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2 Introduction to Organic Chemistry 2 ed William H. Brown 2-1 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Acids and Bases 2-2 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Brønsted-Lowry Definitions • Acid: a proton donor • Base: a proton acceptor H + O H + H Proton donor - O H H O + H H Proton acceptor H Proton donor + N H H H Proton acceptor Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. H H H + O H O H O H + H + N H H 2-3 2 Conjugate Acids & Bases • When an acid donates a proton, it is converted to its conjugate base • When a base accepts a proton, it is converted to its conjugate acid + H3 O Acid + NH3 Base H2 O C onjugate base + NH4 + C onjugate acid 2-4 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Strong Acids and Bases • Strong acid - completely ionized in aqueous solution. Examples are: • HCl, HBr, HI, HNO3, HClO4, and H2SO4 • Strong base - completely ionized in aqueous solution. Examples are: • LiOH, NaOH, KOH, Ca(OH)2, and Ba(OH)2 2-5 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Weak Acids and Bases • Acetic acid is a weak acid • it is incompletely ionized in aqueous solution O CH3 COH + H2 O Acid Base (weaker acid) (weaker base) O CH3 CO - + + H3 O Conjugate base Conjugate acid of CH3CO2 H of H2 O (stronger base) (stronger acid) 2-6 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Weak Acids and Bases • The equation for the ionization of a weak acid, HA, in water and the acid ionization constant, Ka, for this equilibrium are HA + H2 O A - + + Ka = Keq [H 2 O] = + H3 O - [H 3 O ][A ] [HA] pKa = - log Ka 2-7 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Weak Acids and Bases Acid ethanol water pKa Formula CH3 CH2 OH 15.9 H2 O 15.7 bicarbonate ion HCO 3 ammonium ion NH 4 H2 CO 3 9.24 CH3 CO2 H 4.76 carbonic acid acetic acid - + sulfuric acid H2 SO 4 hydrogen chloride HCl 10.33 6.36 Conjugate Base CH3 CH2 O HO 2- CO 3 NH 3 HCO 3 CH3 CO2 -5.2 HSO 4 -7 Cl - - - 2-8 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Structure & Acidity • Electronegativity • the relative acidity of HA within a period is determined by the stability of A• the greater the electronegativity, the greater the stability of A- pKa Electronegativity of A inA-H H3 C-H 51 H2 N -H 38 HO- H 15.7 F-H 3.5 2.5 3.0 3.5 4.0 Increasing acid strength 2-9 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Structure & Acidity • Resonance - delocalization of charge in A• carboxylic acids and alcohols both contain an -OH group • a carboxylic acid is a stronger acid than an alcohol O CH3 COH + H2 O A carboxylic acid O + CH3 CO + H3 O A carboxylate ion CH3 CH2 OH + H2 O An alcohol CH3 CH2 O- + pKa = 4.76 + H3 O pKa = 15.9 An alkoxide ion 2-10 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Structure & Acidity • Resonance - delocalization of charge in A- •• • delocalization of charge stabilizes the carboxylate ion relative to the alkoxide ion •• O CH3 C + H2 O •• O H •• •• •• •• O CH3 •• •• O C CH3 C O •• These contributing structures are equivalent; the carboxylate anion is stabilized by delocalization of the negative charge Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. + H3 O •• •• •• O •• + 2-11 2 Structure & Acidity • Inductive effect • the electron-withdrawing inductive effect of the adjacent carbonyl group weakens the O-H bond of the carboxylic acid O + CH 3 C O H • Carboxylic acids are more acidic than alcohols • delocalization of charge in the carboxylate ion • electron-withdrawing inductive effect of adjacent C=O 2-12 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Position of equilibrium • Favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base HA Stronger acid + B - Stronger base CH3 CO2 H + NH 3 Acetic acid pKa 4.76 (stronger acid) - A HB + Weaker Weaker base acid CH3 CO2 - + NH 4 + Ammonium ion pKa 9.24 (weaker acid) 2-13 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Position of equilibrium • Stronger acid and stronger base react to give weaker acid and weaker base - CH3 CO2 H + HCO3 Acetic acid Bicarbonate ion pKa 4.76 - C6 H5 OH + HCO3 Phenol Bicarbonate ion pKa 9.95 CH3 CO2 Acetate ion - + H2 CO3 Carbonic acid pKa 6.36 - C6 H5 O + H2 CO3 Phenoxide Carbonic acid ion pKa 6.36 2-14 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Position of equilibrium • Stronger acid and stronger base react to give weaker acid and weaker base - C6 H5 OH + HCO3 Phenol Bicarbonate ion pKa 9.95 - C6 H5 O + H2 CO3 Phenoxide Carbonic acid ion pKa 6.36 2-15 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Lewis Acids and Bases • Lewis acid: accepts a pair of electrons to form a new covalent bond • Lewis base: donates a pair of electrons to form a new covalent bond A + B Lewis Lewis acid base A B new covalent bond formed in this Lewis acid-base reaction 2-16 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Lewis Acids and Bases H O H H Lewis acid (accepts an electron pair) - O-H + H- O Lewis base (donates an electron pair) H CH3 -C+ + H- O-H H H + Cl - CH3 A carbocation Chloride ion (a Lewis acid) (a Lewis base) CH3 -C- Cl CH3 2-17 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved. 2 Acids and Bases End Chapter 2 2-18 Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.