Question

The correct order of the basic strength of methyl substituted amines in aqueous solution is:
A. ${{\left( C{{H}_{3}} \right)}_{2}}NH>C{{H}_{3}}N{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{3}}N$
B. ${{\left( C{{H}_{3}} \right)}_{3}}N>C{{H}_{3}}N{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{2}}NH$
C. ${{\left( C{{H}_{3}} \right)}_{3}}N>{{\left( C{{H}_{3}} \right)}_{2}}NH>C{{H}_{3}}N{{H}_{2}}$
D. $C{{H}_{3}}N{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{2}}NH>{{\left( C{{H}_{3}} \right)}_{3}}N$

Answer 1

The basic strength of the amines depends on the degree of the amines as the addition of the bulky groups lead to the steric hindrance in the compounds and change the behavior of the amines in terms of their basic strengths.

Complete step by step solution:
-Many types of compound can be used as electrolytes but the most preferred compounds are acids, bases and salts.
-Acids are the compounds that donate ${{H}^{+}}$ ion to become anion. Bases are compounds that accept the ${{H}^{+}}$ ion and convert into cation. Salts are the compounds formed mainly by the reaction of acids and bases.
-The strength of acidic nature is inversely related to the strength of the basic nature of a compound. It means that strong acid has a weak base and vice versa. So we can solve this question in two ways, either by finding the basic strength directly or by inverting the order of the acidic strengths.
-Amines are the compounds which have nitrogen atoms attached to the carbon atom. They are subdivided as primary, secondary and tertiary amines. They differ in the number of hydrogen atoms associated with the nitrogen atom in the group.
-If there are 2 hydrogen atoms with nitrogen, it is primary amine. If 1 hydrogen and 1 nitrogen atom is linked to a carbon chain, then it is secondary amine. If both the hydrogen atoms are replaced by carbon chains, then the amine is tertiary amine.
-The removal of hydrogen atoms in the amine group increases the bulkiness of the compound which results in more steric crowding of the molecules around the compound. This affects the strength of the compound to attack another compound.
-The basic strength in a compound depends on the electron density of the compound, size of the compound, electronegativity of the atoms or ions, number of delocalizations possible in the compound, aromaticity of the compound and the +I and +M effects of the compound.
-The strength is directly proportional to the electron density and the +I/+M effect and is inversely proportional to all the other mentioned properties.
-So the increment in the size of the molecules decreases the basic strength of the compounds. But the addition of the methyl groups increase the +I effect on the compound and so the order of basic strength becomes
${{\left( C{{H}_{3}} \right)}_{3}}N>{{\left( C{{H}_{3}} \right)}_{2}}NH>C{{H}_{3}}N{{H}_{2}}$
-But in aqueous solution, the other factors come into account. The strength in the aqueous solution depends on the solubility of the compound which is inversely proportional to the size of the compound. So in aqueous solution, the strength order becomes
$C{{H}_{3}}N{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{2}}NH>{{\left( C{{H}_{3}} \right)}_{3}}N$
-Combining both the contradicting results, the actual order of the basic strength of methyl substituted amines in aqueous solution is:
${{\left( C{{H}_{3}} \right)}_{2}}NH>C{{H}_{3}}N{{H}_{2}}>{{\left( C{{H}_{3}} \right)}_{3}}N$

Therefore the correct option is A.

Note: The order of the basic strength is completely contradicting in both the cases; strength in aqueous medium and the strength in the normal conditions. The final order is not guessed or told theoretically. It is experimentally observed and then the conclusion is drawn.