Essential Organic Chemistry Paula Yurkanis Bruice Chapter 2 Acids and Bases

Brønsted–Lowry Acids and Bases 2.1 An Introduction to Acids and Bases Brønsted–Lowry Acids and Bases An acid is a proton donor A base is a proton acceptor acid base base acid base acid acid base Note that water can act as an acid or a base

Brønsted–Lowry Acids and Bases conjugate base conjugate acid The remaining species after the proton has been donated is the conjugate base. The resulting species after the proton has been accepted is the conjugate acid. Every acid–base reaction involving proton transfer has two conjugate acid–base pairs.

Brønsted–Lowry Acids and Bases Acidity: a measure of the tendency of a compound to give up a proton. The stronger the acid, the more readily it gives up a proton. Basicity: a measure of a compound’s affinity for a proton. The stronger the acid, the weaker is its conjugate base.

2.2 pKa and pH

The Acid Dissociation Constant, Ka The degree to which an acid (HA) dissociates is indicated by its acid dissociation constant, Ka product of concentrations of ionized species Ka= concentration of intact acid Ka =

The Acid Dissociation Constant, Ka The Ka implies the concentrations of the acid and the ions The larger the Ka, the stronger the acid. Ka > 1 Ionized products greater than intact acid. Ka < 1 Ionized products less than intact acid. Ka >> 1 Ionization goes to completion (strong acid). (e.g., > 103) Ka << 1 Ionization does not occur to an appreciable amount. (e.g., < 10–3)

The Acid Dissociation Constant, Ka Since the Ka values for various acids have such a wide range, a more manageable way to discuss this measure of acidity is to use pKa = – log (Ka)

Compare pKa and Ka Values strong acids weak acids pKa -2 2 4 6 8 10 12 14 102 10-2 10-6 10-10 10-14 Ka The smaller the value of the pKa, the stronger the acid.

pKa The smaller the pKa, the stronger the acid. pKa < 1 Very strong acids pKa = 1~3 Moderately strong acids pKa = 3~5 Weak acids pKa = 5~15 Very weak acids pKa > 15 Extremely weak acids

pH The concentration of hydrogen ions is used as a measure of acidity This concentration is expressed as pH pH = – log[H3O+] The higher the concentration, the more acidic the solution and the lower the pH

pH pH < 7.00 Acidic solution pH = 7.00 Neutral solution Neutral water: [H3O+] = 1.0 × 10–7 M pH = – log[H3O+] = 7 pH < 7.00 Acidic solution pH = 7.00 Neutral solution pH > 7.00 Basic solution

pH vs. pKa Do not confuse pH and pKa! The pH scale is used to describe the acidity of a solution. The pKa is characteristic of a particular compound, indicating the tendency of the compound to give up its proton.

2.3 Organic Acids and Bases Carboxylic acids Most common organic acids! pKa: 3~5 Weak acids!

Alcohols Compounds having an OH group! pKa~ 16 Much weaker acids!

Amines Compounds with NH2 groups! pKa~ 40 Extremely weak, rarely behave as acids. In fact, amines are the most common organic bases!

Figure: 02-00-16UN.T1 Title: Approximate pKa values Caption: Listed are the approximated pKa values for protonated alcohol, acid and water, a carboxylic acid, protonated amines, and alcohols. Notes: Approximate pKa values for protonated alcohol, acid and water, a carboxylic acid, protonated amines, and alcohols.

2.4 How to Predict the Outcome of an Acid-Base Reaction To determine which of the two reactants of the reaction will be the acid, we need to compare their pKa values! The stronger acid will donate a proton to the weaker acid.

2.5 How to Determine the Position of Equilibrium Compare the pKa value of the acid on the left and right of the arrow! The equilibrium favors: Strong reacts to form weak!

2.6 How the Structure of an acid Affects its pKa The stronger the acid, the weaker is its conjugate base! The stronger the acid, the more stable is its conjugate base! Factors that influence stability of the conjugate base include: Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Electronegativity Elements in the same row are all about the same size, but they have very different electronegativity! When atoms are similar in size, the strongest acid will have its hydrogen attached to the most electronegative atom!

Electronegativity pKa Values 20 15 5 CH4 NH3 H2O HF ~60 36 15.7 3.2 increasing electronegativity pKa Values 20 15 5 CH4 NH3 H2O HF ~60 36 15.7 3.2 RCH3 RNH2 ROH 45 35 18 Consider the conjugate bases

Atomic Size When atoms are very different in size, the strongest acid will have its hydrogen attached to the largest atom!

Atomic Size pKa Values HF HCl HBr HI 3.5 –7 –9 –10 F– 1.36 Å 1.81 Å increasing size pKa Values HF HCl HBr HI 3.5 –7 –9 –10 F– 1.36 Å 1.81 Å 1.95 Å 2.16 Å H2O H2S H2Se H2Te 16 7 4 3 Cl– Br– I– Consider the ionic radii

2.7 How pH Affects the Structure of an Organic Compound A compound will exist primarily in its acidic form if the pH of the solution is less than its pKa. A compound will exist primarily in its basic form if the pH of the solution is greater than its pKa.

2.8 Buffer Solutions A solution of a weak acid (HA) and its conjugate base (A-) is called a buffer solution. A buffer solution will maintain nearly constant pH when small amounts of acid or base are added to it!

The result of a Lewis acid–base reaction is often 2.9 Lewis Acids and Bases A Lewis acid is an electron pair acceptor. (think empty orbital) A Lewis base is an electron pair donor. (think filled orbital) The result of a Lewis acid–base reaction is often called an adduct.

Lewis Acids and Bases Lewis acid : Need two from you! Lewis base : Have pair, will share!

Lewis Acids and Bases Lewis acids don’t have to donate a proton, as long as they can accept a share in an electron pair.

Lewis Acids and Bases Examples of Lewis Acids: Fe3+ BF3 BH3 AlCl3 H3O+ Examples of Lewis Bases: