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In aqueous solution, the basicity order is: $\ce{NH3 > Ph-NH2 > Ph-NH-Ph > (Ph)3N}$

What will be the order in gas phase?

Following are some considerations:

1- In $\ce{NH3}$, $\ce{N}$ is $\ce{sp^3}$ hybridised but in aryl amines, it is between $\ce{sp^3}$ and $\ce{sp^2}$ hybridised. So, does the steric factor is more in ammonia?

2- In aryl amines, $\ce{N}$ is bonded to $\ce{sp^2}$ hybridised electronegative $\ce{C}$ atom. This would withdraw the lp electrons with $\ce{N}$.

3- $\ce{+M}$ effect of $\ce{-NH2, < -NHPh, < -NPh2}$ will reduce the basicity of aryl amines.

Edit: I did find a source[1] citing standard Gibbs energy change values for proton transfer reactions of various amines. (Table from Ref. [1])

Thus, basicity of $\ce{Ph-NH2 > NH3}$.

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Reference

1. J. P. Briggs, R. Yamdagni, and P. Kebarle. "Intrinsic basicities of ammonia, methylamines, anilines and pyridine from gas-phase proton-exchange equilibriums", J. Am. Chem. Soc. (1972), 94(14), 5128–5130. https://doi.org/10.1021/ja00769a081

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  • $\begingroup$ I think you have all you need. Triphenylamines are even flat or rather flat about N, their 3D character is mostly due to a propeller shape. $\endgroup$
    – Alchimista
    Nov 13, 2021 at 14:45
  • $\begingroup$ @Alchimista Mesomeric effect is considered as a very strong effect. Even then, basicity of $\ce{Ph-NH2 > NH3}$ $\endgroup$
    – Apurvium
    Nov 14, 2021 at 1:53
  • $\begingroup$ do you mean why in gas phase aniline is more basic than ammonia? Please make clear what you want to know and what you know already. $\endgroup$
    – Alchimista
    Nov 14, 2021 at 9:40
  • $\begingroup$ @Alchimista Yes, my whole post is about gas phase. $\endgroup$
    – Apurvium
    Nov 14, 2021 at 10:30
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    $\begingroup$ @Alchimista I did find a source. Check the edit part. $\endgroup$
    – Apurvium
    Nov 17, 2021 at 10:05

1 Answer 1

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Reference [1], in the question, indicates that the phenyl group is not really special. In the gas phase substitution onto the nitrogen with either an alkyl (methyl) group or an aryl (phenyl) group increases basicity, and the effect is cumulative when multiple substitution is used. While this reference does not specifically address diphenylamine or triphenylamine, Ref. [2] does. From the abstract:

Using the Brønsted acids $\ce{BH^+}=\ce{NH4^+,CH3NH3^+,(CH3)2NH2^+}$ as reactant ions in the chemical ionization source of [a] mass spectrometer, the general proton transfer reaction $\ce{BH^+ + M -> MH^+ + B}$ was studied. The following order of proton affinities, i.e. gas-phase basicities, was observed: quinineclidine, decylamine, pyridine, 4-N,N-dimethylaminopyridine, N,N-dimethylaniline > dimethylamine > N-methylaniline, m-phenylenediamine, N,N,N- triphenylamine > methylamine, o-phenylenediamine, p-phenylenediamine, N,N-diphenylamine, aniline > ammonia.

This ordering is consistent with the favorable effect of increased substitution indicated by the table quoted from Ref. [1], so that the basicity order

$\ce{Ph3N>Ph2NH>PhNH2>NH3}$

is suggested for a solution to the question.

What appears to be happening is that in gas phase ammonium ions electron donation to the formally cationic nitrogen via hyperconjugation is a controlling factor. Highly substituted ions offer much opportunity for such hyperconjugation whereas the parent cation $\ce{NH4^+}$ offers none.

In water solution, however, solvation is strongest with ions that are compact, more or less spherical and with localized charge -- the exact opposite characteristics of the hyperconjugated ions that are most stable in the gas phase; the solvation effect appears to win out.

References

1. J. P. Briggs, R. Yamdagni, and P. Kebarle. "Intrinsic basicities of ammonia, methylamines, anilines and pyridine from gas-phase proton-exchange equilibriums", J. Am. Chem. Soc. (1972), 94(14), 5128–5130. https://doi.org/10.1021/ja00769a081

2. Ismet Dzidic. "Relative gas-phase basicities of some amines, anilines, and pyridines. Application of some Broensted acids as reactants in chemical ionization mass spectrometry", J. Am. Chem. Soc. (1972), 94, 24, 8333–8335. https://doi.org/10.1021/ja00779a009

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  • $\begingroup$ I couldn't understand "...$\ce{NH4+}$ ions electron donation to the formally cationic nitrogen via hyperconjugation..." In anilinium ion, the carbon bonded to nitrogen, does not have any hydrogen. $\endgroup$
    – Apurvium
    Nov 18, 2021 at 3:06
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    $\begingroup$ There can still be hyperconjygation from carbon-carbon bonds. $\endgroup$ Nov 18, 2021 at 3:16
  • $\begingroup$ This is quite amazing giving that in solid state Ar2NH and Ar3N are planar (about N). I was really expecting that delocalisation would have kicked in with at least two Ar groups. You can mention, if I got it right, that PhNH2 shall rather be compared to cyclohexylamine instead, and so on. $\endgroup$
    – Alchimista
    Nov 18, 2021 at 8:21

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