Copyright © 2010 Pearson Education, Inc. Brønsted–Lowry Acids and Bases  An acid is a proton donor  A base is a proton acceptor acidbase acidbase Note that water can act as an acid or a base

Copyright © 2010 Pearson Education, Inc. Brønsted–Lowry Acids and Bases acidbase  The remaining species after the proton has been donated is the conjugate base. conjugate acid conjugate base Every acid–base reaction involving proton transfer has two conjugate acid–base pairs.  The resulting species after the proton has been accepted is the conjugate acid.

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pH  The concentration of hydrogen ions is used as a measure of acidity  This concentration is expressed as pH pH = – log[H 3 O + ]  The higher the concentration, the more acidic the solution and the lower the pH

Copyright © 2010 Pearson Education, Inc. pH Neutral water:[H 3 O + ] = 1.0 × 10 –7 M pH = – log[H 3 O + ] = 7 pH < 7.00Acidic solution pH = 7.00Neutral solution pH > 7.00Basic solution

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The Acidity Constant, K a  The strength of an acid is represented by its ionization constant (acidity constant), K a Ka=Ka= product of concentrations of ionized species concentration of intact acid K a =

Copyright © 2010 Pearson Education, Inc. The Acidity Constant, K a  The K a implies the concentrations of the acid and the ions K a > 1Ionized products greater than intact acid. K a < 1 Ionized products less than intact acid. K a >> 1Ionization goes to completion (strong acid). (e.g., > 10 3 ) K a << 1Ionization does not occur to an appreciable amount. (e.g., < 10 –3 )

Copyright © 2010 Pearson Education, Inc. pK a = – log (K a ) The Acidity Constant, K a  Since the K a values for various acids have such a wide range, a more manageable way to discuss this measure of acidity is to use

Copyright © 2010 Pearson Education, Inc. Compare pK a and K a Values pKapKa strong acidsweak acids KaKa The smaller the value of the pK a the stronger the acid

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Acid Strength HA + H 2 OH 3 O + + A - HAHA A-A- A-A- Has a strong conj. base (  higher energy) WEAK ACID STRONG ACID ENERGYENERGY ionization easier  The difference between a strong acid and a weak acid can be described by the stability of the conjugate base. Has a weak conj. base (  lower energy)

Copyright © 2010 Pearson Education, Inc. Acid Strength  A more stable conjugate base means a stronger acid. HAHA stabilization ENERGYENERGY A-A- A-A-

Copyright © 2010 Pearson Education, Inc. Acid Strength  Factors that influence stability of the conjugate base include: Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Copyright © 2010 Pearson Education, Inc. Acid Strength  Factors that influence stability of the conjugate base include: Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Copyright © 2010 Pearson Education, Inc. Acid Strength  Factors that influence stability of the conjugate base include: Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Copyright © 2010 Pearson Education, Inc. Acid Strength  Factors that influence stability of the conjugate base include: Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Copyright © 2010 Pearson Education, Inc. Acid Strength  Factors that influence stability of the conjugate base include: Resonance Electronegativity Atomic Size Hybridization Inductive Effects

Copyright © 2010 Pearson Education, Inc. Resonance Effects  More or better resonance structures of the conjugate base lead to a stronger acid.

Copyright © 2010 Pearson Education, Inc. Resonance Effects pK a Values increasing quality of resonance

Copyright © 2010 Pearson Education, Inc. Resonance Effects  The Acetate Ion acetate ion acetic acid Resonance Stabilized Equivalent structures (charges on oxygens)

Copyright © 2010 Pearson Education, Inc. Resonance Effects - More resonance structures, but not more stable than acetate Nonequivalent structures (note charges on carbon and oxygen)  The Phenolate Ion

Copyright © 2010 Pearson Education, Inc. Electronegativity  Placing the negative charge on a more electronegative element (from the same period) in the conjugate base leads to a stronger acid.

Copyright © 2010 Pearson Education, Inc. Electronegativity pK a Values increasing electronegativity CH 4 NH 3 H 2 O HF > RCH 3 RNH 2 ROH Consider the conjugate bases

Copyright © 2010 Pearson Education, Inc. Electronegativity pK a Values Consider the conjugate bases increasing electronegativity CH 4 NH 3 H 2 O HF > RCH 3 RNH 2 ROH

Copyright © 2010 Pearson Education, Inc. Atomic Size  Placing the negative charge on a larger atom (from the same group) in the conjugate base leads to a stronger acid.

Copyright © 2010 Pearson Education, Inc. Atomic Size pK a Values increasing size H 2 O H 2 S H 2 Se H 2 Te Consider the ionic radii HF HCl HBr HI 3.5 – 7 – 9 – 10

Copyright © 2010 Pearson Education, Inc. F–F– Cl – I–I– Br – Electronegativity pK a Values Consider the ionic radii increasing size HF HCl HBr HI 3.5 –7 –9 –10 H 2 O H 2 S H 2 Se H 2 Te Å 1.81 Å 1.95 Å 2.16 Å

Copyright © 2010 Pearson Education, Inc. Hybridization More s character in the orbital bearing the negative charge in the conjugate base leads to a stronger acid.

Copyright © 2010 Pearson Education, Inc. Hybridization sp 3 sp 2 sp > As electrons in hybrid orbitals become closer to the nucleus, they are lower in energy : : : pKapKa pKapKa

Copyright © 2010 Pearson Education, Inc. Inductive Effects  Electron-withdrawing effects due to differences in electronegativity pull electron density away from the negatively charged end of the conjugate base, lowering the energy and stabilizing the conjugate base, making the acid stronger.

Copyright © 2010 Pearson Education, Inc. Inductive Effects  Electron-donating effects due to differences in electronegativity push electron density toward the negatively charged end of the conjugate base, increasing the energy and destabilizing the conjugate base, making the acid weaker.

Copyright © 2010 Pearson Education, Inc. Inductive Effects Electron-withdrawing Groups F, Cl, Br, O, N R, CH 3, B, Si electronegative elements pull electron density away from carbon alkyl groups and elements less electronegative than carbon push electron density toward carbon Remember, the electron-withdrawing and -donating groups work through the  bond system, while resonance groups work through the  system. Electron-donating Groups

Copyright © 2010 Pearson Education, Inc. Inductive Effects Chlorine helps to stabilize – CO 2 – by withdrawing electrons This effect diminishes with distance—it extends for about 3 bonds

Copyright © 2010 Pearson Education, Inc. Inductive Effects pK a Values increasing electronegativity increasing substitution

Copyright © 2010 Pearson Education, Inc. Inductive Effects pKa:pKa:  Increasing substitution 0.65