Amines

Amines

Question

Account for the following:
(i) Primary amines (R-NH2) have higher boiling point than tertiary amines (R3N).
(ii) Aniline does not undergo Friedel - Crafts reactions:
(iii) (CH3)2NH is more basic than (CH3)3N in an aqueous solution.

OR
straight i right parenthesis space straight C subscript 6 straight H subscript 5 space rightwards arrow with Sn space plus HCl on top space straight A space rightwards arrow from 273 space straight K to space NaNO subscript 2 plus HCl of space straight B space space rightwards arrow with straight H subscript 2 straight O on top space straight C

ii right parenthesis space CH subscript 3 CN space rightwards arrow with straight H subscript 2 straight O divided by straight H to the power of plus on top space straight A space rightwards arrow from increment to NH subscript 3 of space straight B space rightwards arrow with Br plus KOH on top space straight C

Answer

(i) The greater the extent of intermolecular hydrogen bonding, greater is the boiling point of the compound. Primary amines are involved in intermolecular hydrogen bonding, whereas tertiary amines do not have any available hydrogen atoms, and hence they are not involved in any intermolecular hydrogen bonding. Thus, primary amines (R-NH2) have higher boiling point than tertiary amines (R3N).

(ii) A Friedel-Crafts reaction is carried out in the presence of a lewis acid such as AlCl3. Aniline being basic in nature reacts with AlCl3 to form a salt (as shown in the following equation).

 

Due to the positive charge on the N-atom, electrophilic substitution in the benzene ring is deactivated. Hence, aniline does not undergo the Friedel-Crafts reaction.

(iii) The order of basic strength of aliphatic amines in the aqueous phase, is based on following factors:

Steric factor: Alkyl group is larger than hydrogen atom that causes steric hindrance to attack of acid. As the number of alkyl group increases from primary to tertiary. The steric hindrance also increases in the same direction.

Solvation of ions: when amines are dissolved in water, they form hydrogen bonds with a water molecule and release hydration energy; in the process converting themselves to the protonated amines to get stabilized. Greater the extent of hydrogen bonding, greater is the hydration energy released and more is the stability of the protonated amine; thus greater is the tendency of the amine to form cation leading to the greater basic strength of the amine. So, the actual order of basic strength in aqueous phase is found to be:

Secondary > Tertiary > Primary

Therefore, secondary amines are the strongest base in aqueous solution.


Or
straight i right parenthesis space straight C subscript 6 straight H subscript 5 space rightwards arrow with Sn space plus HCl on top space stack straight C subscript 6 straight H subscript 5 NH subscript 2 with Aniline below space rightwards arrow from 273 space straight K to space NaNO subscript 2 plus HCl of space stack straight C subscript 6 straight H subscript 5 minus straight N subscript 2 superscript plus Cl to the power of minus with Benzediazonium space chloride below space space rightwards arrow with straight H subscript 2 straight O on top space stack straight C subscript 6 straight H subscript 5 OH with phenol below

ii right parenthesis space CH subscript 3 CN space rightwards arrow with straight H subscript 2 straight O divided by straight H to the power of plus on top CH subscript 3 COOH space rightwards arrow from increment to NH subscript 3 of space CH subscript 3 CONH subscript 2 space rightwards arrow with Br plus KOH on top space CH subscript 3 NH subscript 2

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