Question

A compound with molecular mass 180 is acylated with $C{{H}_{3}}COCl$
to get a compound with molecular mass 390. The number of amino groups present per molecule of the former compound is:
A– 5
B– 4
C– 6
D– 2

Answer 1

Hint: Acylation is the process in which an acyl group ($-COC{{H}_{3}}$) is added in the compound. And the compound which provides acyl groups is known as acylating agent. In this question, there will be addition of acyl group and removal of hydrogen group. The number would increase by 42 in molecular mass by acylation In this way, we can approach the question.

Complete step by step solution:
Before calculation, let us understand the concept of acylation.
Acylation reaction is the process of inserting acyl group ($-COC{{H}_{3}}$) in the compound. It mainly reacts with the amines to form amides, or with alcohols leading to the formation of esters. Acyl halides and anhydrides of carboxylic acids is a common acylating agent.
We can easily understand with the help of reaction which is given below-
\[R-N{{H}_{2}}+C{{H}_{3}}COCl\to R-NH-COC{{H}_{3}}+HCl\]
As we can see from the reaction, there is an addition of one ($-COC{{H}_{3}}$) group and removal of one hydrogen.
Basis of the calculation
The above calculation is based on the law of conservation of mass. Only the atoms arrangement and the structure is changed from amine to amide group.
Mathematically we can say that-
No. of amino groups = $\dfrac{\text{Mass of Product (amino-acid)-Mass of Amine}}{\text{Mass added per N}{{\text{H}}_{2}}-\text{group with acylation}}$
Calculation-
Mass of one ($-COC{{H}_{3}}$) group=43
Mass of Hydrogen atom (H) = 1
Therefore, mass added per ($-N{{H}_{2}}$) group with acylation = 42
As it is given the question,
Molecular mass of the amine= 180
Molecular mass of the product formed=390
Therefore,
Molecular mass that is increased in the product = 390-180 = 210
We can therefore get the number of amino-acid by dividing the molecular mass that is increased in the product by mass added per ($-N{{H}_{2}}$) group with acylation.
Thus, the number of amino groups = $\dfrac{210}{42}=5$
Hence, the correct option is (A)-5

Note Don’t get confused between the concept of increase of mass by units. As we know, in this reaction there is displacement of the H atom, and addition of acyl group. Thus, when the compound is formed, from that we calculate the increase in mass of the $-N{{H}_{2}}$ group by subtracting the actual mass.