Guided by: H.S.Tailor Pacific school of engineering, Surat Sub :Application of following reaction with mechanism

Index  Sandmayer reaction  Hoffman  Diazotisation  Wolff kishner  Baeyer-Villiger  Cannizzaro reaction

Sandmeyer Reaction The substitution of an aromatic amino group is possible via preparation of its diazonium salt and subsequent displacement with a nucleophile (Cl-, I-, CN-, RS-, HO-). Many Sandmeyer Reactions proceed under copper(I) catalysis, while the Sandmeyer-type reactions with thiols, water and potassium iodide don't require catalysis.

Mechanism of the Sandmeyer Reaction The Sandmeyer Reaction is a very important transformation in aromatic chemistry, because it can result in some substitution patterns that are not achievable by direct substitution.

Hoffman Hoffman Elimination is an organic reaction in which anamine is converted to tertiary amine and alkene by treatment of excess of methyl iodide and then, further with excess of silver oxide. This reaction is named after a German chemist August Wilhelm von Hofmann. The common procedure for the preparation of alkene from an amine by the application of Elimination reaction is an important method in organic chemistry and it is employed by Hofmann Elimination.

Hofmann Elimination Mechanism The Hoffman elimination reaction mechanism has been proposed as the chemical pathway for the degradation of the quat. A strong base (the hydroxyl ion) is necessary to facilitate the elimination step.

Diazotisation The nitrosation of primary aromatic amines with nitrous acid (generated in situ from sodium nitrite and a strong acid, such as hydrochloric acid, sulfuric acid, or HBF4) leads to diazonium salts, which can be isolated if the counterion is non-nucleophilic. Diazonium salts are important intermediates for the preparation of halides (Sandmeyer Reaction, Schiemann Reaction), and azo compounds. Diazonium salts can react as pseudohalide-type electrophiles, and can therefore be used in specific protocols for the Heck Reaction or Suzuki Coupling.

Diazotisation The intermediates resulting from the diazotization of primary, aliphatic amines are unstable; they are rapidly converted into carbocations after loss of nitrogen, and yield products derived from substitution, elimination or rearrangement processes.

Mechanism of Diazotisation

WOLFF KISHNER In Wolff-Kishner reduction, the carbonyl compounds which are stable to strongly basic conditions can be reduced conveniently to alkanes. The C=O group is converted to CH2 group. The carbonyl compound is first treated with excess of hydrazine to get the corresponding hydrazone which upon heating, in presence of a base, furnishes the hydrocarbon. A high-boiling hydroxylic solvent, such as diethylene glycol (DEG), is commonly used to achieve the temperatures needed.

Wolff Kishaner Mechanism The Wolff-Kishner reduction is complementary to Clemmensen reduction, which is used to reduce base sensitive compounds.

Baeyer-Villiger The Baeyer-Villiger Oxidation is the oxidative cleavage of a carbon-carbon bond adjacent to a carbonyl, which converts ketones to esters and cyclic ketones to lactones. The Baeyer-Villiger can be carried out with peracids, such as MCBPA, or with hydrogen peroxide and a Lewis acid. The regiospecificity of the reaction depends on the relative migratory ability of the substituents attached to the carbonyl. Substituents which are able to stabilize a positive charge migrate more readily, so that the order of preference is: tert. alkyl > cyclohexyl > sec. alkyl > phenyl > prim. alkyl > CH3. In some cases, stereoelectronic or ring strain factors also affect the regiochemical outcome.

Mechanism of the Baeyer-Villiger

CANNIZZARO REACTION The disproportionation reaction of aldehydes without α-hydrogens in presence of a strong base to furnish an alcohol and a carboxylic acid is called Cannizzaro reaction. One molecule of aldehyde is reduced to the corresponding alcohol, while a second one is oxidized to the carboxylic acid.

CANNIZZARO REACTION The applicability of Cannizzaro reaction in organic synthesis is limited as the yield is not more than 50% for either acid or alcohol formed. In case of aldehydes that do have α-hydrogens, the aldol condensation reaction takes place preferentially. The α,α,α-Trihalo aldehydes undergo haloform reaction in strongly alkaline medium. E.g. Choral will give chloroform in presence of an alkali.

MECHANISM OF CANNIZZARO The cannizzaro reaction is initiated by the nucleophilic attack of a hydroxide ion to the carbonyl carbon of an aldehyde molecule by giving a hydrate anion. This hydrate anion can be deprotonated to give an anion in a strongly alkaline medium. In this second step, the hydroxide behaves as a base. Now a hydride ion, H- is transferred either from the monoanionic species or dianionic species onto the carbonyl carbon of another aldehyde molecule. The strong electron donating effect of O- groups facilitates the hydride transfer and drives the reaction further. This is the rate determining step of the reaction.

MECHANISM OF CANNIZZARO

Thus one molecule is oxidized to carboxylic acid and the other one is reduced to an alcohol. When the reaction is carried out with D2O as solvent, the resulting alcohol does not show carbon bonded deuterium. It indicates the hydrogen is transferred from the second aldehyde molecule, and not from the solvent. The overall order of the reaction is usually 3 or 4. The Cannizzaro reaction takes place very slowly when electron-donating groups are present. But the reaction occurs at faster rates when electron withdrawing groups are present.

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