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According to me, ammonia should be the stronger base for two reasons:

  1. $\ce{-C_6H_5}$ group shows $-I$ effect, due to which electron density on $\ce{N}$ atom of benzylamine decreases
  2. steric hindrance in benzylamine is more than that in ammonia

But their $\mathrm{pK_b}$ values are: benzylamine: $4.70$ and ammonia : $4.75$. High $\mathrm{pK_b}$ value of ammonia shows that it is actually less basic than benzylamine, contrary to my reasoning. Why is it so?

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  • $\begingroup$ See nitrogen inversion $\ce{->}$Wikipedia $\endgroup$ – Avnish Kabaj Feb 8 '18 at 14:38
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Everything you said is correct, except that it is more relevant to consider the inductive effect of methyl on nitrogen: electron density is donated to nitrogen.

The inductive effect of phenyl is on the methyl, decreasing its ability to donate electron density to nitrogen, but apparently not completely.

I suppose that if you increased the negative inductive effect of the phenyl group with a couple of nitro groups, the basicity of the compound would decrease.

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