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NCERT Class XII Chemistry
Chapter - Aldehydes, Ketones and Carboxylic Acids
Questions with Solutions

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Question : 1
Total: 28
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 (viii) Imine (ix) 2,4-DNP derivative (x) Schiff’s base
Solution:  
(i) Carbonyl compounds in the presence of an alkali, react with HCN to form an addition product called cyanohydrin.
HCN+
−
O
H⇌H2O+
−
C
N
(ii) Acetals are gem-dialkoxy compounds formed when aldehydes react with monohydric alcohols in the presence of dry HCl gas.
(iii) Semicarbazone is formed by the reaction between carbonyl compounds and semicarbazide.
(iv) β-Hydroxy aldehydes are called aldol. Aldehydes and ketones containing at least one aα-H atom undergo a reaction in the presence of dilute alkali to form aldol compounds. e.g.,
2CH3CHO
dil‌
−
O
‌H
⇌
CH3
C
|
OH
H−CH2CHO
3−HYDROXYBUTANAL
Δ
––––––––▶
−H2O
CH3CH=CHCHO
But−2−enal
(v) Hemiacetals are gem-alkoxyalcohols which are produced from the addition of one molecule of monohydric alcohol to an aldehyde in the presence of HCl gas.
(vi) Oximes are formed by reaction between aldehydes or ketones and hydroxylamine. The reaction is carried out in mildly acidic condition.
(vii) Ketals are cyclic products formed by the reaction of ketones with ethylene glycol in the presence of dry HCl gas.
(viii) Imines are formed when carbonyl compounds react with ammonia derivative.
(ix) 2, 4-DNP derivatives are formed when 2, 4- dinitrophenylhydrazine reacts with aldehydes or ketones.
(x) Reaction between aldehydes or ketones with 1° aliphatic or aromatic amines yields a Schiff’s base.
R−CHO+H2N−R′
very‌dil‌H+
––––––––▶
RCH=N−R′
Schiff′s‌base
+H2O
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