13.13 Arrange the following: (i) In decreasing order of the pKb values: C2H5NH2 , C6H5NHCH3 , (C2H5 ) 2NH and C6H5NH2 (ii) In increasing order of basic strength: C6H5NH2 , C6H5N(CH3)2 , (C2H5)2NH and CH3NH2 (iii) In increasing order of basic strength: (a) Aniline, p-nitroaniline and p-toluidine (b) C6H5NH2, C6H5NHCH3 , C6H5CH2NH2 (iv) In decreasing order of basic strength in gas phase: C2H5NH2 , (C2H5)2NH, (C2H5)3N and NH3 (v) In increasing order of boiling point: C2H5OH, (CH3 )2NH, C2H5NH2 (vi) In increasing order of solubility in water: C6H5NH2 , (C2H5 ) 2NH, C2H5NH2 .

Pkb value is the negative logarithm of the basicity constant (Kb) .i.e., pKb = -logKb Evidently, smaller the value of pKb , stronger is the base (strong tendency to donate electrons). Aliphatic amines(R-NH2) are more basic(tendency to donate electrons) than aromatic amines(C6H5NH2) because of the following reasons: =>In aliphatic amines, alkyl groups are present. Alkyl groups are electron releasing groups, hence they increase the elecron density of N-atom and thus is easily available to donate electrons. This poperty makes aliphatic amines more basic. As a result of resonance, the lone pair of electrons on the nitrogen atom gets delocalized over benzene ring. As a result, electron density on the nitrogen decreases and thus is less easily available to donate electrons making it less basic. Hence aliphatic amines(R-NH2) are more basic than aromatic amines (C6H5NH2). Now, in C2H5NH2, one ethyl group (alkyl) is present and in (C2H5)2NH2 two ethyl groups are present. As we know that more alkyl groups are present, more basic will be the amine. Hence, (C2H5)2NH2 is more basic than the C2H5NH2 Now in C6H5NH2, due to delocalisation of lone pair of electrons of the N-atom over the benzene ring, makes it less basic than (C2H5)2NH2 and C2H5NH2 Now, in C6H5NHCH3, due to presence of CH3 group, makes it more basic than C6H5NH2 but less basic than (C2H5)2NH2 and C2H5NH2 due to the presence of aromatic ring which is responsible for the delocalisation of lone pair of electrons of N-atom over the benzene ring. Combining all these facts, the relative basic strength of these four amines decrease in the order: (C2H5)2NH2 > C2H5NH2 > C6H5NHCH3 > C6H5NH2 Since, a stronger base has a lower pkb value, therefore, pKb values decrease in the reverse order: C6H5NH2 > C6H5NHCH3 > C2H5NH2 > (C2H5)2NH2 In (C2H5)2NH2, two ethyl groups are present and in CH3NH2 one methyl group is present. As we know that more alkyl groups are present, more basic will be the amine. Hence, (C2H5)2NH2 is more basic than the CH3NH2 Now in C6H5NH2, due to delocalisation of lone pair of electrons of the N-atom over the benzene ring (decreases the electron density of N-atom) makes it less basic than (C2H5)2NH2 and CH3NH2 Now, in C6H5N(CH3)2 due to presence of two CH3 groups (increases the electron density of N- atom) makes it more basic than C6H5NH2 but less basic than (C2H5)2NH2 and CH3NH2 due to the presence of aromatic ring which is responsible for the delocalisation of lone pair of electrons of N-atom over the benzene ring (decreases the electron density of N-atom) Combining all these facts, the relative basic strength of these four amines increases in the order: C6H5NH2 < C6H5NHCH3 < CH3NH2 < (C2H5)2NH (a) Electron-donating groups such as –CH3, -OCH3, -NH2, etc. increase the basicity and electron- withdrawing groups such as –NO2, -CN, -SO3H, -COOH, -X(halogen), etc. decrease the basicity of Explanation: Electron-donating groups releases electrons, stabilizes the conjugate acid (cation) and thus increases the basic strength. Electron-withdrawing groups withdraws electrons, destabilizes the conjugate acid (cation) and thus decreases the basic strength. In p-nitroaniline, NO2 group is present. As we know that NO2 group is an electron withdrawing group which decreases the basic strength of amine. In p-toluidine, CH3 group is present and as we know that CH3 group is an electron donating (releasing) group which increases the basic strength of amine. Hence, p-toluidine is more basic than p-nitroaniline Now in C6H5NH2 (aniline), due to delocalisation of lone pair of electrons of the N-atom over the benzene ring (decreases the electron density of N-atom) makes it less basic than p-toluidine but more basic than p-nitroaniline (NO2 group is present which decreases the density of aniline) Combining all these facts, the relative basic strength of these three amines increases in the order: p-nitroaniline< aniline < p-toluidine (b) C6H5NH2, C6H5NHCH3, C6H5CH2NH2 In C6H5NH2 and C6H5NHCH3, the N-atom is directly attached to the aromatic ring. Hence, they both show resonance in which delocalization of lone pair of electrons N-atom takes place. As a result the electron density of N-atom decreases. Hence both are weaker bases. However in C6H5NHCH3, CH3 group (electron withdrawing) is present which increases the overall density of electrons. Hence, C6H5NHCH3 is more basic than C6H5NH2 In C6H5CH2NH2, the N-atom is not directly attached to the aromatic ring. As a result, it does not show resonance. There is no effect on the electron density of lone pair of electrons of N-atom. Hence, it is more basic than C6H5NH2 and C6H5NHCH3 Combining all these facts, the relative basic strength of these three amines increases in the order: C6H5NH2 < C6H5NHCH3 < C6H5CH2NH2 Amines in gas phase or in non-aqueous solvents, there is no solvation effect (getting influenc

13.13 Arrange the following:

(i) In decreasing order of the pKb values: C₂H₅NH₂ , C₆H₅NHCH₃ , (C₂H₅ ) ₂NH and C₆H₅NH₂

(ii) In increasing order of basic strength: C₆H₅NH₂ , C₆H₅N(CH₃)₂ , (C₂H₅)₂NH and CH₃NH₂

(iii) In increasing order of basic strength: (a) Aniline, p-nitroaniline and p-toluidine (b) C₆H₅NH_2,C₆H₅NHCH₃ , C₆H₅CH₂NH₂

(iv) In decreasing order of basic strength in gas phase: C₂H₅NH₂ , (C₂H₅)₂NH, (C₂H₅)₃N and NH₃

(v) In increasing order of boiling point: C₂H₅OH, (CH₃ )₂NH, C₂H₅NH₂

(vi) In increasing order of solubility in water: C₆H₅NH₂ , (C₂H₅ ) ₂NH, C₂H₅NH₂ .

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Vishal Baghel | Contributor-Level 10

8 months ago

Pk_b value is the negative logarithm of the basicity constant (K_b) .i.e., pK_b = -logK_b

Evidently, smaller the value of pK_b , stronger is the base (strong tendency to donate electrons). Aliphatic amines(R-NH₂) are more basic(tendency to donate electrons) than aromatic amines(C₆H₅NH₂) because of the fol