Unit-I (a) Study of following reactions with mechanism

Unit-I

(a) Study of following reactions with mechanism:

1. Dieckmann Condensation (Dieckmann Reaction):

The base-catalyzed intramolecular condensation of a diester. The Dieckmann Condensation works well to produce 5- or 6-membered cyclic ß-keto esters, and is usually catalysed by sodium alkoxide in alcoholic solvent.

Mechanism of Dieckmann Condensation

2. Benzoin Condensation ( Benzoin Reaction):

The Benzoin Condensation is a coupling reaction between two aldehydes that allows the preparation of α-hydroxyketones.

Mechanism of Benzoin Condensation

3. Favorskii reaction:

Favorskii Rearrangement is a base catalysed organic reaction which converts $\alpha$-halo ketones or cyclopropanones to carboxylic acid derivatives. If the reaction involves cyclic $\alpha$-halo ketones, then ring contraction will take place during rearrangement. This reaction is named after Russian chemist Alexey Yevgrafovich Favorsky.

The main condition for this reaction is the α-halo ketones should contain α-hydrogens. The rearrangement takes place in the presence of a base.

Mechanism of Favorskii reaction

In the first step, deprotonation of α-carbon takes place to form enolate ion. Intra molecular nuclophilic substitution takes place followed by production of cyclopropanone by ring closure. The cyclopropanone formed thus, undergo nucleophilic addition by a base at the carbonyl group followed by the clevage of CO-C bond.This clevage occurs to give less substituted and more stable carbanion.

Favorskii rearrangement leads to ring contraction by one carbon atom. This type of reaction found application in the key step for the synthesis of cubane for the construction of cubane carbon skeleton. The most important application of this reaction is that it is used in the synthesis of steroids.

4. Reimer-Tieman Reaction:

The Reimer–Tiemann reaction is a chemical reaction used for the ortho-formylation of phenols. The simplest example is the conversion of phenol to salicylaldehyde. The reaction was discovered by Karl Reimer and Ferdinand Tiemann.

Mechanism of the reaction

5. Stobbe condensation:

Mechanism

6. Diels-Alder Reaction and Mechanism:

Mechanism:

7. Robinson annulation:

The Robinson annulation is a chemical reaction used in organic chemistry for ring formation. It was discovered by Robert Robin in 1935 as a method to create a six membered ring by forming three new carbon–carbon bonds. The method uses a ketone and a methyl vinyl ketone to form an α,β-unsaturated ketone in a cyclohexane ring by a Michael addition followed by an aldol condensation. This procedure is one of the key methods to form fused ring systems.

Mechanism:

8. Chichibabin Reaction:

Mechanism:

9. Simon- Smith Reaction:

The Simmons–Smith reaction is an organic cheletropic reaction involving an organozinc carbenoid that reacts with an alkene (or alkyne) to form a cyclopropane. It is named after Howard Ensign Simmons, Jr. and Ronald D. Smith. It uses a methylenefree radical intermediate that is delivered to both carbons of the alkene simultaneously, therefore the configuration of the double bond is preserved in the product and the reaction is stereospecific.

Mechanism:

10. Ullmann coupling:

The Ullmann reaction is an organic reaction used to couple two molecules of aryl halide to form a biaryl using copper metal and thermal conditions. The mechanism for the Ullmann reaction is not fully under-stood but there are two popular mechanisms.

Mechanism

11. Mc. Murry reaction:

The McMurry coupling is the reaction of two carbonyl functional groups to establish a new double bond between the carbons of the carbonyl groups. The reaction is mediated by low-valent titanium reagents, which may be generated through the combination of titanium chlorides with any of a number of reducing agents. The McMurry coupling is useful for the construction of sterically hindered alkenes, but has limited scope due to a lack of stereochemical control and statistical mixtures of products in mixed-coupling reactions

Mechanism

12. Dakin Reaction: 

Mechanism

Nucleophilic addition reaction takes place to the carbonyl carbon. Tetrahedral intermediate is formed, followed by elimination, generating phenyl ester. Hydrolysis of phenyl ester forms the corresponding product.