Acids and Bases. Chapter 4. Arrhenius Acids and Bases

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1 Acids and Bases Chapter 4 1 Arrhenius Acids and Bases In 1884, Svante Arrhenius proposed these definits acid: A substance that produces 3 + s aqueous solut. base: A substance that produces - s in aqueous solut. this definit of an acid is a slight modificat of the original Arrhenius definit, which was that an acid is a substance that produces + in aqueous solut. today we know that + reacts immediately with a water molecule to give a hydronium ( l) 3 + ( aq) ydronium 2 1

2 Brønsted-Lowry Definits Acid: proton donor. Base:proton acceptor. 3 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 react: A proton-transfer react. Conjugate acid-base pair: Any pair of molecules or s that can be interconverted by transfer of a proton. conjugate acid-base pair conjugate acid-base pair Cl( aq) + 2 ( l) Cl - ( aq) ( aq) ydrogen Water Chloride ydronium chloride (acid) (base) (conjugate base of Cl) (conjugate acid of 2 ) 4 2

3 Conjugate Acids & Bases Brønsted-Lowry definits do not require water as a reactant. Consider the following react between acetic acid and ammonia. conjugate acid-base pair conjugate acid-base pair C 3 C + N 3 C 3 C - + Acetic acid Ammonia Acetate (acid) (base) (conjugate base acetic acid) N 4 + Ammonium (conjugate acid of ammonia) 5 Acids & Bases Use curved arrows to show the flow of electrons in an acid-base react. : : C 3 -C- : : Acetic acid (proton donor) : : + : N C 3 -C- : N- Ammonia Acetate Ammonium (proton acceptor) : : 6 3

Resonance in Acids Many organic molecules have two or more sites that can act as proton acceptors. In this chapter, discuss is limited to carboxylic acids, esters, and amides.

4 Resonance in Acids Many organic molecules have two or more sites that can act as proton acceptors. In this chapter, discuss is limited to carboxylic acids, esters, and amides. In these molecules, the favored site of protonat is the one in which the charge is more delocalized. Quest: Which oxygen of a carboxylic acid is protonated? C 3 -C S 4 C 3 -C- - or C 3 -C- - + S 4 A B (protonated (protonated on the on the carbonyl oxygen) hydroxyl oxygen) 7 π Electrons As Basic Sites Proton-transfer reacts occur with compounds having π electrons, as for example the π electrons of carboncarbon double and triple bonds. The pi electrons of 2-butene, for example, react with Br by proton transfer to form a new C- bond. + C 3 -C= C-C Br C 3 -C- C-C 3 2-Butene sec-butyl cat (a 2 carbocat) Br 8 4

5 π Electrons As Basic Sites The result is format of a carbocat, a species in which one of its carbons has only six electrons in its valence shell and carries a charge of C 3 -C= C-C Br C 3 -C- C-C 3 2-Butene sec-butyl cat (a 2 carbocat) Br

7 π Electrons As Basic Sites Sample Problem: Draw Lewis structures for the two possible carbocats formed by proton transfer from Br to 2-methyl-2- butene. C 3 C 3 -C=C-C 3 + -Br 2-Methyl-2-buten e 13 Acids & Base Strengths The strength of an acid is expressed by an equilibrium constant. the acid dissociat of acetic acid is given by the following equat. C 3 C + 2 C 3 C - A cetic acid Water Acetate ydronium 14 7

8 Weak Acids and Bases An equilibrium express for the dissociat of any uncharged acid, A, is written as: A + 2 A K eq = [ 3 + ] [A - ] [ A][ 2 ] 15 Weak Acids and Bases Water is a solvent and its concentrat is a constant equal to approximately 55.5 mol/l. We can combine these constants to give a new constant, K a, called an acid dissociat constant. [ K a K eq [ 2 ] 3 + ][A - ] = = [A] 16 8

9 pk a Weaker acid Stron ger acid Acid Fo rm ula pk a Co nju gate Bas e - Ethan e C 3 C 3 51 C 3 C 2 S trong er Ethy lene C 2 =C 2 44 C 2 =C - conju gate - base Am monia N 3 38 N 2 yd rogen Acety lene C C 25 C C - Ethan ol C 3 C C 3 C 2 - Water Methylamm on ium C 3 N C 3 N 2 Bicarbonate - C C 3 Phen ol C C Am mo niu m io n + N N 3 yd ro gen su lfid e 2 S 7.04 S - Carbonic acid 2 C C 3 Acetic acid C 3 C 4.76 C 3 C - Benzoic acid C 6 5 C 4.19 C 6 5 C - Phosp horic acid 3 P P 4 yd ro niu m io n Sulfuric acid 2 S S 4 yd rogen ch loride Cl -7 Cl - yd ro gen brom ide Br -8 Br - Weaker co nju gate yd ro gen iod ide I -9 I - base 17 Acid-Base Equilibria Equilibrium favors react of the stronger acid and stronger base to give the weaker acid and weaker base. C 3 C N 3 C 3 C N 4 Acetic acid p K a 4.76 (stronger acid) Ammonia (stronger base ) Acetate (weaker base) Ammonium p K a 9.24 (weaker acid) Favors the side with the highest pka (i.e weaker acid) 18 9

10 Acid-Base Equilibria Consider the react between acetic acid and sodium bicarbonate. write the equilibrium as a net ic equat. omit Na + because it does not undergo any chemical change in the react. C 3 C + - C 3 C 3 C - + Acetic acid Bicarb on ate Acetate pk a 4.76 (stronger acid) Equilibrium lies to the right. Carbonic acid forms, which then decomposes to carbon dioxide and water. 2 C 3 Carbonic acid pk a 6.36 (weaker acid) 19 Thermochemistry & Mechanisms React mechanism: A step-by-step descript of how a chemical react occurs. Thermochemistry: The study of the energy of an entire system at each and every instant of a react. React coordinate diagram: A graph showing the energy changes that occur during a chemical react. Energy is plotted on the vertical axis and react progress (time) is plotted along the horizontal axis

11 Thermochemistry & Mechanisms Most chemical reacts occur via colliss. During colliss, the structure of a molecule contorts and flexes. Colliss of higher energy lead to larger distorts in structure. During colliss, the kinetic energy of the reactants is converted to potential energy, which is stored in the chemical structures in the form of structural strains. 21 Thermochemistry & Mechanisms During a collis process that yields a react, a transit state ( ) is formed which possesses partially broken and partially formed bonds. Chemists use react coordinate diagrams to show the changes in energy for the molecules involved in a chemical react

12 Thermochemistry & Mechanisms A react coordinate diagram for a one step react between -A and B. 23 Thermochemistry & Mechanisms For reacts at constant pressure, the change in Gibbs free energy, G, is used. This kind of energy controls the rates and equilibrium of reacts. The change in Gibbs free energy between reactants and products is related to the equilibrium constant through this equat: G = -RT ln Keq where R = 8.31J/K mol and T is the temperature in kelvins. Free energy of activat: The difference in energy between reactants and the transit state

13 Molecular Structure and Acidity The overriding principle in determining the relative acidities of uncharged organic acids is the stability of the an, A -, resulting from the loss of a proton. the more stable the an, the greater the acidity of A 25 Molecular Structure and Acidity Ways to stabilize ans include having the negative charge: 1. on a more electronegative atom. 2. on a larger atom. 3. delocalized through resonance. 4. delocalized by the inductive effect. 5. in an orbital with more s character

negative charge, the more strongly its electrons are held, the more stable the an is, and the stronger the acid. 27 Size of the atom 2.

14 Electronegativity 1. 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 an is, and the stronger the acid. 27 Size of the atom 2. Size of the atom bearing the negative charge The larger the atom bearing the charge, the greater its stability. C 3 S Methanethiol pk a 7.0 (stronger acid) + C 3 C 3 S + C 3 Methoxide (stronger base) Methanethiolate (weaker base) Methanol pk a 16 (weaker acid) 28 14

15 Resonance delocalizat 3. Resonance delocalizat of charge in A - The more stable the an, the farther the posit of equilibrium is shifted to the right. Compare the acidity alcohols and carboxylic acids. Ionizat of the - bond of an alcohol gives an an for which there is no resonance stabilizat: C 3 C C 3 C pk a = 15.9 An alcohol An alkoxide 29 Resonance delocalizat Ionizat of a carboxylic acid gives a resonance-stabilized an. The pk a of acetic acid is 4.76 C 3 C + C 3 C C 3 C equivalent contributing structures; the carboxylate an is stabilized by delocalizat of the n egative ch arge. Carboxylic acids are stronger acids than alcohols as a result of the resonance stabilizat of the carboxylate an

16 e - withdrawing inductive effect 4. Electron-withdrawing inductive effect The polarizat of electron density of a covalent bond due to the electronegativity of an adjacent covalent bond. C-C 2 - Ethanol pk a 15.9 F F C-C 2 - F 2,2,2-Tri fl uoroethano l pk a e - withdrawing inductive effect 4. Electron-withdrawing inductive effect Stabilizat by the inductive effect falls off rapidly with increasing distance of the electronegative atom from the site of negative charge in the conjugate base. CF 3 -C 2 - CF 3 -C 2 -C 2 - CF 3 -C 2 -C 2 -C 2-2,2,2-Trifluoroethan ol (pk a 12.4) 3,3,3-Trifluoro-1- prop anol (pk a 14.6) 4,4,4-Trifluoro-1- butan ol (pk a 15.4) 32 16

17 e - withdrawing inductive effect the operat of the inductive effect in the acidity of halogen substituted carboxylic acids is also observed: Butanoic acid pk a 4.82 Cl 4-Ch lorob utan oic acid pk a 4.52 Cl 3-Ch lorobutan oic acid pk a 3.98 Cl 2-Chlorob utanoic acid pk a ybridizat (of the an) 5. ybridizat For ans differing only in the hybridizat of the charged atom, the greater the percentage s character to the hybrid orbital of the charged atom, the more stable the an. Weak Acid Water Alkyne - C C C C Ammonia 2 N- 2 N 38 Alkene Alkane Conjugate Base C 2 = C- C 2 = C C 3 C 2 - C 3 C 2 pk a Increasing acidity 34 17

18 In summary: 1.Negative charge on the more electronegative atom. 2.Negative charge on a larger atom. 3.Delocalizat of the negative charge through resonance. 4.Delocalizat of the negative charge onto electron-withdrawing groups by the inductive effect. 5.ave the negative charge in an orbital with more s character

Chapter 2 Polar Covalent Bonds: Acids and Bases

John E. McMurry www.cengage.com/chemistry/mcmurry Chapter 2 Polar Covalent Bonds: Acids and Bases Modified by Dr. Daniela R. Radu Why This Chapter? Description of basic ways chemists account for chemical