of the original
ketone now migrates from the carbon to
the nitrogen belonging to the imine. This results in
the elimination of dinitrogen.
Now, water is used to attack the resulting
compound, and the subsequent deprotonation
yields a tautomer of the required amide.
Ketone
Amide
© Dr. Atul R. Bendale
