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Department of Chemistry Sri Sarada Niketan College of Arts & Science for Women Kanavaipudur.

UNIT IV

Molecular Rearrangements

The atoms or groups within a molecule move from one atom to another, thus forming structural

isomer of the compound called a rearrangement molecule.

The molecular rearrangement may be defined as the reaction which involves reshuffling of the

sequence of the atoms to form a new structure.

Types of molecular rearrangement

There are two types of molecular rearrangement. They are,

1. Intermolecular rearrangement

2. Intramolecular rearrangement

1. Intermolecular rearrangement

In any rearrangement the migrating group actually becomes free even for a small fraction of

time is called intermolecular rearrangement.

In such reactions the migration group gets completely detached from the molecule during the

rearrangement and then it may go to the migration terminus of the other site of the molecule.

2. Intramolecular rearrangement

In any rearrangement the migrating group remains attached to the molecule in one or the

otherway throughout the process of rearrangement is called intramolecular rearrangement.

Anionotropy

In a molecular rearrangement if the shifting atom or group is an anion the rearrangement is

known as anionotropy.

Cationotropy


In a molecular rearrangement if the shifting atom or group is a cation the rearrangement is

known as cationotropy.

Pinacol-pinacolone Rearrangement

Conversion of pinacol (tetramethyl dihydroxy ethane) into pinacolone (trimethyl acetone) in the

presence of an acid is known as pinacol-pinacolone rearrangement. It is an example of 1,2-shift.

The migrating group may be alkyl or aryl.

Mechanism

1. A proton is added to one of the hydroxy groups.

2. The protonated hydroxy group is lost as water water producing a carbonium ion.

3. The migrating group (alkyl group) migrates.

4. A proton is lost we get the final product.

In the case of unsymmetrical pinacols:


Explanation

Since –OH group is removed as water any polar effect in the molecule which weakens one C-O

bond more than the other will fecilitate the release of that –OH group.

In the above example the phenyl group has a powerful conjugative effect whereas the methyl

group has only a weak inductive effect.

Therefore the –OH group lost should be the one on the phenyl side and consequently it will be

the methyl group that migrates.

Mechanism

Beckmann Rearrangement


Conversion of aromatic ketoximes to an acid amide on treatment with reagents such as

phosphorous pentachloride, sulphuric acid, polyphosphoric acid etc., is known as Beckmann

rearrangement. This is also an example of 1,2-shift.

Mechanism

1. A proton is added to the hydroxy group of the oxime. Intermediate (I) is formed.

2. The protonated hydroxy group is lost as water, producing an intermediate (II) analogous to

carbonium ion.

3. The migrating group Ar, migrates resulting in the carbonium ion (III).

4. The carbonium ion adds water yielding the oxonium cation (IV).

5. The oxonium cation loses a proton and forms an enol derivative of the amide (V).

6. The enol V is unstable and ketonises spontaneously giving the final product, the amide (VI).

Benzidine Rearrangement


Conversion of hydrazobenzene into benzidine and other related products in the presence of acid

is known as benzidine rearrangement.

The other products that may be produced along with benzidine are,

Mechanism

1. The rearrangement is intramolecular. Two protons are added to hydrazobenzene. We get the

deprotonated species (I).

2. The link between the singly charged nitrogen atoms in (I) starts to break in such a way that the

electron pair is shifted towards one of the nitrogen atoms. A new bond begins to form between

the para-position of the benzene nuclei. An unstable intermediate compound (II) is formed.

3. Finally two protons are eliminated in a fast step resulting benzidine.


Hofmann Rearrangement

Conversion of an amide into a primary amine with one carbon atom less by means of bromine

(or chlorine) and alkali is known as Hofmann rearrangement.

Since the product contains one carbon atom less than the parent amide this reaction is also

known as Hofmann degradation. This is an example of 1,2-shift.

Mechanism

1. The amide is brominated to give the bromamide.

2. A proton is eliminated from the bromamide and forms the anion (I).

3. Br- from (I) is lost so that the nitrogen atom is left with a sextet of electrons.

4. Now the migrating group R migrates from carbon to nitrogen resulting in the formation of

isocyanate.

5. The isocyanate is finally hydrolysed to give the amine. This arrangement is intramolecular.


Curtius Rearrangement

Conversion of acid azide into N-alkyl substituted urethane when boiled with an alcohol is known

as curtius rearrangement. It is an example of 1,2-shift.

Mechanism

1. The azide first loses a molecule of nitrogen giving (I) which contains a nitrogen atom with sextet

of electrons.

2. Now the migrating group R migrates from carbon to nitrogen resulting in the formation of

isocyanate.

3. The isocyanate finally reacts with the alcohol giving the N-alkyl substituted urethane. This is also

intramolecular rearrangement.

Lossen Rearrangement

Hydroxamic acids undergo rearrangement to form isocyanates either on treatment with bases

or sometimes thermally, in a reaction known as Lossen rearrangement. The isocyanate may be further

converted into an amine.


Mechanism

1. Abstraction of the proton from the nitrogen atom of the hydroxamic acid derivative.

2. The elimination of the hydroxyl group forming the intermediate (I).

3. The migration of the R with its bonded electrons to form isocyanate.

4. Hydrolysis of isocyanate produces primary amine.

Schmidt Rearrangement

Carboxylic acids on treatment with hydrazoic acid and dil.sulphuric acid give amines. This

reaction takes place on warming the mixture. It is known as Schmidt rearrangement.

Similarly aldehydes and ketones react to give the corresponding compounds.

Mechanism


1. In the presence of sulphuric acid, the carboxylic acid undergoes dissociation to form the stable

acyl ion (I).

2. This ion reacts with one molecule of hydrazoic acid to form the intermediate (II).

3. One molecule ofnitrogen and H+ is eliminated from the intermediate to form the compound (III).

4. Rearrangement takes place in (III) so as to form the isocyanate which on hydrolysis yields a

primary amine.

Benzilic Rearrangement

The conversion of benzil into benzilic acid by heating with ethanolic potassium hydroxide is

known as benzilic rearrangement.

Mechanism

1. The hydroxide ion is added in a fast reversible step to benzil.

2. The phenyl group along with the bonded pair of electrons migrates in a slow step.

3. A proton is transferred and we get the product.


Fries Rearrangement

Phenolic esters on heating with aluminium trichloride (Lewis acid) give o- and p-acyl phenol. This

is known as Fries rearrangement.

Mechanism

1. Exact mechanism is still not clearly understood. There is evidence of both intermolecular and

intramolecular reaction mechanisms. It is therefore suggested that both mechanisms are

operating simultaneously.

2. At first, AlCl3 complexes with the oxygen of the phenoxy group from which the acylium ion is

generated.

3. The acylium ion then attacks the benzene ring and gives the product.


Cope Rearrangement

When 1,5-dienes are heated, they isomerize in a rearrangement is known as Cope

rearrangement.

Mechanism

Cope rearrangement is a concerted step intramolecular process involving a six-membered cyclic

transition state (II).


Note

1. When 1,5-diene is symmetrical this rearrangement gives a product identical with the starting

material.

2. When the diene is not symmetrical, for example 3-methyl-1,5-hexadiene gives 1-methyl-1,5-

hexadiene in this rearrangement.

unit - iv w

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