Chapter 4 Outline: Acids Bases - Chemistry Courses:...

C341/Spring 2010 Chapter 4: Acids & Bases

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Chapter 4 Outline: 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: 4.9 – 4.11, 4.16 - 4.22, 4.24 – 4.29, 4.32, 4.33, 4.36, 4.38, 4.41, 4.46 - 4.48, 4.54. Review this material from C117 (thermochemistry/thermodynamics; section 4.5):


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


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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


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Amphoteric = Drawing CA and CB pairs

OH

O

H3C

N

NH


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2. Acid Dissociation Constants: Ka vs. pKa All acid-base reactions are said to be in equilibrium. What does this mean? Review: writing equilibrium constant expressions H2SO4 H+ + SO4

2- K = CH3COOH CH3COO- + H+ K =

o For STRONG acids, the reactions lie to the right. Hence, the reaction is

product favored and the Ka is

o For WEAK acids, the reactions lie to the left. Hence, the reaction is product

favored and the Ka is


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How do we “define” the strength of an acid? Take a reaction that happens in your stomach: H—Cl + H2O What is the approximate Ka of this reaction? What does this mean? Inorganic acids: Ka typically range from 106 – 101 Organic acids: Ka typically range from 10-5 – 10-50 How do we relate Ka to its acid strength? pKa = -log Ka

Ka = 104 → pKa = Ka = 10-4 → pKa =


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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? H2B—H H3C—H H2N—H HO—H compare to: CH3CO2H H—F


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3. Relative Strengths of Acids & Bases: you need to memorize these !

Full table of species online, too.


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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.


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(1) Provide products, (2) Predict the direction in which the following equilibria lie, and (3) Estimate the equilibrium constant.

OONa

+H


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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:


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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:


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c. Resonance Delocalization of negative charge

Compare (you must compare CB stability):

Compare (you must compare CB stability):


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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?


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e. Hybridization


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Which is more acidic? Trend?

O

OHCl3C

O

OH H3C CH3 HC CH NH3 CH4 HBr HI

CH3CH3CH3

H H


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Discuss the order of acidic protons on the following compounds:


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Which is more basic & WHY?

CH3S- or CH3O-

O

O or OH-

NH3 or H2O

CH3CH2O- or H-

O CH3CH2O


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6. Lewis Acids & Bases Pi bond electrons, not just lone pairs, can also act as a Lewis bases:

+ H Br


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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.

OH OH

OH H

O

NH2 O

O

Alcohol Alkoxide Water Hydroxide

Amine Carboxylate

O

Carbonyl

NH

Amide

Na Na

NaNa

O

R C C NaNa


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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 (OR 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.


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Identify the electrophilic or nucleophilic sites on each molecule:

OBr O O

H

H

H

OH NH

HH

Identify the following as either nucleophiles (Nu), electrophiles (E), or neither (Ne) in the boxes provided.

Chapter 4 Outline: Acids Bases - Chemistry Courses:...

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