Models of Acids and Bases. Acid-Base Equilibrium Conjugate Acid/Base Pairs. Weak Acids QUESTION: 1. Acids & Bases/ Organic Chemistry

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1 Chapter 1 Structure and Bonding Acids and Bases Acids & Bases/ rganic Chemistry Dr. Ron Rusay Fall 2009 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 Cl acid base Comprehensive Tutorial Acid & Base Principles in rganic Chemistry ighly recommended viewing Acid-Base Equilibrium Conjugate Acid/Base Pairs A(aq) + 2 (l) 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. ConjugateAcidBaseActivity.swf QUESTI: 1 Aniline, C 6 5 2, was isolated in the 1800s and began immediate use in the dye industry. What is the formula of the conjugate acid of this base? A. C B. C C. C 6 5 D. C E. C 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]

2 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 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 Which is the stronger acid? methanol Which is the stronger base? methylamine QUESTI: 2 QUESTI: 3 Use information on the table above to determine which of the following bases would have the weakest conjugate acid: Use information on the table above to determine the order of increasing base strength for the following bases: A. C 6 5 B. C C. Cl D. 3 C 6 5 ; C ; Cl ; 3 C 6 5 ; C ; Cl 2 ; 3 A. C 6 5 < 3 < C < Cl 2 B. C < Cl 2 < 3 < C 6 5 C. Cl 2 < C < 3 < C 6 5 D. 3 < C 6 5 < C < Cl 2

3 Acid and Base Equilibria QUESTI: 4 Consider the following equilibria. Identify the weaker of the two: acid vs. its conjugate acid in each reaction. Which reactions favor formation of product? The equilibrium favors the weaker of the acid vs. its conjugate acid or base vs. its conjugate. Weak wins! Weak is favored! C3C pka=4.7 C3C C!C + 2 pk a= pka=50 Cl + C3 pka= C pka=9 Cl + pk a < 15 Cl + Cl + C32 pka-= 7 pka= 2.5 C3C2 pka=36 pk a= C!C + 3 pka=36 C pka=36 A. All do: 1-6 B. 1, 4, 5 C. 2, 3, 6 D. 1, 5 E. one do. Worksheet 5: Acids & Bases 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. This can be used in predicting how molecules will react--- or donʼt react, and the products of reactions. Base / nucleophile Formal Charge / Acids & Bases Electrophiles / ucleophiles / Reactivity 3 C C3 C3-1 : : C 3 C 3 3 C :: C3 : : 3 C 3 C C: : : 3 C :: : 3 C C 3 C 3 :: : : 3 C 3 C C 2 :: : : 3 C :: : C +1 Acid / electrophile C 2 Worksheet 1 Structure and Acid-Base Properties Important factors that effect acidity in binary compounds, eg Cl (having only two elements): Bond Length (shorter = stronger bonds; lower acidity) Bond Strength (longer = weaker bonds; higher acidity: more dissociation, 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 xyacids (Three atoms: ternary vs. binary) Push-Pull electronic effects on the proton. Therefore Br is a stronger acid than F. QUESTI: 5 Rank 1.0M solutions of Br, I and Cl in order of increasing acidity. Br, K a = 2.1 x 10-8 I, K a = 2.3 x Cl, K a = 3.0 x 10-8 A) Br < I < Cl B) I < Br < Cl Question: 6 True (A) / False (B) Br 4 is a weaker acid than Br 2. Cl 2, K a = 1.2 x 10-2 Br, K a = 2.06 x 10-8 I, K a = 2.3 x Cl, K a = 3.0 x 10-8 C) Cl < Br < I D) I < Cl < Br QUESTI: 7 Rank 1.0M solutions of Br, I and Cl in order of increasing p Br, K a = 2.1 x 10-8 I, K a = 2.3 x Cl, K a = 3.0 x 10-8 A) Br < I < Cl B) I < Br < Cl C) Cl < Br < I D) I < Cl < Br QUESTI: 8 Use the concept of push-pull. Rank the following organic acids in order of decreasing acidity. 1) Br-C 2 C 2) I-C 2 ) C A) 2 > 1 > 3 B) 3 > 2 > 1 C) 2 > 3 > 1 D) 1 > 2 > 3

5 QUESTI: 9 Rank the following acids in order of decreasing acidity. A) ClC 2 C B) Cl 2 CC C) Cl 3 CC D) C 1) A > B > C > D 2) C > B > A > D 2) D > C > B > A 3) B > C > A > D Question: 10 True (A) / False (B) Trichloroacetic acid, Cl 3 CC, is more acidic than trifluoroacetic acid, 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 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 + Cl - 3 Solubility: C 2 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?

6 Acid -Base Reactions Acid Base Reactions + Cl - C 2 C 3 C 2 + Cl 2 C 2 C Cocaine ydrochloride + Cl - C 2 2 C + - C 2 2 C "Crack" Cocaine Which form, Acid or its Conjugate Base? The p of a solution (its surroundings) determines which and is related by the following equation. pk a = p + log [ A] [ A ]! A compound will exist primarily in its acidic form if the p of the solution is < than its pk a A weak acid RC 2 with a pka = % acid 50% base A compound will exist primarily in its basic form if the p of the solution is > than its pk a TE: A buffer solution maintains a nearly constant p within certain parameters. 99% acid 1% base 90% acid 10% base 10% acid 90% base 1% acid 99% base QUESTIS from Worksheet: The pka of a general anesthetic, sodium pentothal, is 7.4. If a patient is given sodium pentothal orally instead of iv, will it put the patient to sleep? What information is needed to answer this fundamental anesthesiology ( gas passer ) question? A drug has a pka of 7.8 and is a known teratogen. If given iv to a pregnant woman whose blood p is within normal levels, will this drug cross the placenta and affect the baby? What information is needed to answer this anesthesiology ( gas passer ) question? Lewis Acids and Bases Lewis Acid: electron pair acceptor Lewis Base: electron pair donor Example: Al Al 6 3+

7 Question: 11 Select the chemical equation that depicts the correct movement of electrons in the reactionof ammonia with hydrogen chloride. A) B) C) D) Acid-Base Reactions Showing a reaction with arrows Acid-Base Reactions Predicting Products C3SC3 + BF3 () 2S BF 3 C + C B() 3 + () 3 C3 + C3S C3 + C3S 3 + C3CC C3CC3 3C C 3C + MgCl2 C!C + 2C= C!C C2 2S4 + C3C3 C2=C2 + Cl C2 C3 + Cl BF 3 + I + C3Br IC3 + Br C + AlCl3 Worksheet 5: Acids & Bases Worksheet 5: Acids & Bases

8 Predicting if reactions occur and the products if they do a. + Br Br + C3 + Br + Br b. C!C + 2C= C!C C2- C!C + 2C= C!C C2 c, 2 + C3 2 C C3 2 + C3 Worksheet 5: Acids & Bases