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How would you arrange the conjugate bases of the following amines in order of electron donating ability as EDGs as substituents on an aromatic ring?

$\ce{NH3}$, $\ce{(C2H5)(C2H4CN)NH}$, $\ce{(C2H4OH)2NH}$, $\ce{(C2H5)(C2H4OH)NH}$

I would think that the order would be:

$\ce{(C2H4OH)2NH}$ > $\ce{(C2H5)(C2H4OH)NH}$ > $\ce{(C2H5)(C2H4CN)NH}$ > $\ce{NH3}$

based on the reasoning that alkyls cause an inductive effect which increases electron density.

I think that pKa would be able to measure the electron donating ability of the conjugate base.

However, I found that $\ce{NH3}$ has a higher pKa (9.25) than $\ce{(C2H4OH)2NH}$ (8.96), which would mean that the conjugate base of $\ce{NH3}$ is a stronger base, which I don't understand.

Is my reasoning for the order incorrect? Or is the usage of pKa to measure electron donating ability of the conjugate base incorrect?

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  • $\begingroup$ Hmm. Neither the amines nor their conjugate bases can be substituents on a ring; that role is reserved for the corresponding amino groups. I’m also not sure whether the $\mathrm pK_\mathrm a$ values have anything to do with it. $\endgroup$ – Jan Nov 5 '17 at 13:02
  • $\begingroup$ Ah my bad, I am trying to refer to the corresponding amino group. If pKa values don't have anything to do with electron donating ability of the amino groups as EDGs, based on what is the order of the strength of EDGs determined? $\endgroup$ – oswinso Nov 5 '17 at 13:58

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