Lewis Acids and Bases

Lewis Acids and BasesLewis acids and bases play an important role in chemical reactions because, except oxidation-reduction reactions, almost every reaction could be categorized as an acid-base reaction. Bases in water solutions show certain specific characteristics: bitter taste, feel slippery, and turn litmus paper blue. Acids in water solutions show these most common characteristics: sour taste, react with metals, and turn litmus paper red. The understanding of chemical reactions, such as acid and base interactions, derives from acceptance of the Lewis definition of acids and bases, which alternated the earlier Brønsted-Lowry concept. According to Lewis definition, acids (electrophiles) are electron pair acceptors and bases (nucleophiles) are electron pair donors.

AlCl3 + (CH3)3N ↔Al-Cl3N+(CH3)3In the above example, AlCl3 is Lewis acid because it have unoccupied valence orbital and can receive electron pairs from (CH3)3N, which is Lewis base. In a Lewis base, nitrogen donates an electron pair to an empty valence orbital of an aluminum atom. A curved arrow shows the way of electron-pair flow from the electron-rich Lewis base to the electron-poor Lewis acid. The arrow begins on a pair of nonbonding electrons on the Lewis base (nitrogen atom) and points to the Lewis acid (aluminum atom) with which it reacts.

In the below example number one, hydrochloric acid is Lewis acid and trimethylamine is Lewis base. A curved arrow originates from nitrogen atom and point to hydrogen atom. The initial product of the acid-base reaction is trimethylnitronium chloride.

In the below example number two, aluminum chloride is Lewis acid and dimethyl ether is Lewis base. A curved arrow originates from oxygen atom and point to aluminum atom. The initial product of the acid-base reaction is aluminum dimethyl ether chloride.

(1)(CH3)3N + HCl→(CH3)3N+H + Cl-(2)AlCl3 + CH3OCH3→Al-Cl3O+(CH3)2ReferencesWikipedia, the free encyclopedia. Lewis Acid.

http://en.wikipedia.org/wiki/Lewis_acidWikipedia, the free encyclopedia. Lewis base.

http://en.wikipedia.org/wiki/Lewis_base