Organic name reactions play an indispensable role in the
preparation of GPAT. Most of the students ignore this section as they find
organic chemistry bit tough. In this section, I am discussing the important
name reactions in simplified way. Every weightage of organic chemistry in the
GPAT is increasing with an average of 5-10 questions every year in the exam. So
in my suggestion student should prepare for organic chemistry thoroughly to
qualify GPAT with flying colors. Now, coming to the main theme of the article,
here is the discussion of important name reactions from the point of view of
GPAT.
Benzoin condensation
Benzoin is an organic compound consisting of an ethylene
bridge flanked by phenyl group on both sides & with –OH & -C=O group.
Benzoin condensation involves condensation between 2 molecules of aromatic
aldehydes under the catalytic influence of cyanide ion forming a α-hydroxy ketone (benzoin).
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| Benzoin condensation This image can be used by only giving a link to this article. |
In this reaction, 2 molecules of aldehydes behave differently; 1 of them donates the aldehydic hydrogen to the oxygen of other.
Hence former is known as donor & latter is known as the acceptor.
This reaction can be split into 2 categories-
a) Symmetric benzoin condensation- In this reaction both of
the molecules are of same reactants to give one single product.
b) Assymetric benzoin condensation- In these reactions, two
different molecules of aromatic aldehydes react to give a mixture of products.
Mechanism of benzoin condensation
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| Mechanism of benzoin condensation. This image can be used by only giving a link to this article. |
Important points of benzoin condensation
a) Use of cyanide ion- Cyanide ion is used as a catalyst due
to its following properties.
·
Good nulcleophile.
·
Electron withdrawing ability permits loss of CHO
proton.
·
CN- group takes part in stabilizing carbanions
as formed through resonance.
·
Good leaving group.
b) This reaction is a completely reversible process.
Cannizaro’s reaction
Cannizaro’s reaction is defined as a chemical reaction that
involves base induced disproportiaion (self-redox); of an aldehyde lacking
H-atom in the α-position. The
oxidation product is a carboxylic acid and reduction product is an alcohol.
Bases used in the reaction include 50% con. NaOH, 30%
aq./alcoholic NaOH or KOH.
Genral scheme of the reaction is represented as-
(HO- )
2RCHO ----------> RCH2OH + RCOOH
(HO- )
2RCHO ----------> RCH2OH + RCOOH
Mechanism if cannizaro’s reaction
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| Mechanism of Cannizaro's reaction. This image can be used by only giving a link to this article. |
Important points of cannizaro reaction-
a) Only aldehydes which do not have α-hydrogen undergo cannizaro’s reaction whereas aldehydes with
α-hydrogen give aldol condensation.
b) cannizaro’s
reaction taking place between two dissimilar aldehydes is known as crossed
cannizaro’s reaction. In practice, reductant aldehyde used is HCHO having high
reducing ability.
c) Effect of
substituents on the rate of reaction- Electron withdrawing groups increase the
rate of reactions whereas, electron donating groups decrease the rate of
cannnizaro’s reaction.
Aldol condensation
Under the
influence of dilute base or dilute acid, 2 molecules of an aldehyde or a ketone
combine to form a β hydroxyl ketone. This reaction is named as aldol
condensation.
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| Aldol condensation. This image can be used by only giving a link to this article. |
In every
reaction, the product results from addition of 1 molecule of aldehyde to a 2nd
molecule in such a way that the α-carbon of the first becomes attached to the
carbonyl carbon to the second.
Mechanism of aldol condensation
Riemer-tieman reaction
The formylation
of an activated aromatic ring compound with chloroform in alkaline solution is
known as Riemer-tieman reaction. The method is useful only for phenols &
certain heterocyclic compounds such as pyrroles & indoles.
It leads
preferentially to the formation of an ortho-formylated product; when the ortho
product is blocked, incoming group occupies para position.
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| Riemer-tieman reaction This image can be used by only giving a link to this article. |
If CCl4 is used
instead of CHCl3, carboxylation occurs instead of formylation.
Mechanism of riemer-tieman reaction
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| Mechanism of riemer-tieman reaction. This image can be used by only giving a link to this article. |
Important points of riemer-tieman reaction
a) Ortho
predominated products are formed in the riemer-tieman reaction.
b) Involvement of
dichloromethane as the reaction intermediate receives support from the fact
that the certain substrates like pyrrole, cresol etc. also gives side products
under the reaction conditions.
Wolff rearrangement
Wolff
rearrangement is also known as Arndt-eistert reaction. It involves the
conversion of carboxylic acid to its homologous through enhancing the length of
carbon chain by one methyl species.
Various steps of
the reaction include-
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| Wolff rearrangement. This image can be used by only giving a link to this article. |
The conversion of
α-diazoketone into corresponding ketene intermediate proceeds through a
rearrangement known as wolff rearrangement. This rearrangement can be brought
about by photolysis/thermolysis/AgO.
The highly
reactive ketene gives various products depending on the nucleophile present-
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| Different products in wolff rearrangement. This image can be used by only giving a link to this article. |
Mechanism of wolff rearrangement
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| Mechanism of wolff rearrangement. This image can be used by only giving a link to this article. |
Important points of wolff rearrangement
a) Excess of
diazomethae is to be used to consume the liberated HCl.
RCOCl +
CH2N2 -------------> RCONH2 + HCl
CH2NH2 + HCl ------------->
CH3Cl + N2
Otherwise,
RCONH2 + HCL -------------> RCOCH2Cl + N2
b) The migrating
group R retains its configuration in the corresponding product of Arndt-Eistert
homologation.
c) In
photochemistry catalysed reaction, one more pathway involving oxirene
intermediate can also take place in addition to the normal one.
Claisen condensation
When carboxylic
esters containing an α-hydrogen are treated with a strong base, two molecules
of the substrate undergo self-condensation to give β-ketoester & an
alcohol. This reaction is known as claisen condensation.
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| Claisen condensation. This image can be used by only giving a link to this article |
One molecule of
the ester gives to an enolate and second ester acts as acylating agent.
Classical example of this reaction is-
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| Example of claisen condensation. This image can be used by only giving a link to this article |
Mechanism of claisen condensation
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| Mechanism of claisen condensation. This image can be used by only giving a link to this article. |
Important points of claisen condensation
a) From the
reaction mechanism, it is revealed that to get a good yield of the β-ketoester,
the product must have to convert to its enolate in the basic reaction mixture.
If the β-ketoester
is not converted completely to its enolate , it is attacked at the ketonic
functional group by an alkoxide leading to its decomposition.
Hence, atleast 2
protons must be present at the α-carbon of an esterin order of the equilibrium to
favour product formation. This simply
means that claisen condensation is possible for the esters of type RCH2COOR
but not for RCH(R)COOR.
Perkin reaction
The aldol type
base catalysed condensation of an aromatic aldehyde with a carboxylic acid
anhydride is referred to as perkin reaction. When the anhydride contains 2 α-hydrogen
atoms, the product is the α ,β-unsaturated carboxylic acid.
However, in some
cases β-hydroxy carboxylic acid is isolated when the anhydride bears only one α-hydrogen
atom because dehydration cannot take place.
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| Perkin reaction. This image can be used by only giving a link to this article. |
Base used is a weak base & usually an alkali salt of
the carboxylic acid corresponding to the acid anhydride. Other bases which can
be used are Na2CO3, Quinoline, Pyridine and (C2H5)3N.
Mechanism of perkin reaction
i) CH3COO- furnished by CH3COONa abstracts α-hydrogen from aliphatic to form
carbanion.
ii) Carbanion I is added to the carbonyl of aromatic aldehyde to form intermediate II.
iii) Protonation of the intermediate II at the anion oxygen produces hydroxyl acid anhydride III.
iv) Internal proton transfer leads to elimination of H2O to form α, β unsaturated mixed anhydride IV.
v) Hydrolysis of
unsaturated mixed anhydride IV family yield cinnamic acid.
Important points of perkin reaction
a) E & Z
isomers are anticipated from the reaction but the formation of E isomer is
preferred over Z isomer.
b) Cs salt of –COOH
yields better results.
c) Reactivity due
to para substitution follows the following orde-
NO2>Cl>H>OCH3>Alkyl
Pinacol-pinacolone rearrangement
The acid
catalyzed rearrangement of 1,2-glycols to aldehydes or ketones is called the
pinacol rearrangement.
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| Pinacol-pinacolone rearrangement. This image can be used by only giving a link to this article. |
Mechanism of pinacol-pinacolone rearrangement
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| Mechanism of pinacol-pinacolone rearrangement. This image can be used by only giving a link to this article. |
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