In aqueous solution, the basicity of amines is determined by three factors:

1. Inductive Effect: Alkyl groups are electron-donating ($+I$), increasing electron density on Nitrogen. Order: $3^{\circ }>2^{\circ }>1^{\circ }>N{H}_3$.
2. Solvation Effect: The stability of the conjugate acid ($R_{n}N{H}_{4-n}^{+}$) through H-bonding with water. Order: $1^{\circ }>2^{\circ }>3^{\circ }$ (more H-atoms mean better solvation).
3. Steric Hindrance: Bulky alkyl groups hinder the approach of a proton. Especially significant for $3^{\circ }$ amines.

• Result: The interplay of these effects results in the order: Secondary ($2^{\circ }$) > Primary ($1^{\circ }$) > Tertiary ($3^{\circ }$) > $N{H}_3$ (for methyl substituted amines).