8
Question
Explain the following reactions with suitable example:
(a) Reimer-Tiemann reaction
(b) Cannizaro reaction
(c) Aldol condensation
(d) Sandmeyer reaction
(a) Reimer-Tiemann reaction
(b) Cannizaro reaction
(c) Aldol condensation
(d) Sandmeyer reaction
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Solution
(i) Reimer-Tiemann reaction → When phenol is treated with chloroform in the presence of sodium hydroxide, a −CHO group is introduced at the ortho position of benzene ring. This reaction is known as Reimer-Tiemann reaction. The intermediate is hydrolysed in presence of alkalis to produce salicylaldehyde. (Image attached)(b) Cannizzaro reaction → Aldehydes which do not have a α-hydrogen atom, undergo self-oxidation and reduction (disproportionation) reaction on heating with concentrated alkali.In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidized to carboxylic acid salt.Ex- 2HCHOFormaldehyde+KOHConc.Δ−→CH3−OH+HCOOK(c) Aldol condensation → Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.Ex- 2CH3−CHOEthanaldil. NaOH−−−−−−→CH3−OH|CH−CH2−CHOAldolΔ−−−−→−H2OCH3−CH=CH−CHOAldl condensation product(d) Sandmeyer Reaction → It is used to synthesize aryl halides from aryl diazonium salts. It is an example of a radical-nucleophilic aromatic substitution.When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed. Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by −Cl or −Br.C6H5−NH2NaNO2+HX−−−−−−−−→273−278KC6H5−N2+X−⎧⎪
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⎪⎨⎪
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⎪⎩CuCl−−−−→C6H5−ClCuBr−−−−→C6H5−BrCuCN−−−−→C6H5−CN
(b) Cannizzaro reaction → Aldehydes which do not have a α-hydrogen atom, undergo self-oxidation and reduction (disproportionation) reaction on heating with concentrated alkali.
In this reaction, one molecule of the aldehyde is reduced to alcohol while another is oxidized to carboxylic acid salt.
Ex- 2HCHOFormaldehyde+KOHConc.Δ−→CH3−OH+HCOOK
(c) Aldol condensation → Aldehydes and ketones having at least one α-hydrogen undergo a reaction in the presence of dilute alkali as catalyst to form β-hydroxy aldehydes (aldol) or β-hydroxy ketones (ketol), respectively. This is known as Aldol reaction.The aldol and ketol readily lose water to give α,β-unsaturated carbonyl compounds which are aldol condensation products and the reaction is called Aldol condensation.
Ex- 2CH3−CHOEthanaldil. NaOH−−−−−−→CH3−OH|CH−CH2−CHOAldolΔ−−−−→−H2OCH3−CH=CH−CHOAldl condensation product
(d) Sandmeyer Reaction → It is used to synthesize aryl halides from aryl diazonium salts. It is an example of a radical-nucleophilic aromatic substitution.
When a primary aromatic amine, dissolved or suspended in cold aqueous mineral acid, is treated with sodium nitrite, a diazonium salt is formed. Mixing the solution of freshly prepared diazonium salt with cuprous chloride or cuprous bromide results in the replacement of the diazonium group by −Cl or −Br.
C6H5−NH2NaNO2+HX−−−−−−−−→273−278KC6H5−N2+X−⎧⎪
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⎪⎨⎪
⎪
⎪⎩CuCl−−−−→C6H5−ClCuBr−−−−→C6H5−BrCuCN−−−−→C6H5−CN
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