[shehri]

Hi friends,

                 Is it correct that only Formaldehyde & Benzaldehyde undergo Cannizarro reaction,as they've no alpha-hydrogen attached to alpha carbon?Plz. clarify me.Thanks.

[movies]

It's most common with benzaldehydes, but it can happen with aliphatic aldehydes as well.

[m3lm4k]

If you have formyl group connected to a furan ring it would work as well. General rule is - as movies said - that molecule musn't posses alpha-hydrogens.

[shelanachium]

Aliphatic aldehydes such as pivaldehyde or dimethylethanal (CH3)3CHO will also undergo Cannizzaro reactions as they have no H on the C adjacent to the aldehyde group.

The reason it does not work with aldehydes having alpha-hydrogens is that under the strongly alkaline conditions of the Cannizzaro reaction alpha-hydrogens are removed to produce enolate ions, for example with acetaldehyde:

CH3CHO + OH- <=> CH2=CHO- + H2O

The enolate ion is strongly nucleophilic (reactive to positively-charged or electron-deficient  centres) and attacks the electron-deficient C atom of the aldehyde group on another molecule:

CH2+CHO- + CH3CHO +H2O ---> CH3CH(OH)CH2CHO 'aldol' + OH-

The aldol, or its dehydration product crotonaldehyde or but2-enal can also form enolate ions which react with further aldehyde molecules and you end up with a gooey polymer.

Often 'crossed Cannizzaro' reactions are used, particularly with formaldehyde, one of the most reactive aldehydes, acting as reductant. Thus benzaldehyde with excess formaldehyde in strong alkali reduces benzaldehyde to benzyl alcohol:

C6H5CHO + HCHO + OH- ---> C6H5CH2OH + HCO2-

[movies]

The reaction can, and does, take place with aldehydes that contain alpha-hydrogens as well!  It is often an undesired side-reaction, but it can take place.  I certainly wouldn't recommend it as a designed step in a synthesis though!