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266 Organic Acids and Bases

266 Organic Acids and Bases - Organic Acids and Bases This...

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Dr. Steven Forsey 1 Organic Acids and Bases This document has been written to provide you with an overview of the fundamental concepts of organic acid-base chemistry. Many organic compounds have not been included such as the basicity of pyrrole or the acidity of hydrogens of carbonyl compounds. These topics are generally covered in the second and third organic chemistry courses. 1.0 Review of acids and bases 1 2.0 Trends in acid-base strengths 3 2.1 Shifting of the acid-base equilibrium 3 2.2 Electronegativity 4 2.3 Size 5 2.4 Resonance Stabilization 7 2.5 The proximity of electron-withdrawing and electron-donating groups: Inductive effects. 9 2.6 Hybridization 13 3.0 Base strength of Amines and Amides 14 3.1 Amines 14 3.2 Amides 17 4.0 Some exceptions to the general trends of acidity and basicity 17 pK a Tables 19
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Dr. Steven Forsey 2 Organic Acids and Bases 1.0 Review of acids and bases Acid-base reactions are the first step in understanding why reactions between organic molecules occur. This topic is studied in depth because acid-base reactions are so fundamental, that the theory will be encountered in almost every topic in organic chemistry and when looking at the broadest sense of Lewis acids and bases, most organic reactions are acid-base reactions. In this section you will learn why an acid or base is stronger or weaker than another. You will not be asked to memorize the dissociation constants but understand the trends that we see in acids and bases. This is a fundamental concept that will help you understand Organic Chemistry. This section can be difficult for some students because there is no set order to memorize, you must understand the concepts, look at the molecules of concern and understand the differences in the molecule and then make your decision. Many organic reactions involve proton (H + ) transfer or electron pair transfer. Thus, it will be helpful to review the Bronsted-Lowry and Lewis theories of acids and bases. In the Bronsted-Lowry Theory an acid is a proton donor and a base is a proton acceptor. In the Lewis Theory an acid is an electron pair acceptor and a base is an electron pair donor. In the Bronsted-Lowry theory, an acid must contain an ionizable H atom and a base must contain a lone pair of electrons onto which a proton can bind. For this reason, an acid is often represented in this theory by the general formula HA and a base is represented by :B. An acid HA will produce H 3 O + ions in aqueous solution and a base :B will produce OH ions because of the following ionization reactions. HA(aq) + H 2 O(l) H 3 O + (aq) + A (aq) 3 a [H O ] [A ] [HA] K B(aq) + H 2 O(l) HB + (aq) + OH (aq) b [HB ] [OH ] [B] K The equilibrium constants for the ionization reactions for HA and B: are called “ionization constants” and are denoted as K a or K b . The strength of an acid (or a base) is determined ultimately by the value of K a (or K b ). A strong acid (or base) has a large ionization constant and a weak acid (or base) has a small ionization constant. Because the values of K a and K b can span an enormous range of values (10 30 to 10
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