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Question
What is meant by the following terms ? Give an example of the reaction in each case.
(i) Cyanohydrin (ii) Acetal (iii) Semicarbazone
(iv) Aldol (v) Hemiacetal (vi) Oxime
(vii) Ketal (vii) Imine (ix) 2,4-DNP-derivative
(x) Schiffs base
(i) Cyanohydrin (ii) Acetal (iii) Semicarbazone
(iv) Aldol (v) Hemiacetal (vi) Oxime
(vii) Ketal (vii) Imine (ix) 2,4-DNP-derivative
(x) Schiffs base
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Solution
(i) Cyanohydrin :
-OH group and cyano group are present on same carbon atom. Addition of HCN to carbonyl group in weakly acidic medium forms cyanohydrin.
CH3CHO+HCNpH9−10−−−−−→CH3CH(OH)CN
(ii) Acetal :
Terminal C atom has two alkoxy groups. Two equivalents of monohydric alcohol add to 1 equivalent of aldehyde in presence of dry HCl gas.
CH3CHO+2CH3OHDryHCl−−−−−→CH3CH(OCH3)2+H2O
(iii) Semicarbazone :
Derivative of aldehyde or ketone obtained on treatment with semicarbazide in weakly acidic medium.
H3CCHO+H2NNHCONH2pH3.5−−−−→−H2OCH3CH=NNHCONH2
(iv) Aldol :
Beta hydroxy aldehyde or ketone obtained by condensation of 2 molecules of aldehydes or ketones containing alpha hydrogen atoms in presence of dilute aqueous base.
2CH3CHOOH−−−−→CH3CHOHCH2CHO
(v) Hemiacetal :
Gem alkoxy alcohols obtained by addition of one molecule of monohydric alcohol to aldehyde in presence of dry HCl gas.
CH3CHO+CH3OH⇌CH3CH(OH)OCH3
(vi) Oxime :
Aldehydes or ketones react with hydroxyl amine in weakly acidic medium
CH3CHO+H2NOH→CH3CH+NOH
(vii) Ketal :
Gem dilkoxy alkanes obtained by the reaction of ketone with ethylene glycol in presence of dry HCl. Two alkoxy groups are present on same C atom.
CH3COCH3+HOCH2CH2OH→(CH3)2C(OCH2−)2
(vii) Imine :
They contain -CH=N- group obtained by the reaction between aldehydes and ketones with ammonia derivatives such as hydrazine, hydroxylamine, phenyl hydrazine, semicarbazide etc.
CH3COCH3+PhNH−NH2→(CH3)2C=NNHPh
(ix) 2,4-DNP-derivative :
Derivatives obtained by the reaction of aldehydes and ketones with 2,4-DNP (2,4-dinitrophenylhydrazone) in weakly acidic medium.
CH3COCH3+H2NNH−C6H4(NO2)2pH=3.5−−−−→−H2O(CH3)2C+NNH−C6H4(NO2)2
(x) Schiffs base :
Azomethines obtained by the reaction of aldehydes and ketones with primary amines (aliphatic or aromatic).
PhCHO+PHNH2H+−−→ΔPhCH=NHPh+H2O
-OH group and cyano group are present on same carbon atom. Addition of HCN to carbonyl group in weakly acidic medium forms cyanohydrin.
CH3CHO+HCNpH9−10−−−−−→CH3CH(OH)CN
(ii) Acetal :
Terminal C atom has two alkoxy groups. Two equivalents of monohydric alcohol add to 1 equivalent of aldehyde in presence of dry HCl gas.
CH3CHO+2CH3OHDryHCl−−−−−→CH3CH(OCH3)2+H2O
(iii) Semicarbazone :
Derivative of aldehyde or ketone obtained on treatment with semicarbazide in weakly acidic medium.
H3CCHO+H2NNHCONH2pH3.5−−−−→−H2OCH3CH=NNHCONH2
(iv) Aldol :
Beta hydroxy aldehyde or ketone obtained by condensation of 2 molecules of aldehydes or ketones containing alpha hydrogen atoms in presence of dilute aqueous base.
2CH3CHOOH−−−−→CH3CHOHCH2CHO
(v) Hemiacetal :
Gem alkoxy alcohols obtained by addition of one molecule of monohydric alcohol to aldehyde in presence of dry HCl gas.
CH3CHO+CH3OH⇌CH3CH(OH)OCH3
(vi) Oxime :
Aldehydes or ketones react with hydroxyl amine in weakly acidic medium
CH3CHO+H2NOH→CH3CH+NOH
(vii) Ketal :
Gem dilkoxy alkanes obtained by the reaction of ketone with ethylene glycol in presence of dry HCl. Two alkoxy groups are present on same C atom.
CH3COCH3+HOCH2CH2OH→(CH3)2C(OCH2−)2
(vii) Imine :
They contain -CH=N- group obtained by the reaction between aldehydes and ketones with ammonia derivatives such as hydrazine, hydroxylamine, phenyl hydrazine, semicarbazide etc.
CH3COCH3+PhNH−NH2→(CH3)2C=NNHPh
(ix) 2,4-DNP-derivative :
Derivatives obtained by the reaction of aldehydes and ketones with 2,4-DNP (2,4-dinitrophenylhydrazone) in weakly acidic medium.
CH3COCH3+H2NNH−C6H4(NO2)2pH=3.5−−−−→−H2O(CH3)2C+NNH−C6H4(NO2)2
(x) Schiffs base :
Azomethines obtained by the reaction of aldehydes and ketones with primary amines (aliphatic or aromatic).
PhCHO+PHNH2H+−−→ΔPhCH=NHPh+H2O
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