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 .

What is meant by the following terms? Give an example of the reaction

1.	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 .
Answer» Solution :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, they are used to prepare a-hydroxyacids. For example, (ii) Compounds in which 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. They are easily hydrolysed by dilute mineral acid to regenerate the original aldehydes. Therefore, they are used for the protection of aldehyde group in ORGANIC synthesis and for the purification of aldehydes. (iii) Semicarbazones are derivatives of aldehydes or ketones and are produced by the action of semicarbazide in weakly acidic medium, These are used for identification and characterisation of aldehydes and ketones. (iv) Aldols are `beta`-hydroxyaldehydes or ketones. They are produced by the 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, (v) Alcohols containing on alkoxy group are called hemiacetals. These are produced by addition of one molecule of a monohydric alcohol to an aldehyde in presence of dry HCl gas. (VI) When aldehydes or ketones react with hydroxylamine in weakly acidic medium, oximes are formed. (VII) Ketals are produced when a ketone is heated with ethylene glycol in presence of dry HCI gas or p-toluenesulphonic acid (PTS). It has a cyclic structure. These are easily hydrolysed by dilute mineral acids to regenerate the original ketones. Therefore, ketals are used for protection of keto groups in organic synthesis. (viii) Compound containg -CH=N-group are called imines. These are produced when aldehydes and ketones reacts with ammonia and its derivatives. `Z = H, Alkyl , aryl, -NH, -OH, -NHC_6H_5, -NHCONH_2, ` etc. (ix) Aldehydes or ketones react with 2,4-dinitrophenylhydrazine in weakly acidic medium to produce 2, 4-DNP derivative. 2,4-Dinitrophenyl Acetone 2,4-Dinitrophenylhydrazone hydrazine (2.4-DNP derivative) 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. In principle, Schiff.s bases are imines.