Download benzylic alcohols

BENZYLIC ALCOHOLS
[1]Nitrogen dioxide [Field’s Reaction]
Primary and secondary benzyl alcohols react with nitrogen tetra oxide in
chloroform at 0C e.g.,
Reactions apparently occurs through [radical] nitrogen dioxide, with the
formation and decomposition of a hydronitro
compound
[2]Hexamethylenetetramine [Sommelet Reaction]
The halide from the benzyl alcohol is treated with
hexamethylenetetramine and the resulting salt is hydrolyzed in the presence of
more hexamethylenetetramine, usually with aqueous acetic acid, to aldeyde
.e.g.,
It is not necessary to isolate the intermediate salt.
Electron-withdrawing substituents decrease the yields and orthosubstituensts hinder the reaction e.g. neither 2, 4 dinitro-nor 2, 6 dimethyl
benzaldehyde can be prepared in this way.
The reaction involves hydride –ion transfer. At the acidity employed, the
quaternary benzyl salt is hydrolyzed to the benzylamine and
hexamethylenetetramine itself is hydrolyzed to ammonia and formaldehyde.
The bezylamine transfers hydride ion to methyleneimine [from formaldehyde
and ammonia], giving an imine which is hydrolysed to the aromatic aldehyde
[3]Duff reaction.
Phenols react with hexamethylenetetramine, usually in acetic acid to
give ortho-aldehydes. Reaction occurs by aminomethylation followed by
sommelet reaction
[4]Krohnke reaction
A benzyl halide is converted into its pyridium salt and thence, with pnitrosodimethylaniline, into a nitrone. Acid hydrolysis gives the aromatic
aldehyde.
Reaction occurs under mild conditions so that is suitable for the
preparation of sensitive aldehydes. In addition, it is facilitated by the electronattracting substituents e.g., 2,4dinitro benzalaldehyde can be prepared in this
way [cf the Sommelet reaction]
1, 2 DIOL
1, 2 diol [Glycols] are cleaved by lead tetra-acetate, phenyliodoso
acetate, and periodic acid or sodium metaperiodate
The first two reagents are used in organic medium, commonly glacial
acetic acid, whereas periodate is used in aqueous solution. Phenyliodoso
acetate is the least powerful and is not often employed, but all three are
entirely specific; the aldehydes formed are not oxidized further. Yields are
usually excellent; e.g. di-n-butyl tartarate and lead tetra-acetate give n-butyl
glyoxylate in up to 87% yield 4].
and butane-2,3-diol and sodium periodate give acetaldehyde quantitatively,
Reaction normally occurs by two electron oxidation Pb [iv] to Pb [ii] and
I[vii] to I[v] within a cyclic intermediate:
So that cis-diols are oxidized faster than trans-diol; e.g. ciscyclohexane-1, 2-diol reacts about 25 times faster than trans isomer. However,
an acyclic path is also followed, e.g.
Example
In addition to diols, α-amino-alcohols, α-ketols, and α–dicarbonyl
compounds are cleaved
The grouping –CH (OH)-–CH (OH)-–CH (OH) - is oxidized to a mixture of
aldehydes and formic acid,
Preiodate has proved a valuable degradative agent in carbohydrate
chemistry, for it reacts nearly quantitatively. Since the grouping --CH (OH)-–CH
2OH—is oxidized is oxidized to formaldehyde and the grouping –CH (OH)-–CH
(OH)--CH(OH)- give one mole of formic acid, the estimation of formaldehyde
with dimedone and formic acid with standard base provide information about
the occurrence of these groups, and the total periodate consumed indicates
the total number of 1,2-diol groups present.
An interesting example of synthetic use of periodate, in the oxidation
of the system CO—CH (OH)-–CH(OH)-,occurs in the synthesis of reserpine. Other
uses are in the Lemieux reagents and Grunmann synthesis
Lead tetra-acetate also affects the bisdecarboxlylation of succinic acids.
ALDEHYDES
Aldehydes can be oxidized to acids by the vigorous reagents such as
chromic [vi] acid and permanganate. This can be satisfactory for compounds
which do not possess sensitive groups; e.g., n-heptanaldehyde give n-heptanoic
acid in 76-78% yield with permanganate in sulphuric acid at 20oC [2]
In most of the cases, however, milder routes are necessary. One method
employs sliver oxide: e.g. thiophen-3-aldehyde gives the 3-carboxylic acid
nearly quantitatively in 5 minutes at 0oC [4]
In general, it is usually more convenient to synthesis acids by routes
which do not involve the aldehyde, such as the carbonation of organicmetallic
compounds or from the malonic ester.
Those aldehydes which do not possess hydrocarbon atoms undergo the
Cannizzaro reaction with base; e.g. benzaldehyde gives benzyl alcohol and
benzoate ion. Finally, ortho and pare hydroxylbenzaldyde undergo oxidative
rearrangement with alkaline hydrogen peroxide
KETONES
THE CO-–CH (OH) - bond in ketone can be oxidize in three ways
[1] Nitric acid or alkaline permanganate.
These powerful conditions give carboxylic acids, reaction occurring
through the enol [acid solution] or the enolate anion [basic solution], e.g
.
Attack can occur on both sides of the carbonyl groups so that, unless
the ketone is cyclic, a mixture pf product is obtained. With cyclic ketones,
however, reasonable yields can be achieved: e.g. cyclohexanol oxidized by hot
50% nitric acid, via, cyclohexanone, to give adipic acid in 60% yield [I]
[2] By halogens in alkali.
Methyl ketones are oxidized by chlorine, bromine, or iodine in alkaline
solution to give acids and the corresponding haloform. Reaction occurs by basecatalyzed halogenations followed by elimination of the conjugate base of the
haloform.
Detailed Mechanism
This provided a particularly useful method for the synthesis of aromatic
acids, for the corresponding methyl ketones are often readily available through
the Friedel-Crafts reaction .For example, the acetylation of naphthalene in
nitrobenzene solution gives β–acetylnaphthalene from which naphthoic acid can
be obtained in 97% yield with chlorine in sodium hydroxide solution at 55 oC
There is also application in the aliphatic series, for example, pinacolone,
available from acetone by reduction and arrangement reacts with bromine in
sodium hydroxide solution at below 10oC to give trimethylacetic acid in over
70% yield [I]
[3]By Peroxide
The oxidation of carboxylic acids with HO and an acid catalyst is the best
general method for the preparation of peroxyacids
Ketone undergo oxidative rearrangement with peracids give esters or
lactones, RCOR RCOOR [Baeyer-Villinger reaction]
α–KETOLS
These systems are very readily oxidized to dicorbonyl compounds. One
electron oxidants in basic solution are effective, for the carbanion formed by
base can donate one electron to the oxidant to give a delocalized radical; loss
of a second electron completes the oxidation.
For example, copper sulphate in pyridine at 95 oC oxidizes benzoin to
benzyl in 86% yield [I]