Acids & Bases/ Organic Chemistry Dr. Ron Rusay Fall 2009 Chapter 1 Structure and Bonding Acids and Bases

Models of Acids and Bases Arrhenius: Acids produce H + & bases produce OH  ion in aqueous solutions. Brønsted-Lowry: Acids are H + donors & bases are proton acceptors. HCl + H 2 O  Cl  + H 3 O + acid base

Comprehensive Tutorial Acid & Base Principles in Organic Chemistry Highly recommended viewing

Acid-Base Equilibrium Conjugate Acid/Base Pairs HA(aq) + H 2 O(l)  H 3 O + (aq) + A  (aq) 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

QUESTION: 1 Aniline, C 6 H 5 NH 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 6 H 5 NH 2 + B.C 6 H 5 NH 3 + C.C 6 H 5 NH – D. C 6 H 5 NH + E. C 6 H 5 NH 2

Weak Acids Weak acids are only partially ionized in solution. or K a is the acid dissociation constant.

Organic Acids & Bases Organic acids are weak acids, eg. Acetic acid. However, there can be substantial differences in their relative strengths. What could you use to compare relative acidities? Organic bases are weak bases and relate to ammonia. However, there can be substantial differences in their relative strengths. What could you use to compare relative basicity?

Conjugates K a x K b = ? What do pK a and pK b refer to? pK a + pK b = ? KwKw -log K a or -log K b 14 Remember: pH + pOH = pK w

Which is the stronger acid? Which is the stronger base? methanol methylamine

QUESTION: 2 Use information on the table above to determine which of the following bases would have the weakest conjugate acid: OC 6 H 5 – ; C 2 H 3 O 2 – ; OCl – ; NH 3 A.OC 6 H 5 – B.C 2 H 3 O 2 – C.OCl – D.NH 3

QUESTION: 3 Use information on the table above to determine the order of increasing base strength for the following bases: OC 6 H 5 – ; C 2 H 3 O 2 – ; ClO 2 – ; NH 3 A.OC 6 H 5 – < NH 3 < C 2 H 3 O 2 – < ClO 2 – B.C 2 H 3 O 2 – < ClO 2 – < NH 3 < OC 6 H 5 – C.ClO 2 – < C 2 H 3 O 2 – < NH 3 < OC 6 H 5 – D.NH 3 < OC 6 H 5 – < C 2 H 3 O 2 – < ClO 2 –

Acid and Base Equilibria Weak is favored! The equilibrium favors the weaker of the acid vs. its conjugate acid or base vs. its conjugate. Weak wins!

QUESTION: 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? A.All do: 1-6 B.1, 4, 5 C.2, 3, 6 D.1, 5 E.None do. Worksheet 5: Acids & Bases

Organic Acids & Bases Organic 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 don’t react, and the products of reactions.

Formal Charge / Acids & Bases Electrophiles / Nucleophiles / Reactivity +1 Base / nucleophile Acid / electrophile Worksheet 1

Structure and Acid-Base Properties Important factors that effect acidity in binary compounds, eg HCl (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: HBr or HF.

Therefore HBr is a stronger acid than HF.

Strength of Oxyacids (Three atoms: ternary vs. binary) Push-Pull electronic effects on the proton.

QUESTION: 5 Rank 1.0M solutions of HBrO, HIO and HClO in order of increasing acidity. HBrO, K a = 2.1 x HIO, K a = 2.3 x HClO, K a = 3.0 x A) HBrO < HIO < HClOB) HIO < HBrO < HClO C) HClO < HBrO < HIOD) HIO < HClO < HBrO

Question: 6 True (A) / False (B) HBrO 4 is a weaker acid than HBrO 2. HClO 2, K a = 1.2 x HBrO, K a = 2.06 x HIO, K a = 2.3 x HClO, K a = 3.0 x 10 -8

QUESTION: 7 Rank 1.0M solutions of HBrO, HIO and HClO in order of increasing pH HBrO, K a = 2.1 x HIO, K a = 2.3 x HClO, K a = 3.0 x A) HBrO < HIO < HClOB) HIO < HBrO < HClO C) HClO < HBrO < HIOD) HIO < HClO < HBrO

QUESTION: 8 Use the concept of push-pull. Rank the following organic acids in order of decreasing acidity. 1) Br-CH 2 COOH 2) I-CH 2 COOH 3) CH 3 COOH A) 2 > 1 > 3 B) 3 > 2 > 1 C) 2 > 3 > 1 D) 1 > 2 > 3

QUESTION: 9 Rank the following acids in order of decreasing acidity. A) ClCH 2 COOH B) Cl 2 CHCOOH C) Cl 3 CCOOH D) CH 3 COOH 1) A > B > C> D2) C > B > A > D 2) D > C > B > A3) B > C > A > D

Question: 10 True (A) / False (B) Trichloroacetic acid, Cl 3 CCOOH, is more acidic than trifluoroacetic acid, F 3 CCOOH.

An Organic 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. However, 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….

Acid -Base Chemistry (Physical Properties) m.p. 98 o C b.p. (very volatile > 90 o C) Solubility: Water: 1.67 x g/mL CHCl 3 : 1.43 g/mL Ether: 0.29 g/mL What structural feature makes cocaine a base? What simple compound can you relate it to?

“Regular” Cocaine Conjugate Acid of Cocaine (Physical Properties) m.p. >195 o C Solubility: Water: 2.5 g/mL CHCl 3 : 0.08 g/mL Ether: insoluble What accounts for the differences in solubilities of the base and conjugate acid?

Acid -Base Reactions

Acid Base Reactions

Which form, Acid or its Conjugate Base? A compound will exist primarily in its acidic form if the pH of the solution is < than its pK a A compound will exist primarily in its basic form if the pH of the solution is > than its pK a NOTE: A buffer solution maintains a nearly constant pH within certain parameters. The pH of a solution (its surroundings) determines which and is related by the following equation.

A weak acid RCO 2 H with a pKa = % acid 1% base 90% acid 10% base 50% acid 50% base 10% acid 90% base 1% acid 99% base

QUESTIONS 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 pH 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:

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 Worksheet 5: Acids & Bases

Acid-Base Reactions Predicting Products Worksheet 5: Acids & Bases

Predicting if reactions occur and the products if they do Worksheet 5: Acids & Bases