3.5 Lewis Acids and Lewis Bases

The Brønsted-Lowry definition works well for the reactions we learned so far, however it also limits the scope of acid-base reactions in a way where the proton H+ must be involved. Lewis acids and Lewis bases are defined in a more inclusive way that was first introduced by G. N. Lewis in 1923.


Lewis Acid: a species that can accept an electron pair;

Lewis Base: a species that can donate an electron pair.


All Brønsted-Lowry acids and bases fit into the Lewis definition, because the proton transfer process is essentially the reaction where the base uses its electron pair to accept a proton, as indicated by the mechanism arrow that we learned earlier. Therefore in the following reaction, the BL acid, H+, is also the Lewis acid, and BL base, NH3, also fits to the definition of the Lewis base.


NH3 (BL Base donates electron pair) while the (H+) (BL acid accepts electron pair to create (NH4+)
Figure 3.5a Lewis base & Lewis acid reaction

However, the Lewis definition is broader and covers more situations. For the following reaction, B(CH3)3 is the Lewis acid because boron has an incomplete octet, and the empty 2p orbital on boron is able to accept electrons. (CH3)3N behaves as the Lewis base with the lone pair electron on N that is able to be donated.


Figure 3.5b LA-LB adduct

The product between Lewis acids and Lewis bases is usually a species that has the acid and base joined together, and the product is called the “LA-LB adduct”.

Other examples of Lewis acids include electron-deficient species, such as H+, M+, M2+, BH3, BF3, AlCl3 etc. Lewis bases can be: amine, ether or other species that have lone pair electrons to donate.


Exercises 3.3

Show the product of the following LA-LB reaction:
BF3 + CH2CH3-O-CH2CH3 =
Answers to Practice Questions Chapter 3


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