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Mixture

Arrange the f...

Question

Arrange the following in increasing order of their basic strength:

$A)$ $C_{2} H_{5} N H_{2}, C_{6} H_{5} N H_{2}, N H_{3},$
$C_{6} H_{5} C H_{2} N H_{2} & (C_{2} H_{5})_{2} N H$

$B)$ $C_{2} H_{5} N H_{2}, (C_{2} H_{5})_{2} N H,$
$(C_{2} H_{5})_{3} N, C_{6} H_{5} N H_{2}$

$C)$ $C H_{3} N H_{2}, (C H_{3})_{2} N H, (C H_{3})_{3} N,$
$C_{6} H_{5} N H_{2}, C_{6} H_{5} C H_{2} N H_{2}$

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Solution

$A)$ $Basic strength of amines$

Since we know, basicity

$\propto + R > + H > + I \propto \frac{1}{- R > - H > - I}$

Considering, the inductive effect, solvation effect and steric hindrance of the alkyl group which decides the basic strength of alkylamines in solution.

Order of basic strength in ethyl substituted amine follow as-

$N H_{3} < C_{2} H_{5} N H_{2} < (C_{2} H_{5})_{2} N H$

Order of basic strength in benzene substituted ring-

$C_{6} H_{5} N H_{2} < N H_{3} < C_{6} H_{5} C H_{2} N H_{2}$

Aromatic amine (aniline) are weaker base than ammonia due to electron withdrawing nature of aryl group. Also, Due to the $+ R$ effect of $- N H_{2}$ group in $C_{6} H_{5} N H_{2}$ lone pair of nitrogen is involved in resonance with benzene ring and thus it is not available for the donation to electron deficient species or Lewis’s acid.

While Benzylamine is stronger base than ammonia because due to $+ I$ effect of alkyl group leads to high electron density on nitrogen atom and thus increases it’s basicity.

Hence, overall order of basic strength follows as-

$C_{6} H_{5} N H_{2} < N H_{3} < C_{6} H_{5} C H_{2} N H_{2} <$
$C_{2} H_{5} N H_{2} < (C_{2} H_{5})_{2} N H$

Final answer: $C_{6} H_{5} N H_{2} < C_{6} H_{5} C H_{2} N H_{2} <$
$N H_{3} < C_{2} H_{5} N H_{2} < (C_{2} H_{5})_{2} N H$

$B)$ $Basic strength of amines$

Since we know, basicity

$\propto + R > + H > + I \propto \frac{1}{- R > - H > - I}$

Considering the cumulative effect of inductive effect, solvation effect and steric hindrance of the alkyl group which decides the basic strength of alkylamines. order of basic strength in ethyl substituted amine follow as-

$C_{2} H_{5} N H_{2} < (C_{2} H_{5})_{3} N < (C_{2} H_{5})_{2} N H$

Aromatic amine (aniline) are weaker base than aliphatic amines due to electron withdrawing nature of aryl group. Also, Due to the $+ R$ effect of $- N H_{2}$ group in $C_{6} H_{5} N H_{2}$ lone pair of nitrogen is involved in resonance with benzene ring and thus it is not available for the donation to electron deficient species or Lewis’s acid.

Hence, overall order of basic strength follows as-

$C_{6} H_{5} N H_{2} < C_{2} H_{5} N H_{2} < (C_{2} H_{5})_{3} N <$
$(C_{2} H_{5})_{2} N H$

Final answer: $C_{6} H_{5} N H_{2} < C_{2} H_{5} N H_{2} <$
$(C_{2} H_{5})_{3} N < (C_{2} H_{5})_{2} N H$

$C)$ $Basic strength of amines$

Since we know, basicity

$\propto + R > + H > + I \propto \frac{1}{- R > - H > - I}$

Considering the cumulative effect of inductive effect, solvation effect and steric hindrance of the alkyl group which decides the basic strength of alkylamines. order of basic strength in methyl substituted amine follow as-

$(C H_{3})_{3} N < C H_{3} N H_{2} < (C H_{3})_{2} N H$

Order of basic strength in benzene substituted ring-

$C_{6} H_{5} N H_{2} < C_{6} H_{5} C H_{2} N H_{2}$

Aromatic amine (aniline) are weaker base than aliphatic amines due to electron withdrawing nature of aryl group. Due to the $+ R$ effect of $- N H_{2}$ group in $C_{6} H_{5} N H_{2}$ lone pair of nitrogen involved in resonance with benzene ring, and it is not available for the donation to electron deficient species or Lewis’s acid.

But in case of $C_{6} H_{5} C H_{2} N H_{2}$ lone pair of nitrogen is not involved in resonance with benzene ring and it is easily available for the donation to electron deficient species or Lewis’s acid.

So, it is more basic than aryl amine but observed to be less basic than methyl substituted aliphatic amines due to some –I effect of aryl group at neighbouring position, which decreases electron density on $N$ atom and thus makes it less basic.

Hence, overall order of basic strength follows as-

$C_{6} H_{5} N H_{2} < C_{6} H_{5} C H_{2} N H_{2} < (C H_{3})_{3} N <$
$C H_{3} N H_{2} < (C H_{3})_{2} N H$

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Arrange the following:

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