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Give plausible explanation for each of the following . (i) Why are amines less acidic than alcohols of comparable molecular masses ? (ii) Why have primary amines higher boiling points than tertiary amines. (iii) Why are aliphatic amines stronger bases than aromatic amines ? |
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Answer» Solution :(i) Loss of proton from amines gives alkyl amide ION whereas loss of a proton from alcohol gives an alkoxide ion. `underset("Amine")(RNH_2) to underset("Alkylamide")(RNH^(-)) + H^(+)` `underset("Alcohol")(ROH) to underset("Alkoxide ion")(RO^(-)) + H^+` Since O is more electronegative than `N_1` therefore , `RO^(-)` can accommodate the -ve CHARGE more easily `RNH^(-)` . Consequently , `RO^(-)` is more stable than `RNH^(-)` . Thus, alcohols are more acidic than amines. (ii) Primary amines `(RNH_2)` have two hydrogen atoms on the N atom and therefore, form INTERMOLECULAR hydrogen bonding.  Tertiary amines `(R_3N)` do not have hyrogen atoms on the N atom and therefore, these do not form hydrogen bonds. As a result of hydrogen bonding in primary amines, they have higher boiling points than tertiary amines of comparable molecular mass. For EXAMPLE, b.p. of n-butylamine is 351 K while that of tert-butylamine is 319 K. (iii) Both arylamines and alkyamines are basic in nature due to the presence of lone pair of N-atom . But arylamines are LESS basic than alkylamines . For example, aniline is less basic than ethylamine as shown by `K_b` values : Ethylamine : `K_b = 4.7xx10^(-4)` Aniline : `K_b = 4.2xx10^(-10)` The less basic character of aniline can be explained on the basic of aromatic ring present in aniline. Aniline can have the following resonating structures :  It is clear from the above resonating strucutures that three of these (III, IV and V) acquire some positive charge on N atom. As a result, the pair of electrons become less avilable for protonation . hence, aniline is less basic than ethyl amine in which there is no such resonance. |
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