Acids and Bases The Lewis Definition

The Lewis definition of acids and bases is broader and more encompassing than the Brpnsted-Lowry definition because it s not limited to substances that donate or accept protons. A Lewis acid is a substance that accepts an electron pair, and a Lewis base is a substance that donates an electron pair. The donated electron pair is shared between the acid and the base in a covalent bond. 

The fact that a Lewis acid is able to accept an electron pair means that it must have either a vacant, low-energy orbital or a polar bond to hydrogen so that it can donate H+ (which has an empty Is orbital). Thus, the Lewis definition of acidity includes many species in addition to H . For example, various metal cations, such as Mg , are Lewis acids because they accept a pair of electrons when they form a bond to a base. We ll see numerous instances in later chapters of metabolic reactions that begin with an acid-base reaction between Mg as a Lewis acid and an organic diphosphate or triphosphate ion as the Lewis base. 

ACTIVE FIGURE 2.5 The reaction of boron trifluoride, a Lewis acid, with dimethyl ether, a Lewis base. The Lewis acid accepts a pair of electrons, and the Lewis base donates a pair of non bonding electrons. Note how the movement of electrons from the Lewis base to the Lewis acid is indicated by a curved arrow. Note also how, in electrostatic potential maps, the boron becomes more negative (red) after reaction because it has gained electrons and the oxygen atom becomes more positive (blue) because it has donated electrons. Co to this book s student companion site atwww.cengage. com/chemistiy/mcmurry to explore an interactive version of this figure. 

Some further examples of Lewis acids follow  

CHAPTER 2 POLAR COVALENT BONDS ACIDS AND BASES 

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Boron trifluoride (Lewis acid) Dimethyl ether (Lewis base) Acid-base complex 

Interestingly, alanine and other amino acids exist primarily in a doubly charged form called a zwitterion rather than in the uncharged form. The zwitterion form arises because amino acids have both acidic and basic sites within the same molecule and therefore undergo an internal acid-base reaction. 

Lewis acid Lewis base Acid-base complex  

In the same way, compounds of group 3A elements, such as BF3 and AICI3, are Lewis acids because they have unfilled valence orbitals and can accept electron 

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In Chapter H, we introduce a second definition of acids and bases, the Lewis definition, which focuses attention on electron movement rather than proton movement Until then, acid-base always means proton transfer."... 

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