question_answer

  (a) Give plausible explanation for each of the following. (i) Cyclohexanone forms cyanohydrins in good yield but 2,2,6-trimethyl cyclohexanone does not. (ii) These are two ?NH^ groups in semicarbazide. However only one is involved in the formation of semicarbazones. (iii) During the preparation of esters from a carboxylic acid and an alcohol in the presence of an acid catalyst, the water or the ester should be removed as soon as it is formed. (b) Write structural formulas and names of four possible aldol condensation product from propanal and butanal.                                                                                                                                                                                         [3+2] Or (a) How will you bring about the following conversions in not more than two steps? (i) Benzaldehyde to 3-phenyl propan-1-ol. (ii) Benzoic acid to m-nitrobenzyl alcohol. (iii) Propanone to propene. (b) Describe the following. (i) Cross aldol condensation    (ii) Decarboxylation                                                                                                 [3+2]

Answer:

    (a) (i)     In cyclohexanone, cyanohydrin is formed easily as there is no steric hindrance. But in 2, 2, 6- tri methyl cyclohexanone, adjacent positions are occupied. This produces steric hindrance and thus approach of HCN, in nucleophilic addition is prevented.                [1] (ii) Lone pair on this N is delocalized due to presence   of   CO group. Thus, it is less basic as compared to  attached to NH.        [1]  (iii) (alkoxide) is a better leaving group. If or ester is not removed, above reaction becomes reversible and yield of the ester (product) is decreased.               [1]             (b)                                                                                         When and both take part in one reaction. acts as nucleophile then product is                               acts as nucleophile then product is                                                                 Or (a) (i) Benzaldehyde to 3-phenyl propan -1-ol [1] (ii) Benzoic acid (X) to m-nitro benzyl alcohol (Y)    [1] (iii) Propanone (X) to propene (Y) [1] (b) (i) Cross aldol  condensation When  two different aldehydes having at least one a - H are treated with NaOH, there is formation of aldol. One of the aldehyde acts as nucleophile and condensation product can be converted into enal. This type of reaction involving different aldehydes is called cross aldol condensation. Ketones can also undergo this reaction. From                               + From                               + From A and B                                + [1] (ii) Decarboxylation Sodium salt of carboxylic acids on heating with sodalime changes to hydrocarbon with one carbon less. This is called decarboxylation of the acid. Decarboxylation is used for descending a chain.                   [1]