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) Schiff’s base

Explanation

(i) Gem-Hydroxynitriles, i.e., compounds possessing hydroxyl and cyano groups on the same carbon atom are called cyanohydrins. These are produced by addition of HCN to aldehydes or ketones in weakly basic medium.

Cyanohydrins are useful synthetic reagents because they are used to prepare `alpha`-hydroxyacids. for example,
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(ii) gen-Dialkoxyalkanes in which the two alkoxy groups are present on the terminal carbon atom are called acetals. these are produced by the action of an aldehyde with two equivalents of a monohydric alcohol in presence of dry HCl gas.
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These are easily hydrolysed by dilute mineral acids to regerate the original aldehydes. therefore, these are used for the protection of aldehyde group in organic synthesis.
(iii) Semicarbazones are derivatives of aldehydes and ketones and are produced by the action of semicarbazide on them in weakly acidic medium.
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These are used for identification and characterization of aldehydes and ketones.
(iv) Aldols are `beta`-hydroxyalehydes or ketones and are produced bythe condensation of two molecules of the same or one molecule each of two different aldehydes or ketones in presence of a dilute aqueous base. for example.
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(v) `alpha`-Alkoxyalcohols are called hemiacetals. these are produced y addition of one molecule of a monohydric alcohol to an aldehyde in prsence of dry HCl gas.

(vi) Oximes are produced when aldehydes or ketones react with hydroxylamine in weakly acidic medium.

(vii). gem-Dialkoxyalkanes in which the two alkoxy groups are present on the same carbon within the chain are called ketals. these are produced when a ketone is heated with ethylene glycol in presence of dry HCl gas or p-toluenesulphonic acid (PTS).
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These are easily hydrolysed by dilute mineral acids to regenerate the orginal ketones. therefore, ketals are used for protection of keto groups in organic synthesis.
(viii). Compounds containing group are called imines. these are produced when aldehydes and ketones react with ammonia and its derivatives.

`Z=H,` alkyl, aryl, `-NH,-OH,-NHC_(6)H_(5),-NHCOCH_(2)`, etc.
(ix) 2,4-Dinitrophenylhydrazones (i.e., 2,4-DNP derivatives) are produced when aldehydes or ketones react with 2,4-dinitropheylhydrazine in weakly acidic medium.

2,4-DNP derivatives are used for identification and characterisation of aldehydes and ketones. (x) Aldehydes and ketones react with primary aliphatic or aromatic amines in presence of trace of an acid to form azomethines or Schiff's bases. Schiff's bases may also be regarded as imines.