SN-Ar-( NUCLEOPHILIC AROMATIC SUBSTITUTION:

S_N2 (Aromatic) Type: Chloro benzene does not under goes Nucleophilic substitution reaction in ordinary condition due to following reason.

C–X bond in aryl halide is stable due to delocalisation of electrons by resonance. Also (C–X) bond possesses a double bond character like vinyl chloride and is stronger than C–X bond in alkyl halide.

Case-(1): Hence, SN reaction is not possible in benzene nucleus under ordinary conditions. However, under high temperature and pressure, SN reaction is made possible.

 Case-(2): However Nucleophilic substitutions occur in the presence of one or more electron withdrawing (EWG) at ortho and para position. These electron-withdrawing groups must be positioned ortho or para to the leaving group. The greater the number of electron-withdrawing substituents, the easier it will be to carry out the nucleophilic aromatic substitution reactions. 

EXAMPLE:

While electron releasing group (ERG) destabilizes carbanium ion and deactivated.

OTHER EXAMPLES:

SN-Ar-(SUBSTITUTION OF GROUP OTHER THAN HYDROGEN:

Nucleophilic substitution not occurs in aromatic compound but few Nucleophilic substitutions occur in the presence of strong nucleophile. These reactions may be S_N1 -Aromatic unimolecular and S_N2 –Aromatic bimolecular.

S_N1 (Aromatic) Type: The decomposition of diazonium salt in polar medium and formation of different product in the presence of different nucleophile are example of Nucleophilic Aromatic Substitution (S_N1-Ar). The rate of reaction depends upon only concentration of Aryl cation only hence it is unimolecular reaction.

Aryl cation is very reactive and takes up any nucleophile present recombined with elimination of N₂. Therefore reaction is reversible.

Although aryl cation is very unstable , the driving force for it formation of elimination of very stable due to very high bond dissociation of  nitrogen molecule.

SN-Ar-(SUBSTITUTION OF HYDROGEN OF BENZENE:

S_N-Ar : More general attack of nucleophile on unsubstituted benzene ring is very difficult as compared to attack by Electrophile hence nucleophilic substitution of hydrogen from benzene is also difficult because..

(1) The pi electron cloud of benzene nucleus repels the approaching nucleophile.

(2) It is difficult to accumulate negative charge (electron) donated by attacking nucleophile.

However in the presence of one the electron withdrawing group like nitro group (-NO₂) on benzene ring, then it is sufficiently activate ortho and para position

ILLUSTRATIVE EXAMPLE: 

Nitro phenol give ortho nitro phenol  and also little amount of para nitro phenol on treatment with strong NaOH in the presence of oxidizing agents like KNO₃ and K₃ [Fe(CN)₆].

MECHANISM: (SN-Ar):

­­Important note : Since OH- is  better leaving group than hydride ion (H- ) but the presence of oxidizing agents  like  air , KNO₃ and K₃ [Fe(CN)₆] which encourage the elimination of hydride ion.

Other Examples:

CANNIZZARO REACTION: DISPROPORTIONATION:

(1) Introduction:

(2) Mechanism:

(3) Condition for cannizaro reaction:

(4) Illustrative Examples

(5) Crossed Cannizzaro reaction

(6) Intramolecular cannizaro reaction:

(1) INTRODUCTION:

The aldehydes having absence of α-hydrogens when treated with concentrate strong base undergoes to disproportionate reaction and furnish an alcohol and a carboxylic acid is called Cannizzaro reaction. In this reaction one molecule of aldehyde is reduced to the corresponding alcohol, while a second one is oxidized to the carboxylic acid.

 (2) MECHANISM OF CANNIZARO REACTION:

Step – (1):

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.

Step – (2):

In this second step, the hydroxide behaves as a base. Now a hydride ion, H^- is transferred either from the mono anionic 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.

Thus one molecule is oxidized to carboxylic acid and the other one is reduced to an alcohol. 

(3) CONDITION FOR CANNIZARO REACTION:

(1) Primary condition of cannizaro reaction is absence of alpha hydrogen in aldehyde but (CH₃)₂CH-CHO gives cannizaro reaction although it has one alpha hydrogen.

(2) CCl₃-CHO does not give cannizaro reaction while it has no alpha hydrogen it give halo form reaction.

(3) The overall order of the reaction is usually 3.

(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.

(5) Transfer of hydride is rate determining step.

(6) In cannizaro reaction kinetic isotopic  effect is observed

Step-(1):

Step-(2):

(4) ILLUSTRATIONS & EXAMPLES OF CANNIZZARO REACTION

(1) Formaldehyde is disproportionated to formic acid and methyl alcohol in strong alkali.

(3) Furfural gives furoic acid and furfuryl alcohol in presence of strong alkali.

(2) Benzaldehyde can be converted to benzoic acid and benzyl alcohol.

(5) CROSSED CANNIZARO REACTION:

When a mixture of two different aldehyde (alpha hydrogen less)  like formaldehyde and a non enolizable aldehyde (benzaldehyde) is treated with a strong base, the later is preferentially reduced to alcohol while formaldehyde is oxidized to formic acid. This variant is known as crossed Cannizzaro reaction.

Illustrative example: Benzyl alcohol and formic acid are obtained when a mixture of benzaldehyde and formaldehyde is treated with alkali.

Important note:

(1) In cross cannizaro reaction if one of the reactant is formaldehyde, then oxidation of formaldehyde take place , and reduction of another aldehyde take place .The reason may be: the initial nucleophillic addition of hydroxide anion is faster on formaldehyde as there are no electron donating groups on it.

(2) The preferential oxidation of formaldehyde in crossed Cannizzaro reactions may be utilized in the quantitative reduction of some aldehydes. 

(6) INTRAMOLECULAR CANNIZARO REACTION:

 α-keto aldehydes can be converted to α-hydroxy carboxylic acids by an intermolecular Cannizzaro reaction.

Illustrative Example: Phenylglyoxal undergoes intramolecular cannizzaro reaction by giving Mandelic acid (α-hydroxyphenylacetic acid or 2-Hydroxy-2-phenylethanoic acid)

Illustrative Example: Phthalaldehyde can undergo intramolecular Cannizzaro reaction by giving (o-hydroxymethyl) benzoic acid.

MECHANISM OF CANNIZARO REACTION:

Step – (1):

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.

Step – (2):

In this second step, the hydroxide behaves as a base. Now a hydride ion, H^- is transferred either from the mono anionic 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.

Thus one molecule is oxidized to carboxylic acid and the other one is reduced to an alcohol.