cannizzaro Reaction

What is Cannizzaro Reaction?

The Cannizzaro reaction, named after its discoverer Stanislao Cannizzaro.

In this reaction, two molecules of an aldehyde are reacted with strong base to produce a primary alcohol and a carboxylic acid using a hydroxide base.

•  It is a redox reaction means in this reaction; the aldehyde undergoes both the oxidation and reduction. The oxidation product is a salt of a carboxylic acid and the reduction product is an alcohol.

• Only aldehydes that do not have alpha hydrogen show Cannizzaro reaction.(Alpha hydrogen is the hydrogen atom attached to C next to aldehyde functional group i.e. with  (C* - CHO) like formaldehyde HCHO, acetaldehyde CH₃CHO etc).

Mechanism:

 The reaction begins with hydroxide attack on the carbonyl carbon followed by deprotonation to give a dianion.

This unstable intermediate releases a hydride anion which attacks another molecule of aldehyde. In this process the dianion converts to a carboxylate anion and the aldehyde to an alkoxide.

The alkoxide is more basic than water so it picks up a proton from water to gives alcohol as final product.

On the other hand, the carboxylate which is less basic than water, cannot pick a proton from water. Thus, after acid work up it converts into carboxylic acid. 

Friedal Crafts Alkyation

Friedal Crafts Alkylation:

This reaction is developed by Charles Friedel and James Crafts in 1877.

In Friedal crafts alkylation, a haloalkane reacts with benzene in the presence of an aluminium halide catalyst, or any other lewis acid catalyst, and produce an alkylbenzene and a hydrogen halide.

 The reactivity and polarity of haloalkane increase as we move up the periodic table. It means:

RF > RCl > RBr > RI

This means that the Lewis acid used as catalyst in Friedel Craft alkylation reaction tend have similar halogen combination. BF₃, SbCl₅, AlCl₃ and AlBr₃ are commonly used Lewis acid in the reaction.

Mechanism of Friedel Crafts alkylation:

1)    In this step, a carbocation is formed which act as a electrophile in the reaction. This activates the haloalkane.

2)    In step two, an electrophillic attacks on benzene which results in multiple resonance forms. The halogen reacts with the intermediate and picks up the hydrogen to eliminate the positive charge.

3)    The last step shows end of the step two and give the final product.

 

Limitations:

There are few limitations of Friedel Crafts alkylation.

·         The reaction works only with benzene or ACTIVATED benzene derivatives. It will not occur if the benzene ring is deactivated.

·         The reaction works only with ALKYL halides (i.e. chlorides, bromides or iodides), but it does not work with VINYL or ARYL halides.

·         Alkylation reactions are prone to carbocation rearrangement.

·         Over alkylation can be a problem since the product is more reactive than the starting material. This can usually be controlled with an excess of the benzene.

·         The Lewis acid catalyst AlCl₃ often complexes to aryl amines making them very unreactive.