5.2
Lewis Acids and Bases
This lesson delves into Lewis acids and bases in the context of the octet rule for electron-deficient compounds. Here, the concept is discussed, empha…
In the periodic table, all elements in group 13 have three valence electrons. Consequently, they form trivalent compounds in which the central atom has a sextet of electrons and a vacant orbital.
These electron-deficient compounds achieve their octets by receiving electrons from other species in a chemical reaction.
For example, aluminum chloride reacts with ammonia by accepting the lone pair from the nitrogen atom.
This makes aluminum chloride — the electron-pair acceptor — a Lewis acid, and ammonia — the electron-pair donor — a Lewis base.
As the two oppositely charged species attract each other, the electron pair is transferred from the base to the acid. This completes the octet of the Lewis acid, resulting in a Lewis acid—base adduct.
Now, consider another Lewis acid—base reaction between the electron-deficient boron trifluoride and ammonia.
The electrostatic potential map of boron trifluoride indicates a significant positive charge centered on the boron atom.
In the case of ammonia, the nitrogen atom carries a negative charge on account of its nonbonding electron pair.
The opposite charges cause boron trifluoride and ammonia to react. During the reaction, the lone pair of ammonia fills the valence shell of boron to give it an octet of electrons.
In the adduct, boron carries a formal negative charge and nitrogen carries a formal positive charge — thus conserving the net charge of the compound.
Now, examine the reaction between a protic acid, like hydrochloric acid, and a base — ammonia. By the Brønsted–Lowry definition, HCl is an acid owing to its ability to donate a proton.
Although HCl is not an electron-deficient compound, in its reaction with ammonia, the hydrogen atom of HCl loses its shared electrons to chlorine while simultaneously accepting a pair of electrons from ammonia. HCl is, therefore, also a Lewis acid.
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Q1: What makes group 13 elements Lewis acids?
Group 13 elements like boron and aluminum have three valence electrons, forming trivalent compounds with a sextet of electrons and a vacant orbital. These electron-deficient compounds cannot achieve a stable octet independently, so they accept electron pairs from other species to complete their valence shells. This electron-pair acceptance defines them as Lewis acids.
Q2: How do Lewis acids and bases differ from Brønsted-Lowry acids and bases?
Lewis theory expands acid-base chemistry beyond proton transfer. While Brønsted-Lowry acids donate protons, Lewis acids accept electron pairs. All Brønsted-Lowry acids are protic acids, but Lewis acids can be protic or aprotic. For example, hydrochloric acid is both a Brønsted-Lowry acid and a Lewis acid because its hydrogen accepts electrons from ammonia while losing shared electrons to chlorine.
Q3: What happens when aluminum chloride reacts with ammonia?
Aluminum chloride accepts the lone pair of electrons from ammonia's nitrogen atom. The electron pair transfers from ammonia (the Lewis base) to aluminum chloride (the Lewis acid), completing aluminum's octet. This forms a Lewis acid-base adduct where the oppositely charged species attract and bond together.
Q4: Why does boron trifluoride react with ammonia?
Boron trifluoride has a significant positive charge on the boron atom due to its vacant orbital, while ammonia's nitrogen carries a negative charge from its nonbonding electron pair. These opposite charges cause them to attract. The lone pair from ammonia fills boron's valence shell, giving it an octet and forming a stable Lewis acid-base adduct.
Q5: What is a Lewis acid-base adduct?
A Lewis acid-base adduct is the product formed when a Lewis acid accepts an electron pair from a Lewis base. In the adduct, the acid's octet is completed, and formal charges develop on both species to conserve net charge. For example, in the boron trifluoride-ammonia adduct, boron carries a formal negative charge and nitrogen carries a formal positive charge.
Q6: How does the octet rule apply to electron-deficient compounds?
Electron-deficient compounds like those from group 13 elements start with fewer than eight valence electrons and cannot satisfy the octet rule alone. They achieve stable octets by accepting electron pairs from Lewis bases in chemical reactions. This electron-pair acceptance is the defining characteristic of Lewis acids and explains why these compounds are reactive.
Q7: Can a protic acid be a Lewis acid?
Yes. Hydrochloric acid demonstrates this dual nature: it is a protic acid because it donates a proton to ammonia, and it is also a Lewis acid because its hydrogen atom accepts an electron pair from ammonia while losing shared electrons to chlorine. Lewis theory encompasses both electron-pair transfer and proton-transfer mechanisms.