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Question
Explain the following with one example.
a) Williamson's synthesis
b) Kolbe's reaction
c) Hell-Vohard-Zelinsky (HVZ) reaction
d) Aldol condensation
a) Williamson's synthesis
b) Kolbe's reaction
c) Hell-Vohard-Zelinsky (HVZ) reaction
d) Aldol condensation
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Solution
(a) Williamson synthesis:
It is used for the preparation of simple as well as mixed ethers. Alkyl halide is heated with alcoholic sodium or potassium alkoxide to form corresponding ethers.
R−ONa+X−RΔ−→R−O−R′+NaX
Thus, methyl iodide, on heating with alcoholic sodium methoxide forms dimethyl ether.
CH3−ONa+X−CH3Δ−→CH3−O−CH3+NaI
(b) Kolbe's reaction:
Sodium salt of phenol is heated with carbon dioxide under pressure (100 atm, 125 deg C) , followed by acid hydrolysis to obtain salicylic acid.
(c)
Hell Volhard Zelinsky (HVZ) reaction:
This reaction involves alpha bromination of carboxylic acids. Carboxylic acids react with chlorine or bromine in presence of small amount of red phosphorous to give alpha halo carboxylic acids.
CH3−CH2−COOHBr2/P−−−−→CH3−CHBr−COOHBr2/P−−−−→CH3−CBr2−COOHBr2/P−−−−→ No further reaction.
(d) Aldol condensation
It is a reaction between two molecules of carbonyl compounds having alpha hydrogen atoms to form beta hydroxy aldehyde (aldol) or beta hydroxy ketone (ketone). It occurs in presence of dilute base NaOH, KOH or Na2CO3. Thus, ethanal and propanal will give following products.
2CH3−CHOdilNaOH−−−−−−→CH3−CHOH−CH2−CHO(i)H+−−−→(ii)ΔCH3−CH=CH−CHO
It is used for the preparation of simple as well as mixed ethers. Alkyl halide is heated with alcoholic sodium or potassium alkoxide to form corresponding ethers.
R−ONa+X−RΔ−→R−O−R′+NaX
Thus, methyl iodide, on heating with alcoholic sodium methoxide forms dimethyl ether.
CH3−ONa+X−CH3Δ−→CH3−O−CH3+NaI
(b) Kolbe's reaction:
Sodium salt of phenol is heated with carbon dioxide under pressure (100 atm, 125 deg C) , followed by acid hydrolysis to obtain salicylic acid.
(c)
Hell Volhard Zelinsky (HVZ) reaction:
This reaction involves alpha bromination of carboxylic acids. Carboxylic acids react with chlorine or bromine in presence of small amount of red phosphorous to give alpha halo carboxylic acids.
CH3−CH2−COOHBr2/P−−−−→CH3−CHBr−COOHBr2/P−−−−→CH3−CBr2−COOHBr2/P−−−−→ No further reaction.
(d) Aldol condensation
It is a reaction between two molecules of carbonyl compounds having alpha hydrogen atoms to form beta hydroxy aldehyde (aldol) or beta hydroxy ketone (ketone). It occurs in presence of dilute base NaOH, KOH or Na2CO3. Thus, ethanal and propanal will give following products.
2CH3−CHOdilNaOH−−−−−−→CH3−CHOH−CH2−CHO(i)H+−−−→(ii)ΔCH3−CH=CH−CHO
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