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 .

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 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 delocaliz

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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 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 C₆H₅NHCH_3, 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 pk_b value, therefore, pK_b 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

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<p>Pk_b value is the negative logarithm of the basicity constant (K_b) .i.e., pK_b = -logK_b</p><p>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:</p><p>=&gt;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.</p><div><div><picture><source srcset="https://images.shiksha.com/mediadata/images/articles/1687517926phpyRyAgy_480x360.jpeg" media="(max-width: 500px)"><img src="https://images.shiksha.com/mediadata/images/articles/1687517926phpyRyAgy.jpeg" alt="" width="658" height="344"></picture></div><div><p>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).</p><p>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.</p><p>Hence, (C₂H₅)₂NH₂ is more basic than the C₂H₅NH₂</p><br><p>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</p><p>Now, in C₆H₅NHCH_3, 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.</p><p>Combining all these facts, the relative basic strength of these four amines decrease in the order:</p><p>(C₂H₅)₂NH₂ &gt; C₂H₅NH₂ &gt; C₆H₅NHCH₃ &gt; C₆H₅NH₂</p><p>Since, a stronger base has a lower pk_b value, therefore, pK_b values decrease in the reverse order:</p><p>C₆H₅NH₂ &gt; C₆H₅NHCH₃ &gt; C₂H₅NH₂ &gt; (C₂H₅)₂NH₂</p><p>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</p><p>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</p><p>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)</p><p>Combining all these facts, the relative basic strength of these four amines increases in the order:</p><p>C₆H₅NH₂ &lt; C₆H₅NHCH₃ &lt; CH₃NH₂ &lt; (C₂H₅)₂NH</p><p>&nbsp;</p><p><strong>(a) </strong>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</p><p><u>Explanation</u>: 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.</p><div><div><picture><source srcset="https://images.shiksha.com/mediadata/images/articles/1687520888phpEFgNhZ_480x360.jpeg" media="(max-width: 500px)"><img src="https://images.shiksha.com/mediadata/images/articles/1687520888phpEFgNhZ.jpeg" alt="" width="292" height="171"></picture></div></div><p>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</p><p>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)</p><p>Combining all these facts, the relative basic strength of these three amines increases in the order:</p><p>p-nitroaniline&lt; aniline &lt; p-toluidine</p><p>&nbsp;</p><p><strong>(b)</strong> C₆H₅NH₂, C₆H₅NHCH₃, C₆H₅CH₂NH₂</p><p>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</p><p>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</p><p>Combining all these facts, the relative basic strength of these three amines increases in the order:</p><p>C₆H₅NH₂ &lt; C₆H₅NHCH₃ &lt; C₆H₅CH₂NH₂</p><p>Amines in gas phase or in non-aqueous solvents, there is no solvation effect (getting influenc</p></div></div>

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