OXIDATION OF SATURATED HYDROCARBONS
[i] Allylic and benzylic systems
The first oxidation stage, to give a product at the oxidation level of an
alcohol, can be effectively carried out using sulphuryl chloride in the presence
of peroxide: e.g toluene gives benzyl chloride in about 80% yield
Oxidation to carboxylic acid can be brought about with chromium [vi]
oxide, permanganate, or nitric acid .For example, dilute permanganate
oxidizes o-chlorotoluene to o-chlorobenzoic acid in about 65% yield
And concentrated nitric acid oxidizes o-xylene to o-toluic acid in 54%
yield
Substituents on the methyl group are also removed in these conditions
e.g nicotine is oxidized to nicotinic acid by concentrated nitric acid at 70 C
Industrially, benzoic acid is produced by catalytically air oxidation of
toluene. Its main use is in the production of sodium benzoate which is
employed as a preservative for food stuffs and pharmaceuticals. Benzoyl
chloride is used to prepare dibenzoyl peroxide, used for initiating addition
polymerization[and ,on the laboratory scale, for other radical –catalyzed
reactions]and as a bleaching agent.
OXIDATION TO ALDEHYDIC LEVEL.
In the laboratory, three methods are available for the selective
oxidation of ArCH3 to ArCHO
[1]With Chromyl Chloride [Etard Reaction]
A solution of chromyl chloride in carbon disulphide is added cautiously to
the benzylic compound at 25-45 C and the brown complex which separates is
decomposed by water to give the aldehyde and chromic acid,e.g
The aldehyde must be removed rapidly by distillation or extraction to prevent
its further oxidation
[2]With Chromium [vi] oxide in acetic acid in acetic anhydride
Oxidation is carried out with chromium [vi] oxide in a mixture of acetic
acid and acetic anhydride and sulphuric acid at low temperature .As it is
formed, the aldehyde is converted into its gem-diacetate which is stable to
oxidation. This is isolated and recovered into the aldehyde by acid
hydrolysis.For example, p-nitro-toluene gives p-nitro-benzenaldehyde is about
45% overall yield
[3]With p-nitrosodimethylaniline
The benzylic compound is treated with p-nitrosodimetthylaniline and the
resulting Schiff’s base is hydrolyzed .The method is applicable to those
compounds whose methyl groups are strongly activated e.g.2-4dinitrotouene
A closely related method was used in the preparation of quinolinic acid.
The 4-methyl substituent in 4-methyl-6-methoxy quinoline, activated by the
hetro-atom, was condensed with benzaldehyde in the presence of zinc chloride
and the resulting benzylidene derivative was oxidized to quinolinic acid with
permanganate
[II]THE –CH2-CO-SYSTEM
The methylene groups adjacent to carbonyl may be themselves
be oxidized to carbonyl by two ways.
[1] Through the oxime.
The methylene group is activated by the carbonyl group toward
reaction with organic nitrites in the presence of acid or base. The resulting
nitroso compounds tautomerizes to oxime which may be hydrolyzed to αdicarbonyl compound
Methylene group is oxidized in preference to methyl, so that e.g
methyl ethyl ketone gives biacetyl
[2] By selective dioxide [Riley Method]
The reaction occurs through the enolic form of the carbonyl
compound
For example, camphor reacts in refluxing acetic anhydride to
give camphorquionone in 95%
yield
Selenium dioxide preferentially oxidizes methyl rather than
methylene groups, as shown above
Selenium dioxide is of limited scope because it is unseledtive.
Thus, it can bring about the following oxidations
Oxidation of –CH2-CO- and =CH-CO to –CHOH-CO- and =COH-CO- respectively
The carbanion is formed in the presence of presence of a strong
base such as potassium t-oxide and reacts with oxygen to give a hydroperoxide.
This is reduced to the corresponding alcohol with a trialkyl phosphite [tervalent
phosphrous having a very marked affinity for oxygen
Finally the -dicarbonyl system undergoes dehydrogenation by
autoxidation in the presence of base. The reaction occurs through the
tautomeric dienediol whose anion donates two electrons successively to oxygen
to form first a relatively stable anion-radical and the enedione
[iii] Unactivated C-H
The selective oxidation of one unactivated C-H group in a
molecule possessing alternative centre for attack is attended by the difficulty
that the reagent with C-H bond react essentially only free radicals- are
relatively unspecific. However, selectivity is obtained in two circumstances.
First, a relatively unreactive free radical, such as the bromine atom,
discriminates quite sharply in favour of tertiary C-H compared with primary or
secondary C-H; for example, the gas-phase bromination of iso- butane gives tbutyl bromide almost exclusively
(CH 3)3 CH+Br2 (CH 3)3 CH-Br + HBr
Bromine atoms are significantly more selective than chlorine atoms in their
reaction at primary, secondary and tertiary C-H.
The oxidation of tertiary C-H to C-OH can some time be achieved
directly with alkaline permanganate .The reaction occurs with pretension of
configuration.
Secondly, intra molecular free-radical reaction occur specifically through
six-membered cyclic transition, so that in a possible selectively to oxidize -CH
bonds
Examples of the applications of this principle have been
described below.
Photolysis of Nitrites :The Barton Reaction
The irradiation of organic nitrites may also be used to introduce functionality
at an otherwise unreactive aliphatic carbon. The oxy –radical produced by
photolysis abstract hydrogen from -CH bond and the resulting alkyl radical
combines with the nitric oxide liberated in the photolysis a nitroso compound
and hence ,when primary and secondary CH groups are involved,athe
toutomaric oxime
This method has been used in the synthesis of aldosterone 21acetate from corticosstrerone acetate. The nitrite ester of the 11-hydroxyl
group was fromed from the alcohol with nitrosyl chloride and, after
photolysis,the oxime was hydrolysed in mild conditions [nitrous acid]
The very vigorous oxidation of hydrocarbons chain by chromatic
oxide in concerntrated sulphuric acid oxidizes most substances to carbon oxide
and water, but C-methyl groups give mainly acetic acid. This procedure is
usefully applied in determining the number of C-methyl groups in a compound
of unknown structure.
[iv]Aromatization
Alicyclic compounds which are reduction products of aromatic
systems can be dehydrogenated to the respective aromatics in several ways
[1] With sulphur or selenium
Reaction occur with sulphur at about 200 C and with selenium at about
250C,the hydrogen being removed as hydrogen sulphide or hydrogen
selenide.Skeletal rearrangement may occur and carbon atoms may be
removed;in particular, angular methyl group and gem-alkyl substituents are
degraded e.g
Selenium is less destructive than sulphur and is preferred, but
the methods are more value in degradation than synthesis. Distillation with
zinc dust essentially the same effect but is more destructive .Little is known
about the mechanism.
[2] Catalytically.
Alicyclic rings which contain some unsaturation can be
dehydrogenated on those catalyst which are successful for hydrogenation ;
palladium or charcol or asbestos is the most commonly used. The conditions are
far milder than those using selenium and the procedure is widely applied; e.g.
the reduced isoquinoline obtained by the Bischler-Napieralski synthesis are
usually oxidized in this way
other example is
[3] With Quinones
Partially unsaturated alicyclic rings are oxidized by quinines
through hydride ion transfer
and
The driving force results from the conversion of both the quinone
and the alicyclic system into aromatic compounds. Quinone which contain
electron releasing substituents are stabilized relative to their qionol. e.g
and are less powerful oxidant than those containing electron-releasing
sustituents. Chloranil is frequently used , as in the synthesis of p-terphenyl,
and 2,3 dichloro-5,6-dicynobenzoquinone is more powerful reagent