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According to the Khan Academy, the amino functional group is basic because it can remove $\ce{H+}$ from systems. Why is that?

amine

Amino: Charged (forms $\ce{R-NH3+})$ at the pH of most biological systems. Since amino groups can remove $\ce{H+}$ from solution, they are considered basic.

My current understanding:

  • The $\ce{H+}$ atoms bond in a polar covalent way with $\ce{N-}$. So that means the amino acid does not release that many $\ce{H+}$ ions. I can see why this is not acidic.
  • $\ce{N-}$ also takes electrons from $\ce{R}$, which makes $\ce{R}$ become $\ce{R+}$. This should repel $\ce{H+}$ from elsewhere in solutions.

How does this functional group remove $\ce{H+}$ from its solutions?

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  • $\begingroup$ R-NH2 + H+ -> [R-NH3]+ $\endgroup$
    – Karsten
    Apr 14, 2020 at 3:32
  • $\begingroup$ Right! Totally forgot that N requires 3 more electrons to be stable, not 2. Thanks! But what, then bonds the N to the R? $\endgroup$ Apr 14, 2020 at 3:48
  • $\begingroup$ You have that on your own picture. Amin groups are bonded to R. There is equilibrium $\ce{R-NH2 + H2O <=> R-NH3+ + OH-}$ or $\ce{R-NH2 + H3O+ <=> R-NH3+ + H2O}$ It is the same as with ammonia. $\endgroup$
    – Poutnik
    Apr 14, 2020 at 4:28

2 Answers 2

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Polarity and elementary electrostatics have nothing to do with acid-base definitions. Sometimes we try to find a reason, using our current understanding, and that ensues the danger of over-rationalization. At a truly fundamental level perhaps no chemist can tell you why an amine can accept another proton, may be someone can do quantum mechanical calculations and say why. So avoid over-rationalization... accept some observations as facts (temporarily until we develop a full understanding).

One can titrate an amine with an acid, by this experimentally observed fact, exactly just like we titrate NaOH with HCl. So an amine group is indeed a base or say it behaves like base. The acidic proton cannot annihilate itself during a titration, it has to go somewhere. The amine group gets protonated, in return.

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  • $\begingroup$ Is there no prior concept at all in chemistry that explains why this is basic? I have no idea why, for example, some atoms are stable with 2 shells of electrons, but taking that as a given, I can see why an oxygen atom wants 2 more. What is the given in this case? $\endgroup$ Apr 14, 2020 at 4:21
  • $\begingroup$ Perhaps a theoretical chemist can tell you a fundamental reason. I am not one of them. My understanding is basically from an experimental perspective. Amines can be titrated with acids, and some amines can turn red litmus blue. $\endgroup$
    – AChem
    Apr 14, 2020 at 4:26
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  1. According to Lewis acid-base concept, a molecule with central atom having lone pair can act as proton acceptor. In $\ce{R-NH2}$ there are lone pairs over $\ce{N}$ atom, which can combine with $\ce{H+}$ ions.

  2. This can also be explained by Brønsted–Lowry concept, which states if an atom accepts protons (like above), it is a base.

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  • $\begingroup$ The latter came as an anhancement of the original Arrhenius model where acids release H+ and bases OH-. But is does not apply to ammnia/amines, while the B-L model does. $\endgroup$
    – Poutnik
    Apr 14, 2020 at 8:07

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