MCQ
Factors responsible for base strength comparison of amines in aqueous solution?
  • A
    Availability of lone pair on nitrogen
  • B
    Lone pair donating power of nitrogen
  • C
    Stability of conjugate acid formed by solvation through $H -$ bonding.
  • All of the above

Answer

Correct option: D.
All of the above
The basicity of amines depend upon its ability to donate the lone pair of electrons on nitrogen.
Order of basicity of amines in aqueous solution is,
$\ce{NH_3} ​<$ Primary amines$\ce{(RNH_2​)} <$ Tertiary amines$\ce{(R_3​N)} <$ Secondary amines$\ce{(R_2​NH)}$
This is due to the combined effect of $+I$ effect of alkyl group, steric hindrance caused by alkyl groups and solvation of amines through $H -$ bonding.
The power to donate lone pair of electrons increases as the number of alkyl groups increase.
Therefore, making $\ce{R_3​N}$ most basic, followed by $\ce{R_2​NH}$ and $\ce{RNH_2}​.$
But as the number of alkyl groups increase, the steric hindrance caused by them also increases.
Thus, decreasing the electron donating power of amines.
Hence, making $\ce{RNH_2}$​ most basic followed by $\ce{R_2NH}$ and $\ce{R_3N}.$​
Solvation is the formation of protonated amines when they are dissolved in water.
As the number of $H -$ atoms on nitrogen increases the possibility of $H -$ bonding also increases, providing greater stability to the amine.
Thus, primary amine $\ce{(RNH_2)}$ is more stable than secondary amine $\ce{(R_2​NH)},$ which is more stable than tertiary amine $\ce{(R_3​N)}.$
The combined effect of these three factors give the observed order of basicity of amines in aqueous solution.

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