Models of Acids and Bases. Acid-Base Equilibrium Conjugate Acid/Base Pairs. Weak Acids

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1 Chapter 1 Electronic Structure and Bonding Acids and Bases Acids & Bases in rganic Chemistry Dr. Ron Rusay Fall 2004 Models of Acids and Bases Arrhenius: Acids produce + & bases produce - ion in aqueous solutions. Brønsted-Lowry: Acids are + donors & bases are proton acceptors. Cl + 2 Æ Cl acid base Acid-Base Equilibrium Conjugate Acid/Base Pairs A(aq) + 2 (l) Æ 3 + (aq) + A - (aq) conj conj acid 1 base 2 acid 2 base 1 conjugate acid: formed when the proton is transferred to the base. conjugate base: everything that remains of the acid molecule after a proton is lost. Weak Acids Weak acids are only partially ionized in solution. A(aq) + 2 (l) A(aq) [ ][A ] K a = [A] or 3 + (aq) + A - (aq) + (aq) + A - (aq) K a is the acid dissociation constant. [ + ][A - ] K a = [A] rganic Acids & Bases rganic acids are weak acids, eg. Acetic acid. owever, there can be substantial differences in their relative strengths. What could you use to compare relative acidities? rganic bases are weak bases and relate to ammonia. owever, there can be substantial differences in their relative strengths. What could you use to compare relative basicity? Conjugates The equilibrium favors the weaker of the acid vs. its conjugate or base vs. its conjugate. Weak wins! Ka x Kb =? K w What do pk a and pk b refer to? -log K a or -log Kb pka + pkb =? 14 Remember: p + p = pk w 1

Cl + 2 Æ Cl - + 3 + acid base Acid-Base Equilibrium Conjugate Acid/Base Pairs A(aq) + 2 (l) Æ 3 + (aq) + A - (aq) conj conj acid 1 base 2 acid 2 base 1 conjugate acid: formed when the proton is

2 Which is the stronger acid? methanol Which is the stronger base? methylamine Acid and Base Equilibria Strong vs. weak: Weak is favored! Acid-Base Strength & Reactions Consider the following pairs of equilibrium reactions. Identify the weaker of the two: acid vs. its conjugate acid in each reaction. Which reaction in each pair favors formation of product? C pka=4.7 C Cl + pka= C + 3 C2 pka=36 pk a= pka=9 + 2 Cl + C 3 pk a < 15 rganic Acids & Bases rganic molecules in context can be considered as behaving relatively as weak acids or weak bases. Formal Charge is important in considering which. Knowing the Formal Charge allows a prediction. (+) positive atoms behave acid-like, (-) negative atoms behave base-like. Cl + Cl + 2 pka-= 7 pka= 2.5 C C + 2 C C + 3 pk a=26 pka=36 This can be used in predicting how molecules will react--- or don t react, and the products of reactions. + pka=50 2 C2 + 3 pka=36 2

Which reaction in each pair favors formation of product? C pka=4.7 C Cl + pka=15.

3 Formal Charge = # of valence e-s [1/2(# of bonding e-s ) + # of non-bonding e-s ] : : For : = 5-[1/2(4)+4] = -1 Base : C 3 3 C : : C 3 : : : : C: 3C 3C C2 C C 3 : : : C 3 : 3 C : For : = 6-[1/2(6)+2] = +1 C2 Acid Structure and Acid-Base Properties Important factors that effect acidity in binary compounds, eg Cl (having only two elements): Bond Length (shorter = stronger; favors lower acidities) Bond Strength (weak bonds favor higher acidity: more protons [hydronium ions] in solution) Bond Polarity (smaller e.n. differences favor higher acidities) - Select & explain which is the stronger acid: Br or F. Therefore Br is a stronger acid than F. Strength of xyacids (Three atoms: ternary vs. binary) Push-Pull electronic effects on the proton. Strength of xyacids ame and rank Br, I and Cl: (1) in order of increasing p and (2) in order of increasing acidity. Br, Ka = 2.06 x 10-8 I, Ka = 2.3 x Cl, Ka = 3.0 x 10-8 Is Cl 4 a stronger or weaker acid than Cl? Will this help? Cl2, Ka = 1.2 x

Bond Strength (weak bonds favor higher acidity: more protons [hydronium ions] in solution) Bond Polarity (smaller e.n. differences favor higher acidities) - Select & explain which is the stronger acid: Br or F.

4 Strength of Acids Use the concept of push-pull. Rank the following organic acids in order of increasing acidity. Strength of Acids Use the concept of push-pull. Rank the following acids in order of increasing acidity. Cl-C 2 C, Cl 2 -CC, Cl 3 CC, C 3 C Br-C 2 C, I-C 3 C, C 3 C pka= 2.69 pka=3.12 pka=4.75 Is chloroacetic acid more or less acidic than bromoacetic acid? Will its pka be higher or lower than bromoacetic acid? Which is the stronger acid? Cl 3 CC or F 3 CC An rganic Base in Context Erythroxylon spp. It is very valuable. The leaves are chewed by indigenous tribes in the Andes to boost their energy. It has been used as a psycho-therapeutic, an opthalmic anesthetic and was purportedly used in a popular beverage that is at the heart of a $20 billion corporation. owever, both its base and conjugate acid are currently controlled substances under U.S. Federal Regulations: Title 21 secs & (1987). Can you name the beverage and the base? The beverage reportedly produced using the extract of leaves of Erythroxylon coca: The compound: cocaine, is an organic base: Merck Index, #2450, 11th ed.: Caution: May be habit forming. C 3 Acid -Base Chemistry (Physical Properties) C 2 C 3 2 C "Crack" Cocaine What structural feature makes cocaine a base? What simple compound can you relate it to? m.p. 98 o C b.p. (very volatile > 90 o C) Solubility: Water: 1.67 x 10-3 g/ml CCl 3 : 1.43 g/ml Ether: 0.29 g/ml Regular Cocaine Conjugate Acid of Cocaine (Physical Properties) m.p. >195 o C C 3 + Cl - Solubility: C 2 C 3 Water: 2.5 g/ml CCl 3 : 0.08 g/ml 2 C Ether: insoluble Cocaine ydrochloride What accounts for the differences in solubilities of the base and conjugate acid? 4

75 Is chloroacetic acid more or less acidic than bromoacetic acid? Will its pka be higher or lower than bromoacetic acid? Which is the stronger acid?

5 Acid -Base Reactions Acid Base Reactions C 3 C 3 + Cl - C 2 C 3 C 2 C 3 + Cl 2 C 2 C Cocaine ydrochloride C 3 + Cl - C 2 C 3 2 C + - C 3 C 2 C 3 2 C "Crack" Cocaine Which form, Acid or its Conjugate Base? The p of the solution (surroundings) determines it. pk a = p + log [ A] [ A ] - A compound will exist primarily in its acidic form if the p is < than its pk a A weak acid with a pka = 5.2 A compound will exist primarily in its basic form if the p is > than its pk a TE: A buffer solution maintains a nearly constant p within certain parameters. Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor Example: Al Al

pk a = p + log [ A] [ A ] - A compound will exist primarily in its acidic form if the p is < than its pk a A weak acid with a pka = 5.

6 Acid-Base Reactions Showing a reaction with arrows Acid-Base Reactions Predicting Products S + BF3 (C 3) 2S BF 3 C + C3 C C 3 2 B(C 3) 3 + (C 3) 3 + S + S 3 + C C C + MgCl2 C C + 2C= C C C2 2S4 + C2=C2 + Cl C2 + Cl BF3 + I + Br I + Br C 3C + AlCl3 Predicting if reactions occur and the products if they do a. + Br Br + + Br + Br b. C C + 2C= C C C2- C C + 2C= C C C2 c,

C C2 2S4 + C2=C2 + Cl C2 + Cl BF3 + I + Br I + Br C 3C + AlCl3 Predicting if reactions occur and the

Acids and Bases: A Brief Review

Acids and Bases: A Brief Review Acids and : A Brief Review Acids: taste sour and cause dyes to change color. : taste bitter and feel soapy. Arrhenius: acids increase [H ] bases increase [OH ] in solution. Arrhenius: acid base salt water.