C h a p t e r T h r e e: Acids and Bases 17, 21-Dimethylheptatriacontane, a sex attractant pheromone of the tsetse fly
CM 321: Summary of Important Concepts YConcepts for Chapter 3: Acids and Bases I. Acid-base theories A. Brønsted-Lowry acid-base theory 1. Brønsted-Lowry acids are proton donors 2. Brønsted-Lowry bases are proton acceptors 3. Relationships of conjugate acids and bases B. Curved electron arrows illustrate how acids and bases react C pk a 1. pk a = -logk a, where for the reaction A 2 6 3 A -, 2. Strong acids have pk a < 1 3. Moderate acids have pk a..1-10 4. Weak acids have pk a > 16 5. pk a s can be used to predict in which direction an acid-base equilibrium will lie D Acidity as a function of structure 1. Four general factors influencing the stability of an acid s conjugate base determine how strong an acid is going to be: a. Intrinsic properties of the atom bonded to i. Electronegativity ii. Size b. Resonance stabilization of the conjugate base c. Inductive stabilization of the conjugate base d. ybridization effects 2. Trends a. In any given row of the periodic table, acidity increases as EN increases b. In any given column of the periodic table, the weaker the - heteroatom bond, the stronger the acid c. For C- bonds, pk a Csp- > Csp 2 - > Csp 3 - E. Acidity as a function of other factors 1. Solvation a. Increased dipolar solvent-acid interactions decrease pk a 2. Counterion of conjugate base a. Generally not significant F Lewis acid-base theory 35
1. Lewis bases are electron donors a. Lewis bases which, by donating electrons, undergo formation of a covalent bond are called nucleophiles. 2. Lewis acids are electron acceptors a. Lewis acids which, by accepting electrons, undergo formation of a covalent bond are called electrophiles. 36
1. Provide short answers for each of the following: a. If the pk a of an organic compound is large, it must be a relatively acid. b. If the pk a of an organic compound is small, the conjugate base of that compound is a relatively base. c. Borane, B 3, should have a pk a that is than that of ethanol, C 2 5. 2. 2,2,2-Trifluoro-1-ethanol has a pk a of 9.7 and ethanol a pk a of 16.. F 3 C 2,2,2-trifluoroethanol ethanol a. Which is the stronger Brønsted-Lowry acid? b. Use your answer of part a to draw the equilibrium acid-base reaction between these two compounds. Include the curved electron arrows that illustrate how the proton of the acid is transferred to the base. c. Label your equilibrium reaction with the terms stronger acid, weaker acid, stronger base, and weaker base. d. Knowing that the pk a of the conjugate acid of ethanol is 1.5 and that of 2,2,2-trifluoroethanol is -1.5, will the equilibrium lie to the left or right of the reaction you have written? e. Will the alkane ethane, C 3 C 3, have a pk a greater than or less than the pk a of ethanol? 37
3. Rank the following from 1-4 according to their relative pk a values. Use 1 for the most acidic and 4 for the least acidic: N 2 S 4. Consider the following Brønsted-Lowry acid-base reaction: C.. F :.... C C F :.. C.. F : -. : C C F :.. a. Use curved electron arrows to show how the acid and base on the left of the equilibrium arrow undergo proton transfer to result in the products on the right. b. Label the acid, base, conjugate acid, and conjugate base of the reaction. c. Which is the stronger Brønsted-Lowry acid: one with K eq = 3.5 x 10-8 or one with K eq = 1.4 x 10 3? 5. Provide a clear, detailed, correct mechanism using curved electron arrows that describes how a 1:1 mixture of C 3 and 2 S 4 makes (C 3 2 ) and (S 4 ) -. Be sure and use bond-line structural formulas with all lone pairs drawn in: This will make it easier to see the Brønsted-Lowry acid-base chemistry that is occurring. 38
6. a. For the following chemical equilibrium, identify each chemical species on either side of the equilibrium arrow as being the strong acid (SA), weak acid (WA), strong base (SB), or weak base (WB) of the equilibrium. Refer to the table of pk a values to help you in your assigments. - CCF 3 k CCF 3 Species pk a Cl -7 3-1.7 CCF 3 0 2 15.7 10 C 3 C 2 16 b. Will the reaction equilibrium constant, k, be greater or less than 1? 39
7. This is a difficult problem that will really test your ability to use resonance and inductive effects to predict relative acidities! Rank these carboxylic acids according to their pk a values: N 2 highest pk a A B C 3 lowest pk a C D 40
8. a. Examine the following three acid-base equilibrium reactions, and fill in the table below them: reactant A reactant B reactant C reactant D i. N. :N 3 ii. - :N N 3 Equilibrium reaction What are the 2 letters of the reactants acting as acids? What is the letter of the reactant that is the stronger base? Is the equilibrium constant greater than 1, less than 1, or equal to 1? i. ii. b. Use curved electron arrow formalism to show how reactants A and B of equilibrium reaction i. are converted into reactants C and D. Some pk a values Acid 2 S CN N 4 N 3 pk a -1.7 7.0 9.3 9.2 10.0 19 33 41
9. Consider the following chemical reaction: : Br : Mg.... : Br : - Mg.. a. Clearly label the compound acting as a Lewis acid and the compound acting as the Lewis base in this reaction. b. Use curved electron arrow formalism to show how reactants on the left side of the equilibrium arrow form the product on the right side of the equilibrium arrow. 42