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CHEM 109A Organic Chemistry
https://labs.chem.ucsb.edu/zakarian/armen/courses.html
Chapter 2 Acids and Bases Central to Understanding Organic Chemistry
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Draw the conjugate acid of each of the following:
a. CH3CH2CH2OH
b. CH3CH2O
O c. O
d. CH3NH2
e. HCl
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Which is the stronger base? (by comparing pKas)
a. CH3CH2O
b. O
O
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Approximate pKa Values
p Ka ~ 35
RNH2
NH3
estimate the pKa values of the following compounds
+ © 2017 Pearson Education,CH Inc.3 CH2CH2NH2, CH3CH2OH, CH3CH2CH2NH3
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The Acid Dissociation Constant (Ka) is a Measure of the Extent of Dissociation of an Acid
Put the constants on the same side of the equation.
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General representation is of acid-base reaction
simple formalization moving H+ reversible A H + B A + H B charge charge changes changes by -1 by +1 conj. base conj. acid
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General representation is of acid-base reaction
simple formalization moving H+ reversible A H + B A + H B charge charge changes changes by -1 by +1 conj. base conj. acid complicated reality still moving H+ but which one? A H + B
H
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An Alcohol Can Behave as an Acid and as a Base
A curved arrow points from the electron donor to the electron acceptor.
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A Carboxylic Acid Can Behave as an Acid and as a Base
A curved arrow points from the electron donor to the electron acceptor.
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An Amine Can Behave as an Acid and as a Base
A curved arrow points from the electron donor to the electron acceptor. © 2017 Pearson Education, Inc.
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Approximate pKa Values
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Which Reactant is the Acid?
The stronger acid behaves as the acid.
Water is the base. Water is the acid.
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The Position of Equilibrium
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The Position of Equilibrium
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The Position of Equilibrium
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The Strength of an Acid is Determined by the Stability of Its Conjugate Base
The more stable the conjugate base, the stronger the acid.
Stable bases are weak bases.
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Electronegativity Affects pKa Values
When atoms are the same size, the strongest acid has its hydrogen attached to the most electronegative atom.
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Why are Alcohols Stronger Acids than Amines?
Oxygen is more electronegative than nitrogen.
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Why are Protonated Alcohols Stronger Acids than Protonated Amines?
Oxygen is more electronegative than nitrogen.
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Hybridization Affects Electronegativity
The strongest acid has the most stable (weakest) conjugate base.
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Size Affects pKa Values
When atoms differ in size, the strongest acid has its hydrogen bonded to the largest atom.
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Some pKa Values
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Substituents Affect the Strength of an Acid
! INDUCTIVE electron withdrawal (Inductive Effect)
! DELOCALIZED electrons (Resonance Effect)
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Substituents Affect the Strength of an Acid
inductive electron withdrawal
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A Substituent’s Effect on pKa Depends on Distance
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Why is a Carboxylic Acid a Stronger Acid than an Alcohol?
1. Inductive electron withdrawal
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Why is a Carboxylic Acid a Stronger Acid than an Alcohol?
2. Delocalized electrons
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Summary of Factors That Affect Acid Strength
electronegativity and size
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Summary of Factors That Affect Acid Strength
hybridization
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Summary of Factors That Affect Acid Strength
inductive electron withdrawal
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Summary of Factors That Affect Acid Strength
electron delocalization
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The Henderson–Hasselbalch Equation
The Henderson–Hasselbalch equation tells us whether (at a given pH) a compound will be in its acidic form (with its proton) or in its basic form (without its proton).
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A Compound with a pKa = 5.2
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Is a Compound Neutral or Charged?
A carboxylic acid is neutral in its acidic form and charged in its basic form.
An alcohol is neutral in its acidic form and charged in its basic form.
An amine is charged in its acidic form and neutral in its basic form. © 2017 Pearson Education, Inc.
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Water and Diethyl Ether Form Two Layers
Diethyl ether is less dense than water, so diethyl ether is on top.
Neutral compounds dissolve in diethyl ether.
Charged compounds dissolve in water.
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A buffer solution contains an acid and its conjugate base.
A buffer solution maintains nearly constant pH.
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Lewis Acids and Bases
Broader definition than proton acids (Brønsted acids)
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Lewis Acids and Bases
Lewis acid: a species that accepts a share in an electron pair Lewis base: a species that donates a share in an electron pair
All Brønsted acids are Lewis acids. All Brønsted bases are Lewis bases.
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How Chemists Use the Terms
“acid” = a proton-donating acid “Lewis acid” = a non-proton-donating acid
All bases are Lewis bases because they have a pair of electrons they can share.
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An Acid Loses a Proton A Base Gains a Proton
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Most Acid–Base Reactions Are Reversible
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