Organic Chemistry: Concepts and Applications - Headley Allan D. 2020
An Overview of the Reactions of Organic Chemistry
6.2 Acid—Base Reactions
Acids and bases are encountered in just about all aspects of our everyday life; the different salad dressings that are used regularly to make salads taste tangy contain a small amount of an acid, acetic acid. The tart taste of lemons, grapefruits, and other fruits is due to the presence of small amounts of different acids, of which the most commonly known is citric acid. Oxalic acid is used in the bleaching of wood pulp. Benzoic acid is commonly used in cosmetics and dyes, and the salts of these acids are most commonly included in food as a preservative. Some of the everyday kitchen cleaners contain different acids or bases. The liquid that is used to unclog most drains contains a very strong base. Examples of common acids are shown in Figure 6.1.
Figure 6.1 Examples of common organic acids.
Acids react with bases to produce salts and water, and the reaction of benzoic acid and sodium hydroxide is shown in Reaction (6-1).
(6-1)
Acid—base reactions that will be encountered in organic chemistry include bases that are much stronger that those encountered in general chemistry. Bases that were encountered in the general chemistry lab included sodium hydroxide and potassium hydroxide, but in organic chemistry, much stronger bases such as sodium amide and potassium methoxide will be encountered. Similarly, some of the acids in organic chemistry are much stronger than those encountered in general chemistry, but the reaction principles remain the same. In the next sections, a review of the definition and concepts of organic acids and bases is carried out.
6.2.1 Acids
One of the earliest definitions of an acid was coined by Arrhenius, which states that an acid is a substance that when added to water increases the concentration of protons (H+ ions). On the other hand, a base is any substance that when added to water increases the hydroxide ions (OH−) concentration. Over the years, as chemists gained a better understanding of the role of acids and bases in nature and chemical reactions, other definitions have developed. In order to understand the importance of acids and bases and how they work, other definitions of acids and bases have been more useful. Another important chemical definition of acids and bases that is used frequently in organic chemistry is that of Brønsted—Lowry, named after a Danish chemist, Johannes Brønsted and an English chemist, Thomas Lowry, who coined another definition of acids and bases. A Brønsted—Lowry acid is any substance that will donate a proton (H+). A proton is a hydrogen atom without its electron. Recall from Chapter 1 that a hydrogen atom has only one electron and if this electron were removed, a proton is the result. Hydrochloric acid (HCl) is an acid because it can undergo a heterolytic cleavage of the polar covalent bond to give a proton (H+) and the chloride anion (Cl−). If HCl is placed in water, it will donate its proton to the water to form the protonated water and a Cl− anion, also known as the conjugate base of the acid, HCl. The reaction is shown in Reaction (6-2).
(6-2)
Thus, HCl is a proton donor and it can donate its proton to water. In the above example, a H2O molecule accepted the proton from the HCl.
Problem 6.1
i. Which of the following are Brønsted—Lowry Acids?
HI, H2O, NaCN, AlCl3, and KCl.
ii. Write a reaction similar to the one given in Reaction (6-2) for each of the acids identified above with water.
Based on the Brønsted—Lowry definition of an acid, any molecule that has hydrogen atoms is a potential acid. Since organic compounds have mostly carbons and hydrogens, most are potential acids, but to varying degrees. Some organic molecules are fairly acidic, whereas most are extremely weak acids and will require extremely strong bases to abstract a proton.
6.2.2 Bases
A Brønsted—Lowry base is a proton acceptor (H+ acceptor). Thus, water in the presence of HCl will accept the proton from HCl to become H3O+; hence, water in this case is a base and that reaction is shown in Reaction (6-2). Sodium hydroxide is a base since it will accept a proton from an acid such as HCl, as shown in Reaction (6-3).
(6-3)
In this case, water is the conjugate acid of sodium hydroxide. With this knowledge of acids and bases, it becomes clear that for the reaction given in Reaction 6.1, which involves benzoic acid and sodium hydroxide, benzoic acid is the proton donor and sodium hydroxide is the proton acceptor, hence the base. Another application of this concept is shown in Reaction (6-4).
(6-4)
For the above reaction shown in Reaction (6-4), cyclopentanol is the acid (the acidic hydrogen is shown in red), in which the proton goes to the sodium amide, which is the base. Note in this case, the product is a salt and ammonia. Thus, for acid—base reactions that will be encountered in organic chemistry, each reaction has to be carefully analyzed by applying the concepts and definitions of acids and bases. Students should also be able to readily identify an acid and a base and, of course, be able to predict the products of an acid—base reaction.
Problem 6.2
i. Identify the acid and base in the reactants and the conjugate acid and conjugate base in the products for each of the following reactions.
ii. Predict the products of the reactions of the acid—base pair shown below.
There is yet another definition of acids and bases and it is that of G. N. Lewis of the University of California, Berkeley. In 1923, he defined a Lewis acid as an electron pair acceptor. In order for a molecule to be able to accept a pair of electrons, there must be an available empty orbital in which these electrons must be placed. The proton (H+) is the simplest, which is a hydrogen atom without its electron, which means that it has an empty orbital. On the other hand, a Lewis base is defined as an electron pair donor. Thus, a Lewis base must have at least one unshared pair of electrons. Lewis bases include molecules, which have functional groups such as amines (RNH2) and alcohols (ROH) since they have at least one unshared pair of electrons. Bases that we have seen before, such as NaOH and CH3O− K+, are considered bases under the Lewis definition since they have at least one unshared pair of electrons on the oxygen, but this definition now includes a larger number of molecules; the same is true for acids. The reactions of a Lewis base (ammonia) with a proton and the reaction of ammonia with BF3, an electron pair acceptor, are shown in Reactions (6-5) and (6-6).
(6-5)
(6-6)
Note that in these reactions, the lone pair of electrons from the ammonia (shown in red) forms the single bond in the product to the proton, which has an empty orbital. Also, note that since nitrogen in the product has four bonds, it acquires a formal charge of positive one (+1). The same is true for the reaction of ammonia with boron trifluoride. The lone pair of electrons of ammonia bonds to the empty p orbital of boron trifluoride to form a new ionic complex shown in Reaction (6-6); boron of boron trifluoride is sp2 hybridized, with an empty p orbital.
Problem 6.3
Identify the Lewis acid and Lewis base in each of the following reactions.
