Organic Chemistry William H. Brown Christopher S. Foote Brent L. Iverson 4-1 Acids and Bases Chapter 4 4-2 Arrhenius Acids and Bases In 1884, Svante Arrhenius proposed these definitions • acid: a substance that produces H3O+ ions aqueous solution • base: a substance that produces OH- ions in aqueous solution • this definition of an acid is a slight modification of the original Arrhenius definition, which was that an acid produces H+ in aqueous solution • today we know that H+ reacts immediately with a water molecule to give a hydronium ion + H ( aq) + H2 O( l) + H3 O ( aq) Hydronium ion 4-3 Brønsted-Lowry Definitions Acid: a proton donor Base: a proton acceptor H H O: : :O : - : : : + H O H + + H O H : H Proton donor Proton acceptor H H : : + H O H + H H Proton donor :N H H Proton acceptor H O: H + + H N H H 4-4 Conjugate Acids & Bases • conjugate base: the species formed from an acid when it donates a proton to a base • conjugate acid: the species formed from a base when it accepts a proton from an acid • acid-base reaction: a proton-transfer reaction • conjugate acid-base pair: any pair of molecules or ions that can be interconverted by transfer of a proton con jugate acid-bas e pair con jugate acid-bas e pair HCl(aq) Hyd rogen chloride (acid ) + H2 O( l) Water (base) + Cl ( aq) + H3 O (aq) Chlorid e Hydronium ion ion (conju gate (conjugate b ase of HCl) acid of H 2O) 4-5 Conjugate Acids & Bases • Brønsted-Lowry definitions do not require water as a reactant • consider the following reaction between acetic acid and ammonia conju gate acid -base p air conju gate acid -base p air CH3 COOH + NH3 A cetic acid Ammonia (acid ) (base) - + CH3 COO + NH4 Acetate Ammonium ion ion (conju gate b ase (conjugate acid acetic acid ) of ammonia) 4-6 Conjugate Acids & Bases • we can use curved arrows to show the flow of electrons in an acid-base reaction : : H + :N H H A cetic acid Ammonia (proton d on or) (p roton acceptor) :O: CH3 -C-O:- H + H-N-H H Acetate ion Ammonium ion : : :O: CH3 -C-O H 4-7 Conjugate Acids & Bases Many organic molecules have two or more sites that can act as proton acceptors in this chapter, we limit our discussion to carboxylic acids, esters, and amides in these molecules, the favored site of protonation is the one in which the charge is more delocalized question: which oxygen of a carboxylic acid is protonated? + H O CH3 -C-O-H + H2 SO4 O O + CH3 -C-O-H or CH3 -C-O-H + HSO4 H A B (p roton ation (p roton ation on th e on the carbonyl oxygen) h yd roxyl oxygen ) 4-8 Conjugate Acids & Bases for protonation on the carbonyl oxygen, we can write three contributing structures two place the positive charge on oxygen, one places it on carbon + O CH3 -C-O-H H O + CH3 -C-O-H -C A-1 (C an d O have comp lete octets) A-2 (C h as incomplete octet) H H O + CH3 -C=O-H A-3 (C an d O have comp lete octets) A-1 and A-3 make the greater contribution because all atoms have complete octets the positive charge is delocalized over three atoms with the greater share on the two equivalent oxygens 4-9 Conjugate Acids & Bases for protonation on the hydroxyl oxygen, we can write two contributing structures O + CH 3 -C-O-H H B-1 O + CH 3 -C-O-H + H B-2 (ch arge sep aration an d ad jacen t positive charges) B-2 makes only a minor contribution because of charge separation and adjacent positive charges therefore, we conclude that protonation of a carboxylic acid occurs preferentially on the carbonyl oxygen 4-10 Conjugate Acids & Bases Problem 4.3 Does proton transfer to an amide group occur preferentially on the amide oxygen or the amide nitrogen? + H O O CH3 -C-N-H + HCl H A cetamide (an amid e) CH3 -C-N-H or H A (p rotonation on the amide oxygen) OH + CH3 -C-N-H + Cl - H B (protonation on th e amid e nitrogen) 4-11 Pi Electrons As Basic Sites Proton-transfer reactions occur with compounds having pi electrons, as for example the pi electrons of carbon-carbon double and triple bonds the pi electrons of 2-butene, for example, react with HBr by proton transfer to form a new C-H bond CH3 -CH=CH-CH3 + H-Br 2-Butene H + CH3 -C-C-CH3 + Br - HH sec-Butyl cation (a 2° carb ocation ) the result is formation of a carbocation, a species in which one of its carbons has only six electrons in its valence shell and carries a charge of +1 4-12 Pi Electrons As Basic Sites Problem 4.4 Draw Lewis structures for the two possible carbocations formed by proton transfer from HBr to 2-methyl-2-butene CH3 CH3 -C=CH-CH3 + H-Br 2-Methyl-2-buten e 4-13 Acids & Base Strengths The strength of an acid is expressed by an equilibrium constant the acid dissociation of acetic acid is given by the following equation O CH3 COH + H2 O A cetic acid Water O - CH3 CO Acetate ion + + H3 O Hyd roniu m ion 4-14 Weak Acids and Bases We can write an equilibrium expression for the dissociation of any uncharged acid, HA, as: - + Keq = + A + H3 O HA + H2 O - [H3 O ] [A ] [ HA][ H2 O] water is a solvent and its concentration is a constant equal to approximately 55.5 mol/L we can combine these constants to give a new constant, Ka, called an acid dissociation constant + K a = Keq[ H2 O] = - [H3 O ][A ] [HA] 4-15 Acid Weaker Ethan e acid Ethylene Ammonia Hyd rogen Acetylene Ethan ol Water Formula CH3 CH3 CH2 =CH2 NH3 H2 HC CH CH3 CH2 OH H2 O Methylammon ium ion CH3 NH3 + - Bicarbonate ion Phen ol HCO3 C6 H5 OH Ammoniu m ion Hyd rogen su lfid e NH4 H2 S Carbonic acid Acetic acid Benzoic acid Phosp horic acid Hyd roniu m ion Sulfuric acid Hyd rogen ch loride Stron ger Hyd rogen bromide acid Hyd rogen iod ide + H2 CO3 CH3 COOH C6 H5 COOH H3 PO4 + H3 O H2 SO4 HCl HBr HI pK a 51 44 38 35 25 15.9 15.7 10.64 10.33 9.95 Conju gate Bas e CH3 CH2 S tronger - conju gate CH2 =CH base NH2 H HC CCH3 CH2 O - HO CH3 NH2 2- CO3 C6 H5 O 9.24 7.04 6.36 4.76 4.19 NH3 HS HCO3 CH3 COO C6 H5 COO 2.1 -1.74 -5.2 -7 -8 -9 H2 PO4 H2 O HSO4 Cl Br I - Weaker conju gate base 4-16 Acid-Base Equilibria Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base CH3 COOH + - NH3 A cetic acid Ammon ia (stron ger bas e) pK a 4.76 (stron ger acid ) CH3 COO + NH4 + Acetate ion A mmonium ion (w eaker bas e) pK a 9.24 (w eak er acid) pK eq = 4.76 - 9.24 = -4.48 Ke q = 3.0 x 104 4-17 Acid-Base Equilibria Consider the reaction between acetic acid and sodium bicarbonate we can write the equilibrium as a net ionic equation we omit Na+ because it does not undergo any chemical change in the reaction O + HCO3 CH3 COH A cetic acid Bicarb on ate ion pK a 4.76 (stron ger acid ) O CH3 CO + H2 CO3 Acetate ion Carbonic acid pK a 6.36 (w eak er acid) equilibrium lies to the right carbonic acid forms, which then decomposes to carbon dioxide and water 4-18 Molecular Structure and Acidity The overriding principle in determining the relative acidities of uncharged organic acids is the stability of the anion, A-, resulting from the loss of a proton the more stable the anion, the greater the acidity of HA Ways to stabilize anions include having the negative charge on a more electronegative atom on a larger atom delocalized through resonance delocalized by the inductive effect in an orbital with more s character 4-19 Molecular Structure and Acidity A. Electronegativity of the atom bearing the negative charge • within a period, the greater the electronegativity of the atom bearing the negative charge, the more strongly its electrons are held, the more stable the anion is, and the stronger the acid Acid M ethanol CH3 O H pKa 16 M ethylamine CH3 N H pKa 38 H H Ethane CH3 C H pKa 51 H Conjugate base CH3 O – M ethoxide ion CH3 N – M ethylamide ion H H CH3 C – Ethyl anion H 4-20 Molecular Structure and Acidity B. Size of the atom bearing the negative charge • within a column of the Periodic Table, acidity is related to the size of the the atom bearing the negative charge • atomic size increases from top to bottom of a column • the larger the atom bearing the charge, the greater its stability CH3 S H + CH3 O – Methan ethiol Meth oxid e p Ka 7.0 ion (stronger acid ) (stronger bas e) CH3 S – + CH3 O H Methan ethiolate Methanol ion p Ka 16 (w eaker base) (w eaker acid) 4-21 Molecular Structure and Acidity C. Resonance delocalization of charge in A• the more stable the anion, the farther the position of equilibrium is shifted to the right • compare the acidity alcohols and carboxylic acids • ionization of the O-H bond of an alcohol gives an anion for which there is no resonance stabilization CH3 CH2 O-H + H2 O A n alcohol CH3 CH2 O - + + H3 O pK a = 15.9 An alk oxide ion 4-22 Molecular Structure and Acidity • ionization of a carboxylic acid gives a resonancestabilized anion • the pKa of acetic acid is 4.76 O CH3 C O + H2 O O H CH3 C O + + H3 O CH3 C O O equ ivalen t contrib uting structu res ; the carb oxylate anion is stab ilized by delocalization of the n egative ch arge. • carboxylic acids are stronger acids than alcohols as a result of the resonance stabilization of the carboxylate anion 4-23 Molecular Structure and Acidity D. Electron-withdrawing inductive effect • the polarization of electron density of a covalent bond due to the electronegativity of an adjacent covalent H F bond H C-CH2 O-H H F C-CH2 O-H Ethanol pK a 15.9 2,2,2-Tri fl uoroethano l pK a 12.4 F • stabilization by the inductive effect falls off rapidly with increasing distance of the electronegative atom from the site of negative charge CF3 -CH2 -OH CF3 -CH2 -CH2 -OH CF3 -CH2 -CH2 -CH2 -OH 2,2,2-Trifluoroethan ol (pK a 12.4) 3,3,3-Trifluoro-1prop anol (p Ka 14.6) 4,4,4-Trifluoro-1butan ol (pK a 15.4) 4-24 Molecular Structure and Acidity • we also see the operation of the inductive effect in the acidity of halogen substituted carboxylic acids O O OH Bu tanoic acid pK a 4.82 Cl Cl OH O O OH OH Cl 4-Ch lorob utan oic 3-Ch lorobutan oic 2-Chlorob utanoic acid acid acid p Ka 4.52 pK a 3.98 pK a 2.83 4-25 Molecular Structure and Acidity E. Hybridization • for anions differing only in the hybridization of the charged atom, the greater the % s character to the hybrid orbital of the charged atom, the more stable the anion • consider the acidity of alkanes, alkenes, and alkynes (given for comparison are the acidities of water and ammonia) Weak Acid Conjugate p Ka Bas e Water HO-H HO– Alk yn e HC C H HC C – 25 – 38 H2 N – Alken e CH2 =CH-H CH2 =CH Alkan e CH3 CH2 -H CH 3 CH2 – 44 51 In c re a si n g a ci d it Ammonia H2 N-H 15.7 4-26 Lewis Acids and Bases Lewis acid: any molecule of ion that can form a new covalent bond by accepting a pair of electrons Lewis base: any molecule of ion that can form a new covalent bond by donating a pair of electrons A Lewis acid + :B Lewis base - + A B new covalent bond formed in this Lewis acid-base reaction 4-27 Lewis Acids and Bases • examples : H : Br : H H H sec-Butyl cation (a carbocation) CH3 CH 2 + : : + CH3 -C C- CH3 : Br : Bromide ion F B F CH3 -C C- CH3 H H 2-Bromobutane CH3 CH 2 F + :O: + :O B- F CH3 CH 2 F F CH3 CH 2 Diethyl ether Boron trifluoride A BF3-ether complex (a Lewis base) (a Lewis acid) 4-28 Acids and Bases End Chapter 4 4-29