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Monday, 10 September 2012

Important name reactions of organic chemistry: Notes for GPAT


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).
Benzoin condensation
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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

Mechanism of benzoin condensation.
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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

Mechanism if cannizaro’s reaction

Mechanism of Cannizaro's reaction.
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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.
Aldol condensation.
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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.
Riemer-tieman reaction
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If CCl4 is used instead of CHCl3, carboxylation occurs instead of formylation.

Mechanism of riemer-tieman reaction

Mechanism of riemer-tieman reaction.
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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-
Wolff rearrangement.
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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-
Different products in wolff rearrangement.
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Mechanism of wolff rearrangement

Mechanism of wolff rearrangement.
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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.
Claisen condensation.
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One molecule of the ester gives to an enolate and second ester acts as acylating agent. Classical example of this reaction is-
Example of claisen condensation.
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Mechanism of claisen condensation

Mechanism of claisen condensation.
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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.
Perkin reaction.
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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.
Pinacol-pinacolone rearrangement.
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Mechanism of pinacol-pinacolone rearrangement

Mechanism of pinacol-pinacolone rearrangement.
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