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Arrange the following (i) In increasing order of solubility in water C_(6)H_(5)NH_(2),(C_(2)H_(5))_(2)HN,C_(2)H_(5)NH_(2) (ii) In increasing order of basic strength (a) aniline , p- toludine and p- nitroaniline (b) C_(6)H_(5)NH_(2), C_(6)H_(5)NHCH_(3),C_(6)H_(5)NH_(2),p-Cl-C_(6)H_(4)-NH_(2) (iii) .In decreasing order of basic strength in gas phase C_(2)H_(4)NH_(2), (C_(2)H_(5))_(2)NH,(C_(2)H_(4))_(3)N and NH_(3) (C_(2)H_(4)OH,(CH_(3))_(2)NH,C_(2)H_(5)NH_(2) (v) In decreasing order to the pk_(b) values C_(2)H_(5)NH_(2),C_(6)H_(5)NHCH_(3),(C_(2)H_(4))_(2)NH and CH_(3)NH_(2) (vi) Increasing order of basc strength C_(6)H_(4)NH_(2),C_(6)H_(5)N(CH_(3))_(2),(C_(6)H_(5))_(2)NH and CH_(3)NH_(2) (vii) In decreasing order of basic strength |
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Answer» Solution :i. Solubility decreases with increase in molecular mass of amines due to increase in the size fo a hydrophobic hydrocarbon part and with decrease in the number of H-atoms on the N-atom which undergo H-bonding. Now among the given compounds `C_(6)H_(5)NH_(2)` has the highest molecular mass of 93 followed by `(C_(2)H_(5))_(2)NH` with molecular mass of 73 with `C_(2)H_(5)NH_(2)` has the lowesst molecular mass of 45. Thus the solubililty increases in the order in whcih molecular mass decreases. `C_(6)H_(5)NH_(2)lt(C_(2)H_(5))_(2)NHltC_(2)H_(5)NH_(2)` (ii) a. Theelectron donating groups increases the basic strength of amines while the electron- withdrawing groups decrease the basic strength of amines. Therefore p-nitroanilne is the weakest base followed by aniline while p- toluidine, which has methyl group andtherefore it is the strongest base. Basic strength increases in the order. P-nitro aniline `lt` aniline `lt` p-toluidine b. Chlorine atom has both -I effect and +R effect since -I effect out weights the +R effect, therefore p-chloro aniline is weak base than aniline. Alkyl groups are electron-donating groups. As a result the electron DENSITY on the nitrogen atom increases in the ethyalamine and thus they can donate lone pair of electrons more easily. Therefore Ethylamine is more base than aromatic amines. Due to delocalization of lone pair of electrons of the N-atom over the benzene ring, `C_(6)H_(5)NH_(2)` and `C_(6)NHCH_(3)` are far less basic than `C_(2)H_(5)NH_(2)`. Further due to +I effect of the `CH_(3)` group, `C_(6)_(5)NHCH_(3)` is little more basic than `C_(6)H_(5)NH_(2)`, Therefore increasing order basic strength is `P-Cl-C_(5)H_(4)-NH_(4)ltC_(6)H_(5)NH_(2)ltC_(6)H_(5)NHCH_(3)ltC_(2)H_(5)NH_(2)` (iii). In the gas phase, solvent effects i.e., stabilization of the conjugate acids due to H- bonding, are absent. Therefore, in the gas phase, basic strength mainly depends upin the +I effect of the alkyl groups. Since the +I effect increases wtih the number of alky groups, therefore the basic strength of the amines decreases as the number of ethyl groups decreases from three in `(C_(2)H_(5))(3)N` to two `(C_(2)H_(5))_(2)NH` to one in `C_(2)H_(5)NH_(2)` and zero in `NH_(3)`. `:.` Basic strength in the gas phase decreases in the order is `(C_(2)H_(5))_(3)Ngt(C_(2)H_(5))_(2)NgtC_(2)H_(5)NH_(2)gtNH_(3)` iv. Since the electro negativity of O is higher than that of N, therefore, alcohols form stronger H-bonds than amines. In other words, the boiling points ofalcohols are higher than those of amines of comparabel molecular masses. Therefore the boiling point of `C_(2)H_(5)OH(46)` is higher than those of `(CH_(3))_(2)NH(45)` and `C_(2)H_(5)NH_(2)(45)`. Further since the EXTENT of H-bonding depends upon the number of N-atoms on the N-atom. Therefore `1^(@)`- amines with two H- atoms on the N-atom have higher boiling points than `2^(@)` - amines having only one H-atom. Therefore the boiling point of `C_(2)H_(5)NH_(2)` is higher than that of `(CH_(3))_(2)`. `:.` Increasing order of boiling point is `(CH_(3))_(2)NHltC_(2)H_(5)NH_(2)lt C_(2)H_(5)OH` v. Dueto delocalization of lone pair of electrons of the N-atom over the benzene ring, `C_(6)H_(5)NHCH_(3)` is far less basic than `C_(2)H_(5)NH_(2),(C_(2)H_(5))_(2)NH` and `CH_(3)NH_(2)` among `C_(2)H_(5)NH_(2))` and `(C_(2)H_(5))_(2)NH,(C_(2)H_(5))_(2)` is more basic than `C_(2)H_(5)NH_(2)` due to greater +I effect of the two `C_(2)H_(5)` groups and stabilization of its conjugate acid by H-bonding. Compare to Ethyl and methyl group, `C_(2)H_(5)` group has more +I effect than `CH_(3)`- group Therefore methylamine is weak base than ethylamine. Combining all these facts relative basic strengt of these four amines decreases in the orderl, `(C_(2)H_(5))_(2)NHgtC_(2)H_(4)NH_(2)gtCH_(3)NH_(2)gtC_(6)H_(5)NHCH_(3)` Since a stronger base has a lower `pK_(b)` value therefore `pK_(b)` values decrease in the reverse order. `C_(6)H_(5)NHCH_(3)gtCH_(3)NH_(2)gtC_(2)H_(5)NH_(2)gt(C_(2)H_(5))_(2)NH` Due to delocalization of lone pair of electrons of the N-atom over the benzene ring, all aromatic amines are less basic than alkylamines i.e. `CH_(3)NH_(2)` Presence of electron donating groups `(-CH_(3))` on the N-atom increases the basicity of substituted aniline with respect to `C_(6)H_(5)NH_(2)`. In `(C_(6)H_(5))_(2)NH`, the lone pair of electrons on the N-atom is delocalized over two benzene rings instead of one in `C_(6)H_(5)NH_(2)`, therefore `(C_(6)H_(5))_(2)NH` is much less basic than `C_(6)H_(5)NH_(2)`. Combining all the three trends TOGETHER, the basic strength of the four amines increasing in the order: `(C_(6)H_(5))_(2)NHltC_(6)H_(5)NH_(2)ltC_(6)H_(5)N(CH_(3))_(2)ltCH_(3)NH_(2)` Aliphatic amines are more basic than aromatic amines. Therefore `CH_(3)CH_(2)NH_(2)` and `CH_(3)NH_(2)` are more basic. Among the ehtylamine and methylamine, ethylamine was experienced more +1 effect than methylamine and hence ethylamine is more basic than methylamine. Nitrogroup has a powerful electron withdrwing group and they have bot -R effect as well as -I effect. As a result all the nitro anilines are weaker bases than aniline. In P-nitroaniline  both -R effect and -I effect of the `NO_(2)` group decrease the basicity. Therefore decreasing order of basic strength is 
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