Arrange the following:

(i) In decreasing order of the pK _b 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

(i) In decreasing order of the pK _b values:

C₂H₅NH₂, C₆H₅NHCH₃, (C₂H₅)₂NH and C₆H₅NH₂ pk_b value is the negative logarithm of the basicity constant (K _b)  i.e., pKb = -log K_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 following reasons:

 in aliphatic amines, alkyl groups are present. Alkyl groups are electron

releasing groups; hence they increase the electron density of N-atom and thus is easily available to donate electrons. This property makes aliphatic amines more basic.

 

 In aromatic amines, aryl group is present. Aromatic amine shows resonance:

 

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 (C ₂H₅)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 (C ₂H₅)₂NH₂ and C₂H₅NH₂

Now, in C₆H₅NHCH₃, due to presence of CH ₃ 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 pk _b value, therefore, pK_b values decrease in the reverse order:

C₆H₅NH₂ > C₆H₅NHCH₃ > C₂H₅NH₂ > (C₂H₅)₂NH₂ (ii) In increasing order of basic strength:

C₆H₅NH₂, C₆H₅N (CH₃)₂, (C₂H₅)₂NH and CH₃NH₂

In (C₂H₅)2NH₂, two ethyl groups are present and in CH ₃NH₂ 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 CH ₃NH₂

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 CH ₃ groups (increases the electron density of N-atom) makes it more basic than C ₆H₅NH₂ 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  (iii) In increasing order of basic strength:

(a) Aniline, p-nitroaniline and p-toluidine

(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 amines.

 

Explanation: Electron-donating groups releases electrons, stabilizes the conjugate acid (cation) and thus increases the basic strength.

Electron-withdrawing group withdraws electrons, destabilizes the conjugate acid (cation) and thus decreases the basic strength.

 

In p-nitroaniline, NO₂ group is present. As we know that NO ₂ 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 CH ₃ 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 delocalisation 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 C ₆H₅NHCH₃, 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 C ₆H₅NH₂ and

C₆H₅NHCH₃

Combining all these facts, the relative basic strength of these three amines increases in the order:

C₆H₅NH₂ < C₆H₅NHCH₃ < C₆H₅CH₂NH₂

(iv) In decreasing order of basic strength in gas phase:

C₂H₅NH₂, (C₂H₅)₂NH, (C₂H₅)₃N and NH₃

Amines in gas phase or in non-aqueous solvents, there is no solvation effect (getting influenced by the solvent).It means that the stabilization of conjugate acid formed due to the formation of hydrogen bonding is absent.

 

Hence, the basic strength of amines depends only on the +I effect of the alkyl groups. Alkyl groups are electron releasing groups, they release electron to the nitrogen in amine and increase the overall electron density of electrons and thus is easily available to donate electron. This property makes it more basic. As s result, more alkyl groups are attached, the higher the +I effect. Hence, the higher the +I effect, stronger is the base (high tendency to accept electrons)

In (C₂H₅)₃N, 3 alkyl groups are present, hence it is more basic. In (C ₂H₅)₂NH, 2 alkyl groups are present, hence it is less basic than (C ₂H₅)₃N. In C₂H₅NH₂, only one alkyl group is present, hence it is less basic than (C ₂H₅)3N and (C₂H₅)₂NH.

In NH₃, no alkyl group is present, so there is no +I effect. Hence it is less basic among all the amines.

Combining all these facts, the relative basic strength of these four amines decreases in the order:

(C₂H₅)₃N > (C₂H₅)₂NH > C₂H₅NH₂ > NH₃

(v) In increasing order of boiling point:

C₂H₅OH, (CH₃)₂NH, C₂H₅NH₂

As we know that boiling point of compounds depend upon the formation of H bonding. Amines have higher boiling points than hydrocarbons of simple molecular masses. This is due to the reason that amines being polar, form intermolecular H-bonding (except tertiary amine which do not have hydrogen atom linked to N-atom, i.e., R₃N)

Further, since the electronegativity (tendency to attract a shared pair of electrons) of nitrogen in amine is lower (3.0) than that of oxygen (3.5) in alcohol, therefore, amines form weaker H-bonds than electronegative oxygen atom.

 

Intermolecular H-bonding(strong)

Hence, C₂H₅OH has higher boiling point than (CH ₃)₂NH and C₂H₅NH₂. Because in C₂H₅OH, the electronegativity of O-atom is higher than H-atom which makes strong intermolecular H-bonding whereas in (CH₃)₂NH and C₂H₅NH₂, the electronegativity of N-atom is higher than H-atom but lower than O-atom in C₂H₅OH which makes weak intermolecular H-bonding.

Further, since, the extent of H-bonding depends upon the number of H-atoms on the N-atom. More the no. of H-atoms linked to nitrogen, the higher the boiling point. Since in (CH₃)₂NH have one hydrogen atom and in C ₂H₅NH₂ have two H atoms linked to nitrogen, therefore,

C₂H₅NH₂ has higher boiling point than (CH ₃)₂NH.

Combining all these facts, the boiling point of the given three compounds increases in the order:

(CH₃)₂NH < C₂H₅NH₂ < C₂H₅OH

(vi) In increasing order of solubility in water:

C₆H₅NH₂, (C₂H₅)₂NH, C₂H₅NH₂

All the three classes of aliphatic amines form H-bonds with water. As the size of alkyl group increases, the solubility decreases due to a corresponding increase in the hydrophobic part (hydrocarbon part) of the molecule. On the other hand, aromatic amines are insoluble in water due to presence of larger hydrocarbon part (C₆H₅-group) which tends to retard the formation of H-bonds.

 

Intermolecular H-bonding with water

Now, among the given compounds, C ₆H₅NH₂ is insoluble in water due to presence of C₆H₅-group (hydrocarbon part). In (C ₂H₅)₂NH, two alkyl groups are present and in C₂H₅NH₂ only one alkyl group is present, hence C ₂H₅NH₂ is more soluble in water than (C₂H₅)₂NH.

Combining all these facts, solubility of the given three compounds increases in the order:

C₆H₅NH₂ < (C₂H₅)₂NH < C₂H₅NH₂