Question
Explain Cannizzaro reaction.###Write a note on Cannizzaro reaction###
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Answer
- Aldehydes which do not have a-hydrogen atom, on heating with concentrated alkali ($50 \%$ aqueous or ethanolic solution of NaOH or KOH) undergo self oxidation and reduction reaction or redox reaction.
- This self redox reaction or disproportionation reaction is called Cannizzaro reaction.
- In this reaction one molecule of the aldehyde is oxidised to carboxylic acid while the second molecule of the aldehyde is reduced to alcohol (carboxylic acid formed, reacts with alkali, NaOH and forms a salt R – COONa).
- When formaldehyde (methanal) is heated with 50% NaOH solution, methanol (reduction product) and sodium formate (oxidation product) are formed.
- Ketones and aldehydes like acetaldehyde, propionaldehyde, etc. having a – H atom do not give Cannizzaro reaction.
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$Mg _{( s )}\left| Mg _{( aq )}^{2+}(1 M ) \| Br _{( aq )}^{-}(1 M )\right| Br _{2( l )} \mid Pt$
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(ii) $2 AgCl _{( s )}+2 e ^{-} \rightarrow 2 Ag _{( s )}+2 Cl _{\text {(aq) }}^{-}$(Reduction half reaction)
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$Pt \left| Sn _{\text {(aq }}^{2+}, Sn _{\text {(aq) }}^{4+} \| Cl _{\text {(aq) }}^{-}(1 M )\right| AgCl _{( s )} \mid Ag _{( s )}$
(b) $Mg _{( s )}+ Br _{2(l)} \rightarrow Mg _{( aq )}^{2+}+2 Br _{( aq )}^{-}$
The overall reaction takes place into two steps:
(i) $Mg _{( s )} \longrightarrow Mg _{\text {(aq) }}^{2+}+2 e ^{-}$
(Oxidation half reaction)
(ii) $Br _{2()}+2 e ^{-} \longrightarrow 2 Br _{\text {(aq) }}^{-}$
(Reduction half reaction)
Hence the cell is,
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