Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Acid—Base Reactions in Organic Chemistry
7.2 Lewis Acids and Bases

The Arrhenius, Brønsted-Lowry, and the Lewis definitions of acids and bases were covered in the previous chapter. In organic chemistry, the Lewis definition is more frequently used, compared to the other definitions. A Lewis acid is an electron pair acceptor. In order for a molecule to be an acid and be able to accept a pair of electrons, there must be a vacant orbital in which these electrons must be placed. Of course, the simplest Lewis acid is a proton (H⁺) since it has a vacant orbital. On the other hand, a Lewis base is an electron pair donor. Thus, Lewis bases have at least one unshared pair of electrons. A Lewis acid—base reaction involves an electron pair from the base being transferred to the vacant orbital of the acid, as illustrated in Reaction (7-1) for the reaction of methylamine (a Lewis base) with BF₃, a Lewis acid.

(7-1)

For the reaction given in Reaction (7-1), the lone pair of electrons on the methylamine forms a bond to BF₃ using the vacant p orbital of BF₃ to produce the product shown. Note, for the product for Reaction (7-1), the boron has acquired a formal negative charge due to the gain of a pair of electrons from the methylamine. On the other hand, the nitrogen has a positive charge since it has given up a pair of electrons to form a bond to boron.

A carbocation, which is sp² hybridized and has a vacant p orbital, is also a Lewis acid and, as a result, can undergo a reaction with a Lewis base, such as the chloride anion since the chloride anion has four unshared pair of electrons. The reaction of a carbocation and the chloride anion is shown in Reaction (7-2). Acid—base reactions of this type will be encountered frequently throughout organic chemistry.

(7-2)

For the acid—base reaction given in Reaction (7-2), both reactants have formal charges of +1 and −1, respectively, and as a result, the product is neutral. The new bond is a result of using one pair of electrons from the chloride anion to form a bond using the vacant p orbital on the carbocation.

Problem 7.1

Which of the following are bases or acids? Explain your answer.

HI, H₂O, NaCN, AlCl₃, and KCl.

Ralph G. Pearson categorized acids and bases as hard and soft acids and bases (HSAB). Hard Lewis bases have small ionic radii and are typically highly solvated in a solvent; they are also relatively electronegative and nonpolarizable, NH₂^−, and OH⁻ are two examples of hard Lewis bases. On the other hand, soft Lewis bases have large ionic radii, they are relatively large and polarizable, typically not highly solvated; the anions of P, S, and Cl are soft Lewis bases. Hard Lewis acids have small ionic radii and are highly solvated; the proton, H⁺ is considered to be the hardest acid and will be encountered frequently throughout organic chemistry. An example of a soft acid is Cu²⁺. Typically, hard acids bind strongly with hard bases and soft acids bind with soft bases.