The number of structural isomers possible from the molecular formula ${\mathrm{C}}_3{\mathrm{H}}_9 \mathrm{N}$ is

For aliphatic amines structural Isomers 5 and 3 are not possible as there is no compound which can justify the molecular formula ${\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+1}-{\mathrm{NH}}_2 \text{or }{\mathrm{C}}_{\mathrm{n}}{\mathrm{H}}_{2\mathrm{n}+3}-{\mathrm{NH}}_2$(n=no.of carbon atom)
The possible structural isomers are

N,N-Dimethylmethanamine

1. ${\mathrm{CH}}_3{\mathrm{CH}}_2{\mathrm{CH}}_2{\mathrm{NH}}_2\text{-Propanamine}$

$\text{3. }{\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{NH}-{\mathrm{CH}}_3$ 

                           N-Methylethanamine

If n=2, C₆H₅NH₂ two structural isomers are possible they are CH₃CH₂NH₂ and CH₃NHCH₃