chapter 3 outline: acids bases - chemistry courses: about...

C341/Fall 2011 Chapter 3: Acids & Bases

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Chapter 3 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? (read chapter 6.7)

You should be do all the problems at the end of the chapter, but at the very least do these: 3.36 – 3.39, 3.43 – 3.52, 3.57, 3.59, 3.60,


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1. Defining Acids & Bases

Lewis Acid = Brønsted‐Lowry acid =

Lewis Base = Brønsted‐Lowry base =


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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

H

OHH

+

Base Acid

N

H

OHH

+

BaseAcid


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Work with a neighbor to identify the acids and bases in the following

mechanism:


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Amphoteric =

Drawing stable CA and CB pairs

OH

O

H3C


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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

Compare: 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? Ka values range from 10‐50 to 1010 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.

OH

OH

O

H

HH


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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 the relevant pKa values, you can predict which direction an acid/base

equilibrium will favor.

• Why is the equilibrium arrow bigger on top than on bottom?


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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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5. How 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. Orbital 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


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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 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 = the pull of electron density through bonds caused by electronegativity differences in atoms.

How much stronger of an acid is hexafluoroisopropanol?


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e. Orbital 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?


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When making qualitative judgments, use ARIO as your guide loosely. Sometimes it fails as a guide:

For each of the molecules below, rank the labeled Hydrogen atoms in order of increasing pKa value.

O

F F

Hd

HbHa

O

Hc

N

O

Ha

NHb

Hc


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Sometimes acids will carry a formal positive charge.

• For such acids, their conjugate bases will be neutral.

• In such cases, we can use ARIO 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.

A B C D E

Predict the equilibrium for a reaction where pKa’s are not known:

O

H

N

H

O

H

N

H

F

F

F

O

H


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Another important skill is to be able to choose an appropriate REAGENT for a acid/base reaction:

– Choose an ACID that could effectively PROTONATE each of the following molecules.

– Choose a CONJUGATE BASE that could effectively DEPROTONATE each of the following molecules.

O

OO

O


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Another important skill is to be able to choose an appropriate SOLVENT 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 H3O+ cannot be used in water.

• Bases stronger than OH‐ cannot be used in water.

For example, water would not be an appropriate solvent for the following reaction. WHY?

Which would be a better solvent choice?

N+

H

HN+

O

OHor or


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Solvent can have a large effect on reaction conditions:

What explains the difference between these two pKas?

• SOLVATION is critically important in reactions (stabilize transition states, intermediates, and/or products to allow a reaction to occur).

• Discuss with your neighbor why the pKa for acetic acid is 4.75 in water while it is 23.5 in CH3CN.


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Discuss the order of acidic protons on the following compounds:


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Which species 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. 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 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 3 outline: acids bases - chemistry courses: about...

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