Donating protons versus electrons  [ENDORSED]

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Mary Becerra 2D
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Joined: Fri Sep 29, 2017 7:06 am

Donating protons versus electrons

Postby Mary Becerra 2D » Sun Dec 03, 2017 10:22 pm

How can you tell when an atom is donating a proton versus an electron? Does it work as in when a proton is donating electrons it is always donating protons? Therefore a Lewis base is always a Bronsted acid?

K Stefanescu 2I
Posts: 68
Joined: Fri Sep 29, 2017 7:04 am

Re: Donating protons versus electrons  [ENDORSED]

Postby K Stefanescu 2I » Mon Dec 04, 2017 9:49 am

It is the inverse of what you stated. A Bronsted acid is a specific type of Lewis acid. Consider the reaction NH3+HCl<-> NH4+ +Cl-
A Bronsted acid is a proton donor, so it will donate H+. In this case, HCl is the acid, as it donates its H+ to NH3.
A Lewis acid is an electron acceptor. For HCl to dissociate, its already polar bond (with Cl being more electronegative) must be broken. The lone pair on NH3 will be attracted to the H+ in HCl, "pushing" the electrons in the HCl bond ever closer to Cl until a coordinate covalent bond is formed between NH3 and H. This breaks apart HCl, allowing Cl to accept the extra electron from H that was holding the HCl bond together.

In this example, by the same token, NH3 is the Bronsted base, as it accepts the proton (H+). It is also the Lewis base, as it is an electron donor. In this case, Cl does not have an extra orbital with which to accept the electrons, so NH3 does not technically donate to electrons. However, the acid behaves as though the base "donated" electrons, as it gains full "control" over the electrons in its bond with H+. So, for the purposes of differentiating between bases and acids, we consider that the base "donated" electrons, because the acid behaves as if it did.

So, in conclusion, Bronsted acids are Lewis acids and Bronsted bases are Lewis bases. Bronsted definitions focus on the proton transfer and Lewis definitions focus on the transfer of electrons. However, the result of this is always equivalent for both definitions.

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