Question

The molecular formula ${C_3}{H_9}N$ cannot represent :
( A ) $1^\circ$ amine
( B ) $2^\circ$ amine
( C ) $3^\circ$ amine
( D ) Quaternary salt

Answer 1

There are only $3 - C$ atoms present in the molecular formula so only those structures are possible according to the formula which are having $3 - C$ atom. Quaternary salt requires $4 - C$ atoms .

Complete step-by-step answer: First let’s see the structural isomers of this molecular formula :
1. $C{H_3} - C{H_2} - C{H_2} - N{H_2}$ $\to$ $Propan - 1 - a\min e$ ( $1^\circ$ amine )
2. $C{H_3} - NH - C{H_2} - C{H_3}$ $\to$ $N - methylethanamine$ ($2^\circ$ amine )
3. $\to$ $N,N-Dimethylmethanamine\;$( $3^\circ$amine )

4. \to $$$Propan - 2 - amine\;$$ (1^\circ $ amine )

There are 4 structural isomers of the molecular formula${C_3}{H_9}N$ according to the number of carbon atoms , number of hydrogen atoms and number of nitrogen atoms .

As this molecular formula has $3 - C$atom , $9 - H$ atom , $1 - N$ atom . As we can see that molecular formula has only $3 - C$ atoms so only these 4 structures are possible: $1^\circ$ amine , $2^\circ$ amine , $3^\circ$ amine .
While for Quaternary salt, we require $4 -$carbon atoms ,which according to the molecular formula is not possible.
In ( 1 ) structural isomer $- N{H_2}$ group attaches to the one carbon atom and results in the $1^\circ$ amine.
In ( 2 ) structural isomers $- N{H_2}$ group attach to two carbon atoms and result in the formation of $2^\circ$ amine .
In ( 3 ) structural isomer $- N{H_2}$ group attached to three carbon atoms and result in the formation of $3^\circ$amine .
In ( 4 ) structural isomer $- N{H_2}$ group attached to one carbon atom and results in the formation of $1^\circ$ amine.

Note: Structural isomers depend on the number of atoms present in the given molecular formula. So we have to count the number of atoms given in the molecular formula .