Chapter 3: Acids & Bases

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Marian Golden
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1 Chapter 3 utline: Acids & Bases 1. What are acids & bases? 2. ow are acid dissociation constants? 3. ow does one define relative strengths of acids (pka)? 4. ow does one determine equilibria in acid base reactions? 5. ow does molecular structure correlate to acidity? 6. What are Lewis Acids & Lewis Bases? 7. What are Nucleophiles & Electrophiles? (read chapter 6.7) You should be do all the problems at the end of the chapter, but at the very least do these: , , 3.57, 3.59, 3.60, Page 1 of 28

2 1. Defining Acids & Bases Lewis Acid = Brønsted Lowry acid = Lewis Base = Brønsted Lowry base = Page 2 of 28

3 Acid base reactions are central to organic chemistry: Label the acid, base, and the conjugates in the reaction below: 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 Identify A, B, CA, CB Draw reasonable products for the following acid/base reaction, and label the conjugates. N + Base Acid N + Acid Base Page 3 of 28

4 Work with a neighbor to identify the acids and bases in the following mechanism: Page 4 of 28

5 Amphoteric = Drawing stable CA and CB pairs 3 C Page 5 of 28

6 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 Compare: 2 S S 4 2 K = C 3 C C 3 C + + K = o For STRNG acids, the reactions lie to the right. ence, the reaction is product favored and the K a is o For WEAK acids, the reactions lie to the left. ence, the reaction is product favored and the K a is Page 6 of 28

7 ow do we define the strength of an acid? K a values range from to Inorganic acids: K a typically range from rganic acids: K a typically range from ow do we relate K a to its acid strength? pka = log Ka Ka = 10 4 pka = Ka = 10 4 pka = Page 7 of 28

8 Not all A bonds are created equally Compare the polarity of the following A bonds. Which are closer to be ionized than others? Which might you expect to be most acidic? 2 B 3 C 2 N compare to: C 3 C 2 F Page 8 of 28

9 3. Relative Strengths of Acids & Bases: you need to memorize these! Full table of species online, too. Page 9 of 28

10 4. Equilibrium in Acid Base Reactions Do all acids react with all bases? ow 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? ow do you know? With the relevant pk a values, you can predict which direction an acid/base equilibrium will favor. Why is the equilibrium arrow bigger on top than on bottom? Page 10 of 28

11 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 11 of 28

12 Page 12 of 28

13 5. ow does Molecular Structure Correlate to Acidity? Several effects must be considered when relating molecular structure to acidity: a. Atom effects Electronegativity b. Atom effects Size of atom bearing charge c. Resonance delocalization d. Inductive effect e. rbital bearing the charge (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 B/CB is more stable. o The more stable the B/CB is derived from the stronger acid. a. Atom effects electronegativity = Negative charges on electronegative atoms are much more stable than on less electronegative atoms (good trend for atoms in same period). CB Page 13 of 28

14 b. Atom effects size of the atom bearing the charge (good trend for atoms in same group). 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 X acids: Page 14 of 28

15 c. Resonance delocalization of negative charge Compare (you must compare CB stability): Compare (you must compare CB stability): Page 15 of 28

16 d. Inductive Effect = the pull of electron density through bonds caused by electronegativity differences in atoms. ow much stronger of an acid is hexafluoroisopropanol? Page 16 of 28

17 e. rbital that bears the charge (hybridization of orbital) If a strong base were to react with the following molecule, which proton would most likely to react? Why would a very strong base be required? Could the molecule above act as a base in the presence of a strong acid? Page 17 of 28

18 When making qualitative judgments, use ARI as your guide loosely. Sometimes it fails as a guide: For each of the molecules below, rank the labeled ydrogen atoms in order of increasing pk a value. c a c N a b F F d N b Page 18 of 28

19 Sometimes acids will carry a formal positive charge. For such acids, their conjugate bases will be neutral. In such cases, we can use ARI to compare the stability of the acids directly to see which is best at stabilizing its positive charge. Rank the following acids in order of increasing strength. N N F F F A B C D E Predict the equilibrium for a reaction where pka s are not known: Page 19 of 28

20 Another important skill is to be able to choose an appropriate REAGENT for a acid/base reaction: Choose an ACID that could effectively PRTNATE each of the following molecules. Choose a CNJUGATE BASE that could effectively DEPRTNATE each of the following molecules. Page 20 of 28

21 Another important skill is to be able to choose an appropriate SLVENT for a acid/base reaction: The solvent should be able to surround the reactants and facilitate their collisions without reacting itself. Because water can act as an acid or a base, it has a LEVELING EFFECT on strong acids and bases: Acids stronger than 3 + cannot be used in water. Bases stronger than cannot be used in water. For example, water would not be an appropriate solvent for the following reaction. WY? + N + N Which would be a better solvent choice? or or Page 21 of 28

22 Solvent can have a large effect on reaction conditions: What explains the difference between these two pkas? SLVATIN is critically important in reactions (stabilize transition states, intermediates, and/or products to allow a reaction to occur). Discuss with your neighbor why the pk a for acetic acid is 4.75 in water while it is 23.5 in C 3 CN. Page 22 of 28

23 Discuss the order of acidic protons on the following compounds: Page 23 of 28

24 Which species is more basic & WY? C 3 S or C 3 or N 3 or 2 C 3 C 2 or C 3 C 2 Page 24 of 28

25 6. Lewis Acids & Bases Pi bond electrons, not just lone pairs, can also act as a Lewis bases: + Br Page 25 of 28

26 7. 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 are δ, have a lone pair or is the pair of electrons in a π bond. o ence, 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. Na Na Alcohol Alkoxide Water ydroxide Amine N 2 Amide NNa Carbonyl Na Carboxylate Na R C C Na Page 26 of 28

27 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 27 of 28

28 Identify the electrophilic or nucleophilic sites on each molecule: Br N Identify the following as either nucleophiles (Nu), electrophiles (E), or neither (Ne) in the boxes provided. Page 28 of 28

RESONANCE, USING CURVED ARROWS AND ACID-BASE REACTIONS

RESONANCE, USING CURVED ARROWS AND ACID-BASE REACTIONS A STUDENT SHOULD BE ABLE TO: 1. Properly use curved arrows to draw resonance structures: the tail and the head of every arrow must be drawn in exactly