1 Organic Chemistry, Second Edition Janice Gorzynski Smith University of Hawai’i Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prepared by Rabi Ann Musah State University of New York at Albany Chapter 2 Acids & Bases

2 Arrhenius acids and bases: An Arhennius acid yields a proton in solution. An Arhennius base yields a hydroxide ion in solution. Bronstead-Lowry acids and bases: A Brønsted-Lowry acid is a proton donor. A Brønsted-Lowry base is a proton acceptor. Lewis acids and bases: A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor. Acids and Bases Three acid-base systems:

3 Brønsted-Lowry Acids and Bases: Proton donors and acceptors. H + or H 3 O + = a proton Acids and Bases Figure 2.1 Examples of Brønsted - Lowry acids and bases

4 Reactions of Brønsted-Lowry Acids and Bases: A Brønsted-Lowry acid base reaction results in the transfer of a proton from an acid to a base. In an acid-base reaction, one bond is broken, and another one is formed. The electron pair of the base B: forms a new bond to the proton of the acid. The acid H—A loses a proton, leaving the electron pair in the H—A bond on A:. These forms represent conjugate acid/conjugate base pairs. Note the equilibrium arrows. Acids and Bases

5 Acid Strength and pK a : Acid strength is the tendency of an acid to donate a proton. The more readily a compound donates a proton, the stronger is its acidity. The equilibrium constant is a measure of acidity. When a Brønsted-Lowry acid H—A is dissolved in water, an acid-base reaction occurs, and an equilibrium constant can be written for the reaction. Acids and Bases

6 Acid Strength and pK a : Because the concentration of the solvent H 2 O is essentially constant, the equation can be rearranged and a new equilibrium constant, called the acidity constant, K a, can be defined. It is generally more convenient when describing acid strength to use “pK a ” values than K a values. Acids and Bases

7 Acid Strength and pK a : Acids and Bases

8 Factors that Determine Acid Strength: Anything that stabilizes a conjugate base A: ¯ makes the starting acid H—A more acidic. Four factors affect the acidity of H—A. These are: Element effects (trends in periodic table) Inductive effects (electronegativity) Resonance effects (multiple resonance structures) Hybridization effects (sp, sp 2, sp 3 ) Acids and Bases

9 Factors that Determine Acid Strength: No matter which of these factors is discussed, to compare the acidity of any two acids: oAlways look at the conjugate bases. oDetermine which conjugate base is more stable. oThe more stable the conjugate base, the more acidic the acid. The strengths of a conjugate acid and its conjugate base are inversely related. A strong conjugate base has a weak conjugate acid. A weak conjugate base has a strong conjugate acid. Acids and Bases

10 Factors that Determine Acid Strength: Element Effects—Trends in the Periodic Table. Across a row of the periodic table, the acidity of H—A increases as the electronegativity of A increases. Positive or negative charge is stabilized when it is spread over a larger volume. Acids and Bases

11 Factors that Determine Acid Strength: Element Effects—Trends in the Periodic Table. Down a column of the periodic table, the acidity of H—A increases as the size of A increases. Size, and not electronegativity, determines acidity down a column. The acidity of H—A increases both left-to-right across a row and down a column of the periodic table. Although four factors determine the overall acidity of a particular hydrogen atom, element effects—the identity of A—is the single most important factor in determining the acidity of the H—A bond. Acids and Bases

12 Factors that Determine Acid Strength: Inductive Effects An inductive effect is the pull of electron density through  bonds caused by electronegativity differ- ences between atoms. In the example below, when we compare the acidities of ethanol and 2,2,2-trifluoroethanol, we note that the latter is more acidic than the former. Acids and Bases

13 The reason for the increased acidity of 2,2,2- trifluoroethanol is that the three electronegative fluorine atoms stabilize the negatively charged conjugate base. This effect is limited to a three bond distance. Factors that Determine Acid Strength: Inductive Effects Acids and Bases

14 Factors that Determine Acid Strength: Resonance Effects Resonance is a third factor that influences acidity. In the example below, when we compare the acidities of ethanol and acetic acid, we note that the latter is more acidic than the former. Acids and Bases

15 Factors that Determine Acid Strength: Resonance Effects When the conjugate bases of the two species are compared, it is evident that the conjugate base of acetic acid enjoys resonance stabilization, whereas that of ethanol does not. Acids and Bases

16 Resonance delocalization makes CH 3 COO ¯ more stable than CH 3 CH 2 O ¯, so CH 3 COOH is a stronger acid than CH 3 CH 2 OH. The acidity of H—A increases when the conjugate base A: ¯ is resonance stabilized. Acids and Bases Factors that Determine Acid Strength: Resonance Effects

17 The final factor affecting the acidity of H—A is the hybridization. The higher the percent of s-character of the hybrid orbital, the closer the lone pair is held to the nucleus, and the more stable the conjugate base. Let us consider the relative acidities of three different compounds containing C—H bonds. Acids and Bases Factors that Determine Acid Strength: Hybridization Effects

18 Acids and Bases Factors that Determine Acid Strength: Hybridization Effects Figure 2.4 Electrostatic Potential plots

19 Figure 2.5 Summary of the factors that determine acidity Acids and Bases Factors that Determine Acid Strength: Summary of Effects

20 Commonly Used Acids in Organic Chemistry: In addition to the familiar acids HCl, H 2 SO 4 and HNO 3, a number of other acids are often used in organic reactions. Two examples are acetic acid and p-toluene-sulfonic acid (TsOH). Acids and Bases

21 Common strong bases used in organic reactions are more varied in structure. Commonly Used Bases in Organic Chemistry: Acids and Bases Figure 2.6 Common negatively charged bases

22 It should be noted that: Strong bases have weak conjugate acids with high pK a values, usually > 12. Strong bases have a net negative charge, but not all negatively charged species are strong bases. For example, none of the halides F ¯, Cl ¯, Br ¯, or I ¯, is a strong base. Carbanions, negatively charged carbon atoms, are especially strong bases. A common example is butyllithium. Two other weaker organic bases are triethylamine and pyridine. Commonly Used Bases in Organic Chemistry: Acids and Bases

23 Lewis Acids and Bases: The Lewis definition of acids and bases is more general than the Br Ø nsted-Lowry definition. A Lewis acid is an electron pair acceptor. A Lewis base is an electron pair donor. Lewis bases are structurally the same as Br Ø nsted- Lowry bases. Both have an available electron pair—a lone pair or an electron pair in a  bond. All Br Ø nsted-Lowry acids are also Lewis acids, but the reverse is not necessarily true. Acids and Bases

24 Lewis Bases: A Br Ø nsted-Lowry base always donates this electron pair to a proton, but a Lewis base donates this electron pair to anything that is electron deficient. Common examples of Lewis bases (which are also Br Ø nsted-Lowry bases) Acids and Bases

25 Lewis Acids: A Lewis acid must be able to accept an electron pair and can be any species that is electron deficient and capable of accepting an electron pair. Common examples of Lewis acids (which are not Br Ø nsted-Lowry acids) include BF 3 and AlCl 3. Acids and Bases

26 Electrophiles and Nucleophiles: A Lewis acid is also called an electrophile. When a Lewis base reacts with an electrophile other than a proton, the Lewis base is also called a nucleophile. In this example, BF 3 is the electrophile and H 2 O is the nucleophile. A Lewis Acid-Base Reaction: Electrophile Nucleophile Acids and Bases

27 Two other examples are shown below. Note that in each reaction, the electron pair is not removed from the Lewis base. Instead, it is donated to an atom of the Lewis acid and a new covalent bond is formed. Lewis Acid-Base Reactions: Acids and Bases

28 Lewis Acids and Bases: Consider the Lewis acid-base reaction between cyclohexene and H—Cl. The Br Ø nsted-Lowry acid HCl is also a Lewis acid, and cyclohexene, having a  bond, is the Lewis base. Acids and Bases

29 Lewis Acids and Bases: To draw the product of this reaction, the electron pair in the  bond of the Lewis base forms a new bond to the proton of the Lewis acid, generating a carbocation. The H—Cl bond must break, giving its two electrons to Cl, forming Cl ¯. Because two electron pairs are involved, two curved arrows are needed. Acids and Bases

30 Organic Chemistry, Second Edition Janice Gorzynski Smith University of Hawai’i Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Prepared by Rabi Ann Musah State University of New York at Albany End Chapter 2