《有机化学英文课件》由会员分享，可在线阅读，更多相关《有机化学英文课件（29页珍藏版）》请在金锄头文库上搜索。
1、Organic Chemistry,William H. Brown Christopher S. Foote Brent L. Iverson,Acids and Bases,Chapter 4,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 th
2、is 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,Brnsted-Lowry Definitions,Acid: a proton donor Base: a 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 reaction: a proton-transfer reaction conjugate acid-base pair: any pair of molecules or ions that
4、 can be interconverted by transfer of a proton,Conjugate Acids & Bases,Brnsted-Lowry definitions do not require water as a reactant consider the following reaction between acetic acid and ammonia,Conjugate Acids & Bases,we can use curved arrows to show the flow of electrons in an acid-base reaction,
5、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: whi
6、ch oxygen of a carboxylic acid is protonated?,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 A-1 and A-3 make the greater contribution because all atoms have complete octets t
7、he positive charge is delocalized over three atoms with the greater share on the two equivalent oxygens,Conjugate Acids & Bases,for protonation on the hydroxyl oxygen, we can write two contributing structures B-2 makes only a minor contribution because of charge separation and adjacent positive char
8、ges therefore, we conclude that protonation of a carboxylic acid occurs preferentially on the carbonyl oxygen,Conjugate Acids & Bases,Problem 4.3 Does proton transfer to an amide group occur preferentially on the amide oxygen or the amide nitrogen?,Pi Electrons As Basic Sites,Proton-transfer reactio
9、ns 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 the result is formation of a carbocation, a species in which one of its carbons ha
10、s only six electrons in its valence shell and carries a charge of +1,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,Acids & Base Strengths,The strength of an acid is expressed by an equilibrium c
11、onstant the acid dissociation of acetic acid is given by the following equation,Weak Acids and Bases,We can write an equilibrium expression for the dissociation of any uncharged acid, HA, as: water is a solvent and its concentration is a constant equal to approximately 55.5 mol/L we can combine thes
12、e constants to give a new constant, Ka, called an acid dissociation constant,Acid-Base Equilibria,Equilibrium favors reaction of the stronger acid and stronger base to give the weaker acid and weaker base,Acid-Base Equilibria,Consider the reaction between acetic acid and sodium bicarbonate we can wr
13、ite the equilibrium as a net ionic equation we omit Na+ because it does not undergo any chemical change in the reaction equilibrium lies to the right carbonic acid forms, which then decomposes to carbon dioxide and water,Molecular Structure and Acidity,The overriding principle in determining the rel
14、ative 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 r
15、esonance delocalized by the inductive effect in an orbital with more s character,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 ar
16、e held, the more stable the anion is, and the stronger the acid,C,H,H,H,N,H,H,O,H,i,H,H,H,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,Molecular Structure and Acidity,C. Resona
