Chapter 4: Acids & Bases

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Ross Gregory
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1 Chapter 4 utline: Acids & Bases 1. What are acids & bases? 2. How are acid dissociation constants? 3. How does one define relative strengths of acids (pka)? 4. How does one determine equilibria in acid-base reactions? 5. How does molecular structure correlate to acidity? 6. What are Lewis Acids & Lewis Bases? 7. What are Nucleophiles & Electrophiles? You should be do all the problems at the end of the chapter, but at the very least do these: , , , 4.32, 4.33, 4.36, 4.38, 4.41, , Review this material from C117 (thermochemistry/thermodynamics; section 4.5): Page 1 of 23

2 1. Defining Acids & Bases What is an acid? Lewis Acid = Brønsted-Lowry acid = What is a base? Lewis Base = Brønsted-Lowry base = Page 2 of 23

3 Generic acid-base reaction: B: + H A + B H + A - Arrow-pushing is important: Simple example (do these reactions lie to the right or left as written?): ammonia hydronium ion ammonium water By definition, the CA and CB formed are ALWAYS weaker than the ones you start with. With acid-base reactions, you will need to learn how to: Provide products for the reaction Draw arrows for reaction progress Identify each reactant as Nu or E+ Predict the direction of the equilibrium (not all reactions go to the right as written) Identify A, B, CA, CB Page 3 of 23

4 Amphoteric = Drawing CA and CB pairs H 3 C H N N H Page 4 of 23

5 2. Acid Dissociation Constants: K a vs. pk a All acid-base reactions are said to be in equilibrium. What does this mean? Review: writing equilibrium constant expressions H 2 S 4 H + + S 4 2- K = CH 3 CH CH 3 C - + H + K = o For STRNG acids, the reactions lie to the right. Hence, the reaction is product favored and the K a is o For WEAK acids, the reactions lie to the left. Hence, the reaction is product favored and the K a is Page 5 of 23

6 How do we define the strength of an acid? Take a reaction that happens in your stomach: H Cl + H 2 What is the approximate K a of this reaction? What does this mean? Inorganic acids: K a typically range from rganic acids: K a typically range from How do we relate K a to its acid strength? pka = -log Ka Ka = 10 4 pka = Ka = 10-4 pka = Page 6 of 23

7 Not all A H bonds are created equally Compare the polarity of the following A H bonds. Which are closer to be ionized than others? Which might you expect to be most acidic? H 2 B H H 3 C H H 2 N H H H compare to: CH 3 C 2 H H F Page 7 of 23

8 3. Relative Strengths of Acids & Bases: you need to memorize these! Full table of species online, too. Page 8 of 23

9 4. Equilibrium in Acid-Base Reactions Do all acids react with all bases? How do we know when an acid will react with a particular base? Just because you can write down for a reaction paper does not mean that the reaction proceeds that way. Relative acid strength can be understood by familiarizing yourself with pka values. Do you predict this reaction proceeds to the right as written? How do you know? With acid-base reactions, you will need to learn how to: (1) Provide products for the reaction (2) Draw mechanistic arrows for reaction progress (3) Identify each reactant as Base or Acid (4) Predict the direction of the equilibrium (Note: not all reactions go to the right as written) (5) Identify A, B, CA, CB once the reaction direction has been established (6) Estimate an equilibrium constant. Page 9 of 23

10 (1) Provide products, (2) Predict the direction in which the following equilibria lie, and (3) Estimate the equilibrium constant. H + Na Page 10 of 23

11 Page 11 of 23

12 5. How does Molecular Structure Correlate to Acidity? Several effects must be considered when relating molecular structure to acidity: a. Element effects - Electronegativity b. Element effects - Size of atom bearing charge c. Resonance delocalization d. Inductive effect e. Hybridization No matter which factor is discussed, the same procedure is always followed. To compare the acidity of any two acids: o Always draw the CB to assess. o Determine which CB is more stable. o The more stable the CB is derived from the stronger acid. a. Element effects electronegativity = Negative charges on electronegative atoms are much more stable than on less electronegative atoms. CB For example, compare the conjugate base stability: Page 12 of 23

13 b. Element effects - Size of the atom bearing the charge Size, not electronegativity, determines acidity down a column. (Electronegativity is more important only when in same period.) Positive or negative charge is stabilized when it is spread over a larger volume. Compare: Compare pkas for HX acids: Page 13 of 23

14 c. Resonance Delocalization of negative charge Compare (you must compare CB stability): Compare (you must compare CB stability): Page 14 of 23

15 d. Inductive Effect = is the pull of electron density through σ bonds caused by electronegativity differences in atoms. How much stronger of an acid is hexafluoroisopropanol? Page 15 of 23

16 e. Hybridization Page 16 of 23

17 Which is more acidic? Trend? Cl 3 C H H H 3 C CH 3 HC CH NH 3 CH 4 HBr HI CH 3 CH 3 CH 3 H H Page 17 of 23

18 Discuss the order of acidic protons on the following compounds: Page 18 of 23

19 Which is more basic & WHY? CH 3 S - or CH 3 - or H - NH 3 or H 2 CH 3 CH 2 - or H - CH 3 CH 2 Page 19 of 23

20 6. Lewis Acids & Bases Pi bond electrons, not just lone pairs, can also act as a Lewis bases: + H Br Page 20 of 23

21 7. Functional Groups & Reactivity (Nucleophiles & Electrophiles) The majority of organic reactions are dictated by interactions between nucleophiles and electrophiles. The most common observation is that nucleophiles (Nu- = atoms with electrons to give) attack or react with electrophiles (E+ = atoms with a lack of electron density). Acids and bases are just a subcategory of Nu- and E+. o Nucleophiles provide electrons toward bond formation. Nu - are reagents that are seeking a nucleus to attack. o Nu - either have a lone pair or the pair of electrons in a π-bond. o Hence, nucleophiles are either negatively charged species or neutral molecules containing lone pairs of electrons. o Because electron flow ALWAYS begins with the nucleophile, nucleophiles are often said to attack electrophiles. H Na Na H H H Alcohol Alkoxide Water Hydroxide Amine NH 2 Amide NHNa Carbonyl Na Carboxylate Na R C C Na Page 21 of 23

22 Electrophiles o Electrophiles accept electron pair from Nu - toward bond formation. o Electrophiles are electron-seeking and have room in their orbitals to accept an electron pair (R they have an atom on them that will preferentially leave making room for the incoming nucleophile.) o Electrophiles are either positively charged species electron-deficient (δ+) species. Page 22 of 23

23 Identify the electrophilic or nucleophilic sites on each molecule: Br H H H H H N H H Identify the following as either nucleophiles (Nu), electrophiles (E), or neither (Ne) in the boxes provided. Page 23 of 23

ORGANIC CHEMISTRY I PRACTICE PROBLEMS FOR BRONSTED-LOWRY ACID-BASE CHEMISTRY

ORGANIC CHEMISTRY I PRACTICE PROBLEMS FOR BRONSTED-LOWRY ACID-BASE CHEMISTRY RGANIC CHEMISTRY I PRACTICE PRBLEMS FR BRNSTED-LWRY ACID-BASE CHEMISTRY 1. For each of the species below, identify the most acidic proton and provide the structure of the corresponding conjugate base.