hapter 3 Acids and Bases
Basic Definitions Associated with Acids and Bases
Molecular Definitions of Acids and Bases
Molecular Models of Selected Acids
Brønsted-Lowry Theory 1. In a Brønsted-Lowry reaction, an is transferred. (A reaction does not have to take place in aqueous solution.) 2. An acid is an donor; a base is an acceptor. (Base structures contain unshared electron pair.) 3. Acid-Base eaction: -A :B :A - -B
epresentation of Acid-Base eactions with urved Arrows
l 2 l - 3 acid donor base acceptor lone pair attacks
N3 2 N4 - base acceptor acid donor lone pair attacks
onjugate Pairs 1. In a Brønsted-Lowry reaction, the original base becomes an acid in the reverse reaction, and the original acid becomes a base in the reverse process 2. Each reactant and product are a conjugate pair. 3. The original base becomes a conjugate acid, and the original acid becomes a conjugate base
onjugate Acid Base Pairs -A :B :A - -B acid base conjugate conjugate base acid 2 2 2-3 acid base conjugate conjugate base acid 2 N3: - N4 acid base conjugate conjugate base acid
Acid Strength The Meaning of pka
The Acid Dissociation onstant, K A 2! A! 3 " # 3 K = eq $ " % # A! $ % "# A$ % "# $ 2 % A 2! A! 3 K a = K eq " # 3 "# $ 2 % = $ " % # A! "# A$ % $ %
The Base Dissociation onstant, K A - 2 A - Keq = [A] [ - ] [A - ] [2] A - 2 A - Kb = Keq [2] = [A] [- ] [A - ] Ka x Kb = [3 ] x [ - ] = 1.0 x 10-14
The Meaning of pk pka = log Ka The free energy change in an equilibrium is related to log (Keq). ΔG o = T(ln Keq) = -2.302T(log Keq) A lower value of pka indicates a more negative value for ΔG and a stronger acid. Ka x Kb = [3 ] x [ - ] = 1.0 x 10-14 pka pkb = 14
Short pka Table Functional group Example pka Acid Strength onjugate base Base strength Mineral Acid l -8 l arboxylic Acid 5 3 3 Protonated Amine N 10 N Water 16 Alkyne 25 Amine ~35 N N Alkane ~50
Functional group Example pka onjugate base Functional group Example pka onjugate base Mineral acid I -10 I- Malonates 13 Mineral acid Br, l -9, -6 Br-,l- Water 2 16 - Mineral acid 2S4-3 S4- Protonated carbonyl -2.9 N N ydronium Long ion 3-1.7 pka 2 Nitrile Table 25 Sulfonic acid S3-1 S3- Ester 25 () Alcohols 17 Ketones, aldehydes 20-24 () Mineral acid F 3.2 F- Alkyne 25 arboxylic acid 4 Protonated amine N N Phenols 10 S 9-11 Thiols 13 S Sulfoxide 31 S Amine N ~35 Alkene ~43 Alkane ~50 2 S 2 N
The Meaning of pk Acidic Form Basic Form 100 75 % 50 25 0 3.2 4.2 5.2 6.2 7.2 p p = pka -1 p = pka p = pka 1 p = pka -2 p = pka 2
Acid Strength Based on the Stability of the onjugate Base A A- The strength of acid A (the degree to which the acid ionizes ) depends on the stability of A-.
Acid Strength Based on the Stability of the onjugate Base Factors influencing the stability of a conjugate base: 1. n which atom is the negative charge? 2. Does resonance stabilize the conjugate base? 3. Do inductive factors stabilize the conjugate base? 4. In what type of orbital does the negative charge reside?
Factor #1: The atom which bears a negative charge As anions are created from elements further to the right in the periodic table (* greater electronegativity), the atom is better able to stabilize the negative charge, and the anions become more stable. The corresponding binary conjugate acids become stronger.
Factor #1: The atom which bears a negative charge 4 N3 2 F Increasing electronegativity of central atom Increasing ability of central atom to stabilize anion pka 48 36 16 3 Acid strength
Factor #1: The atom which bears a negative charge As anions are created from elements further down a column in the periodic table, they get larger, spread out electron density, the anions become more stable (weaker bases) even though the atoms become less electronegative. The corresponding binary conjugate acids become stronger as they are created from elements further down a periodic-table column.
Factor #1: The atom which bears a negative charge pka 3.2 Increasing Bond length Decreasing Bond strength Increasing stability of anion Increasing Acid strength -7-8 -9
Factor #2: The effect of resonance pka ~50 ~16 X ~5
pka Factor #2: The effect of resonance ~50 ~19 ~9
Which is the most acidic proton in ascorbic acid (Vitamin )?
Factors influencing the stability of a conjugate base: Factor #1: The atom which bears a negative charge Factor #2: The effect of resonance Factor #3: The influence of inductive factors Factor #4: The influence of orbitals
Factor #3: The influence of inductive factors 3 l 2 l l l l l pka 4.75 2.87 1.25 0.70?? If? is electron-withdrawing, the anion will be stabilized, and the acid will be stronger.
Factor #3: The influence of inductive factors I 2 Br 2 l 2 F 2 pka 3.10 2.90 2.87 2.65 3?? pka 4.75 The effect is greater with a more electronegative element.
Factor #4: The influence of orbitals
Factor #4: The influence of orbitals pka Least acidic 50 44 25 Most acidic
Electrons reside in an orbital Electrons sp 3 furthest from nucleus sp 2 Acid strength sp closest to nucleus
hapter 3 omparing the Strength of Acids
Base Strength
The Meaning of pk pka = log Ka The free energy change in an equilibrium is related to log (Keq). ΔG o = T(ln Keq) = -2.302T(log Keq) A lower value of pka indicates a stronger acid. Ka x Kb = [3 ] x [ - ] = 1.0 x 10-14 pka pkb = 14
Acid-Base Equilibria Principle Involved: In an equilibrium between two acid-base conjugate pairs, the equilibrium will favor the formation of the weaker acid and the weaker base. stronger stronger weaker weaker base acid acid base
Predict the position of the following equilibrium:? pka: 16 50 stronger weaker stronger acid acid weaker base base
Predict the position of the following equilibrium: N 2? N pka: 38 4.8 weaker stronger acid weaker stronger acid base base N 2 N
A1 2 A1-3 1 A2-3 A2 2 2 A1 A2- A1- A2 3 Keq 1 = [A1-][3 ] [A1][2] = Ka1/[2] Keq 2 = [A2][2] [A2-][3 ] = [2]/Ka2 Keq 3 = (Keq 1)(Keq 2) log Keq 3 = logka1-logka2 Keq 3 = Ka1/Ka2 log Keq 3 = -(-logka1)(-logka2) pka2 - pka1
elationship Between ΔGº and Keq at 25º ΔGº(kJ/mol) ΔGº(kcal/mol) Keq logkeq 17.1 4.1 0.001-3 11.4 2.7 0.01-2 5.7 1.4 0.1-1 0 0 1 0-5.7-1.4 10 1-11.4-2.7 100 2-17.4-4.1 1000 3 everse reaction goes to completion. Forward and everse reaction proceed to same extent. Forward reaction goes to completion. log Keq 3 = pka2 - pka1
Predict the position of the following equilibrium:? pka: 10 16 stronger stronger weaker weaker acid base base acid
Predict the position of the following equilibrium: N? N pka: 25 38 stronger weaker stronger acid acid weaker base base N N
Predict the position of the following equilibrium: N? N pka: 16 38 stronger weaker stronger acid acid weaker base base N N
Predict the position of the following equilibrium:? pka: 10-1.7 weaker stronger acid weaker stronger acid base base
Acid-Base Equilibria Functional group Example pka Acid Strength onjugate base Base strength l Mineral Acid -8 l arboxylic Acid 5 Protonated Amine 3 N 3 N 10 Water 16 Alkyne 25 Amine ~35 N N Alkane ~50 l can protonate the conjugate base of any acid whose pka is higher than -8. The anion of an alkane can deprotonate any acid whose pka is lower than 50.
hoosing Solvents and eagents
hoosing Solvents and eagents What happens when you dissolve sodium acetate in water?? 3 Na 3 Na pka: 16 5 weaker base weaker acid stronger acid stronger base onclusions: 1. You obtain a slightly basic solution. 2. Water is not acidic enough to protonate the acetate ions.
hoosing Solvents and eagents What happens when you dissolve sodium acetate in aq l?? 3 Na l- 3 pka: -8 5 Nal stronger base stronger acid weaker acid weaker base onclusions: 1. The solution remains acidic. 2. Mineral acid is acidic enough to protonate acetate ions.
hoosing Solvents and eagents What happens when you dissolve sodium methylate in water?? 3 Na 3 water methanol pka: 16 17 Na stronger acid weaker acid?? onclusions: 1. The solution becomes basic. 2. The solution contains a substantial amount of hydroxide which may interfere with reactions of methylate ions. 3. Water is not a good solvent for reactions of methylate.
3 hoosing Solvents and eagents What happens when you dissolve sodium methylate in acetonitrile?? Na Na 3 N 3 N acetonitrile pka: 25 17 methanol weaker base weaker acid stronger acid stronger base onclusions: 1. The solution contains very little of anion from acetonitrile which might interfere with reactions of methylate ions. 2. Acetonitrile is a good solvent for reactions of methylate.
ounterions ow the structure appears What it means ow the structure appears What it means N N M X - M X - M 3 3 X -
Lewis Acids and Bases electron pair donor = Lewis Base = nucleophile electron pair acceptor = Lewis Acid = electrophile Lewis acid-base reactions include all the previously described acid-base reactions plus additional types.
Molecules as Lewis Acids and Bases
The reaction of the Lewis acid BF3 with the Lewis base N3 -
Boron Trifluoride Etherate -