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Arrange the following compounds in increasing order of their property as indicated: (i) Acetaldehyde, Acetone, DI-tert-butyl ketone, Methyl tert-butyl ketone (reactivity towards HCN). (ii) CH_(3)CH_(2)CH(Br)COOH,CH_(3)CH(Br)CH_(2)COOH,(CH_(3))_(2)CHCOOH,CH_(3)CH_(2)CH_(2)COOH (acid strength). (iii) Benzoic acid, 4-Nitrobenzoic acid, 3,4-Dinitrobenzoic acid, 4-Methoxybenzoic acid (acid strength). |
Answer» Solution :(i) The reativity towards HCN addition decreases at the +I-effect of the alkyl group/s increases and/or the steric hindrance to the nucleophilic attach by `CN^(-)` at the carbonyl carbon increases. Thus, the reactivity decreases inthe order.  In other words, reactivity increases in the REVERSE order, i.e., Di-tert-butyl ketone lt tert-butyl methyl ketonelt acetonelt acetaldehyde. (ii) we KNOWN that +I-effect decreases while -I-effect increases the acid strength of carboxylic acids. Since +I-effect of isopropyl group is more tha the of n-propyl group, therefore, `(CH_(3))_(2)CHCOOH` is a weaker acid than `CH_(3)CH_(2)CH_(2)COOH`. Further Since -I-effect decreases with distance, therefore, `CH_(3)CH_(2)CH(Br)COOH` is a stronger acid than `CH_(3)CH(Br)CH_(2)COOH`. thus, the overall acid strength increases in the order: `CH_(3)-overset(CH_(3))overset(|)(C)H-COOH lt CH_(3)CH_(2)CH_(2)- COOH lt CH_(3)-UNDERSET(Br)underset(|)(C)H-CH_(2)-COOH lt CH_(3)-CH_(2)-underset(Br)underset(|)(C)H-COOH` (iii) Since electron-donating GROUPS decrease the acid strength, therefore, 4-methoxybenzoic acid is a weaker acid than benzoic acid. Further since electron-withdrawing groups inicrease the acid strength, therefore, both 4-nitrobenzoic acid and 3-4-dinitrobenzoic acids are stronger acids than benzoic acid further due to the presence of an additional `NO_(2)` group at m-position w.r.t. `COOH` group, 3,4-dinitrobenzoic acid is a little stronger acid than 4-nitrobenzoic acid. thus, the overall acid strength increases in the order: 4-Methoxybenzoic acidltBenzoic acidlt4-Nitrobenzoic acidlt3,4-Dinitrobenzoic acid. |
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