MCQ
A compound with molecular mass $180$ is acylated with $CH_3COCl$ to get a compound with molecular mass $390.$ The number of amino groups present per molecule of the former compound is :
- A$2$
- ✓$5$
- C$4$
- D$6$
✓
Answer
Correct option: B.
$5$
b
During acetylation, one $H$ atom with atomic mass $1\,amu$ of $- NH_2$ group is replaced by an acetyl group $\mathrm{CH}_{3} \mathrm{CO}[\text { molecular mass }=434 \,\mu ]$. Thus
During acetylation, one $H$ atom with atomic mass $1\,amu$ of $- NH_2$ group is replaced by an acetyl group $\mathrm{CH}_{3} \mathrm{CO}[\text { molecular mass }=434 \,\mu ]$. Thus
$-\mathrm{NH}_{2}+\mathrm{CH}_{3} \mathrm{COCl} \longrightarrow-\mathrm{NHCOCH}_{3}+\mathrm{HCl}$
The above equation suggests that, the acylation of each $-\mathrm{NH}_{2}$ group increases the mass by $42 \,\mu$ $[43-1] .$ If the molecular mass of the organic compound is $180 \,\mu$ while that of the acylated product is $390 \,\mu$, then the increase in the mass due to acylation is given by
$390-180=210 \,\mu$
Hence, the number of $-\mathrm{NH}_{2}$ groups $=\frac{210\, \mu}{42\, \mu}=5$
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