Instructor: Prof. Duncan Wardrop Time/Day: T, 4:30-8:15 p.m. September 9, 2010
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Transcript of Instructor: Prof. Duncan Wardrop Time/Day: T, 4:30-8:15 p.m. September 9, 2010
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Instructor: Prof. Duncan Wardrop
Time/Day: T, 4:30-8:15 p.m.
September 9, 2010
Welcome to CHEM 494
UICUniversity of
Illinois at Chicago
2CHEM 494, Fall 2012 SlideLecture 1
Course Website
http://www.chem.uic.edu/chem494
Syllabus
Course Policies
Other handouts
Announcements (Course News)
Course Calendar
UICUniversity of
Illinois at Chicago
3CHEM 494, Fall 2012 SlideLecture 1
Keys to Success
1. Attend all lectures and discussion sections.
2. Don’t fall behind. Organic chemistry is easy, but each topic
builds on the previous.
3. Don’t memorize. Organic chemistry is conceptual.
4. Always ask yourself “Why” for everything you read or hear.
You may not always find the answer, but just asking will help
you to find connections and remember more.
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Introduction
Origins and Examples of Organic Chemistry
UICUniversity of
Illinois at Chicago
5CHEM 494, Fall 2012 SlideLecture 1
Vitalism
Living
vs.
Nonliving.
•posses vital force•compounds derived from are “organic” (coined by J. Berzelius, 1807)
•could not be synthesized in the laboratory
•termed “inorganic”
•derived from nonliving
matter
•can be synthesized in the
lab
UICUniversity of
Illinois at Chicago
6CHEM 494, Fall 2012 SlideLecture 1
Wohler Synthesis Debunks Vitalism
ammonium cyanate(inorganic)
urea(organic)
AgOCN(aq) + NH4Cl(aq) AgCl(s) + NH4OCN(aq)
F. Wohler(Germany, 1828)
N
H
H
HH O C NO
CNH2H2N
Expected Product Actual Product
X
UICUniversity of
Illinois at Chicago
7CHEM 494, Fall 2012 SlideLecture 1
Self Test Question
What is the minimum requirement, today, for a chemical substance to be classified as organic?
A. derived from living matter
B. contains carbon
C. cannot be synthesized
D. bought at Whole FoodsE. combustion yields SO2
UICUniversity of
Illinois at Chicago
8CHEM 494, Fall 2012 SlideLecture 1
Vitalism Lives On. . .
CH3
OH
terpinen-4-olsold as tea tree oil
or Melaleuca oil
fructose“fruit sugar”also in HFCS
HO O
OH
OH
OH
OH
UICUniversity of
Illinois at Chicago
9CHEM 494, Fall 2012 SlideLecture 1
Organic Chemistry Everywhere
H3C O
O
3-methylbutyl acetate(bananas)
O
O
CH3
OH
methyl salicylate(oil of wintergreen)
OHO
O
O
CH3
aspirin(analgesic)
O
O
NH3C
cocaine(analgesic)
O
O
N
procaine, "Novocaine"(analgesic)
UICUniversity of
Illinois at Chicago
10CHEM 494, Fall 2012 Slide
Lecture 1
Natural Products
Order: coleoptera
Family: coccinellidae
Order: Tetraodontoformes
NHN
H2N
OH
OHHO
OO
OHO
(-)-tetrodotoxin
N
C6H13
NCS
fasicularin
3
6 7
3'
A
BC
NH
O
(-)-adalinine
C5H11
O
UICUniversity of
Illinois at Chicago
11CHEM 494, Fall 2012 Slide
Lecture 1
Pharmaceuticals
N
F3C
CH3
S NH2
O
O
Tamiflu(influenza)
Celebrex(arthritis)
Morphine(analgesic)
UICUniversity of
Illinois at Chicago
12CHEM 494, Fall 2012 Slide
Lecture 1
Optics - Transition Lenses?
NC
O
N
NC
N
O
does not absorb visible
light
does absorb visible light
UVlight
heat
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Atomic StructureA General Chemistry
Review
UICUniversity of
Illinois at Chicago
14CHEM 494, Fall 2012 Slide
Lecture 1
particle chargemass
molar masssymbol
proton positive1.6726 x 10-24 g1.0073 g/mol p
neutron neutral1.6750 x 10-24 g1.0087 g/mol n
electron negative9.1096 x 10-38 g
5.486 x 10-4 g/mol
e–
Atomic Composition
+++
-
-
-
+
Bohr Model-
UICUniversity of
Illinois at Chicago
15CHEM 494, Fall 2012 Slide
Lecture 1
Atomic Number & Mass Number
+++
-
--
XA
Z
A = protons + neutronsZ = protons
Li6
3For neutral molecules, the
number of electrons equals the number of protons.
UICUniversity of
Illinois at Chicago
16CHEM 494, Fall 2012 Slide
Lecture 1
Self Test Question
How many neutrons are in the following atom:
A. 14B. 6C. 8D. 20E. cannot be determined
C14
6
UICUniversity of
Illinois at Chicago
17CHEM 494, Fall 2012 Slide
Lecture 1
Tenets of Schrӧdinger Equation
• electrons have wave properties• wave equation gives energy of electron at a
location• solutions to wave equation are wave functions (Ψ);
a.k.a orbitals• probability distribution = Ψ2 (Heisenburg
uncertainty principle)
UICUniversity of
Illinois at Chicago
18CHEM 494, Fall 2012 Slide
Lecture 1
Probability Distribution vs. Boundary Surface
Probability distribution (Ψ2)(“electron cloud”)
Boundary Surface (1s orbital)
(where the probability = 90-95%)
1s 1s
UICUniversity of
Illinois at Chicago
19CHEM 494, Fall 2012 Slide
Lecture 1
Wave Function Values:Quantum Numbers
quantumnumber
principle orbital magnetic spin
symbol n l ml ms
values 1, 2, 3. . . 0, 1, . . . n-1(ml = −l, −l+1 ...
0 ... l−1, l)+1/2 or -1/2
examples/abbreviatio
ns
l= 0, sl = 1, pl = 2, dl = 3, f
s: 1 orbitalp: three orbitalsd: five orbitals
f: seven orbitals
•Shrodinger equation → wave function (orbital, Ψ)
•many solutions for Ψ, energies of electrons in atom
UICUniversity of
Illinois at Chicago
20CHEM 494, Fall 2012 Slide
Lecture 1
s-Orbitals• spherically symmetric
• possible for n≥1
• 1 s-orbitals for each value of n
• 1s = no nodes, 2s = 1 nodes, 3s = 2 nodes, etc.
• probability of finding s-electron at nodal surface = 0
• s-orbital energy increases with increasing nodes (as n increases)
UICUniversity of
Illinois at Chicago
21CHEM 494, Fall 2012 Slide
Lecture 1
p-Orbitals• shaped like dumbells
• not possible for n = 1 (n≥2)
• 3 p-orbitals for each value of n; they are degenerate (equal in energy)
• wave function changes sign at the nucleus (node)
• probability of finding p-electron at nodal plane = 0
• higher in NRG than s-orbitals of the same shell
UICUniversity of
Illinois at Chicago
22CHEM 494, Fall 2012 Slide
Lecture 1
Relative Energies of Orbitals
1s
2s2p 2p 2p
3s3p 3p 3p
n = 1
3d 3d 3d 3d 3d
n = 2
n = 3/4
4s
En
erg
y
UICUniversity of
Illinois at Chicago
23CHEM 494, Fall 2012 Slide
Lecture 1
Order of Orbitals from Periodic Table
1s2s 2p3s 3p4s 3d 4p5s6s7s
4d5d6d
5p6p7p
4f5f
Read left to right starting at the top left; just like a
typewriter.
UICUniversity of
Illinois at Chicago
24CHEM 494, Fall 2012 Slide
Lecture 1
Electron Configuration: Filling Orbitals
1s
2s2p 2p 2p
3s3p 3p 3p
n = 1
3d 3d 3d 3d 3d
n = 2
n = 3/4
1. Aufbau principle: fill lowest energy orbitals first.
2. Pauli exclusion principle: two electrons with same quantum numbers cannot occupy a single orbital. Electrons must have opposite spins in the same orbital.
3. Hund’s rule: degenerate orbitals filled singly first.
spin (ms = +1/2)
spin (ms = –1/2)
4s
En
erg
y
UICUniversity of
Illinois at Chicago
25CHEM 494, Fall 2012 Slide
Lecture 1
Self Test Question
Which of the following orbitals is highest in energy?
A. 3fB. 3pC. 2sD. 3sE. 2d
UICUniversity of
Illinois at Chicago
26CHEM 494, Fall 2012 Slide
Lecture 1
Self Test Question
Which is the correct electron configuration for carbon?
A. 1s2, 2s8
B. 1s2, 2s4
C. 1s2, 2s2, 2s2
D. 1s2, 2s2, 2p2
E. none of the above
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Bonding and Molecular Structure
UICUniversity of
Illinois at Chicago
28CHEM 494, Fall 2012 Slide
Lecture 1
What is a Covalent Chemical Bond?
Forces involved:
•electron-electron repulsion
•nucleus-nucleus repulsion
•electron-nucleus attraction
valent = bondingcovalent = electrons shared between two
nuclei
electron-electron repulsion < electron-neutron attraction
UICUniversity of
Illinois at Chicago
29CHEM 494, Fall 2012 Slide
Lecture 1
Three Models of BondingLewis Model:
•atoms gain, lose or share electrons in order to achieve a closed-shell electron configurations (all orbitals fully occupied)•closed-shell for row 2 = 8 electrons (octet rule)•only valence electrons (in outermost shell) are involved in bonding
Atomic StructuresMolecular
Lewis StructuresMolecular
Lewis Structures
H–H
F–F
UICUniversity of
Illinois at Chicago
30CHEM 494, Fall 2012 Slide
Lecture 1
Three Models of BondingValence Bond Model:
•in-phase overlap of two half-filled orbitals•in-phase = constructive interference•increases probability electrons between two nuclei
H H H–HBoundar
y Surfaces
Electrostatic Potential
maps:red =
negativeblue = postive
UICUniversity of
Illinois at Chicago
31CHEM 494, Fall 2012 Slide
Lecture 1
Three Models of BondingMolecular Orbital (MO) Model:
•combine atomic orbitals (AO) of all atoms, then extract molecular orbitals•number of atomic orbitals in equals the number of molecular orbitals out•combination possibilites:•additive = produces bonding molecular orbitals•subtractive = produces antibonding molecular orbitals
UICUniversity of
Illinois at Chicago
32CHEM 494, Fall 2012 Slide
Lecture 1
Molecular Orbital Diagram
1s AO 1s AO
antibonding (σ*) MO
bonding (σ) MO
nodal planee- probability = 0
= no bond
H H
Rules Still Apply:1. Aufbau2. Pauli3. Hund
UICUniversity of
Illinois at Chicago
33CHEM 494, Fall 2012 Slide
Lecture 1
Self Test Question
Which is the correct Lewis dot structure for CH4O?
A. H C
H
H O H
O
CH H
HH
B.
C
H
O
H
H H
H C O
H
H
HC.
D.
E. none of the above
UICUniversity of
Illinois at Chicago
34CHEM 494, Fall 2012 Slide
Lecture 1
Structural Bond-line Condensed
•format varies; typically most covalent bonds are drawn out•bonds are lines•only lone-pair (nonbonding) electrons are drawn as dots
•only atoms written are those that are not C or H bound to C•intersection of two lines is C•terminus of a line is -CH3 group•# H atoms is assumed for C•chains drawn as “zig-zag”
•all or most covalent bond lines are omitted•groups are separated by parentheses (infers group is attached to carbon with available valency on left or right)
Structural, Bond-line and Condensed Formulas
CH3CH2CH2CHOCC
CC
H
O
HH
HHH
H H
CC
CH
HH
HOH
H H
H OHCH3CH(OH)CH3
CC
CC
HH
HH
H H
HHH
C
HH
O
OH
O
O
H (CH3)2CHCH2CH2CO2H
UICUniversity of
Illinois at Chicago
35CHEM 494, Fall 2012 Slide
Lecture 1
VSEPR: Quick Review
UICUniversity of
Illinois at Chicago
36CHEM 494, Fall 2012 Slide
Lecture 1
Self Test Question
Which bond-line drawing correctly represents the following condensed formula?
A.
B.
C.
D.
CH3CH(OCH2CH3)CH2Br
O
Br
O
Br
O
Br
O Br
Br
OE.
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Functional Groups
UICUniversity of
Illinois at Chicago
38CHEM 494, Fall 2012 Slide
Lecture 1
Memorize These Functional Groups NOW
Table 4.1, page 140
functional group: a defined group of atoms with a specific connectivity •responsible for properties•predictable reactivity•well-defined nomenclatureMemorizing: “What is the minimum number of atoms needs to define a functional group and in what order are they bonded?” Flashcards!
UICUniversity of
Illinois at Chicago
39
CHEM 494, Fall 2010 SlideLecture 2
Formal Charge - Example 1My logic (no memorizing equations):
1.Recognize that opposing charges cancel each other2.Determine difference in protons vs. electrons by asking:a.How many valence (outer shell) electrons does the atom have?b.How many valence (outer shell) electrons does the atom “want” (groups #)?1.If atom has more than it “wants,” negatively charged.2.If atom has less than it “wants,” positively charged.
O16
8--
-
+
+++
+ +
-
1s
2s
2p
-
-
-
-
-
-
+
+Has? =
Wants? =
Difference? =
8
6
2 extra
2–
UICUniversity of
Illinois at Chicago
40
CHEM 494, Fall 2010 SlideLecture 2
Formal Charge - Example 2My logic (no memorizing equations):
1.Recognize that opposing charges cancel each other2.Determine difference in protons vs. electrons by asking:a.How many valence (outer shell) electrons does the atom have?b.How many valence (outer shell) electrons does the atom “want” (groups #)?If atom has more than it “wants,” negatively charged.If atom has less than it “wants,” positively charged.
Has? =
Wants? =
Difference? =
5
6
1 less“formal” = count one electron for
each bond
H
O
H
H
UICUniversity of
Illinois at Chicago
41
CHEM 494, Fall 2010 SlideLecture 2
Shortcut for Some 1st and 2nd Period Atoms•For each additional bond
= +1•For each “missing” bond = –1
AtomBonds Forme
dGroup
H 1 I
C 4 IV
N 3 V
O 2 VI
F (X) 1 VII
H3CC
CH3
CH3
H3C
O
O
N
UICUniversity of
Illinois at Chicago
42
CHEM 494, Fall 2010 SlideLecture 2
Self Test Question
What is the formal charge on the carbon atom in red?
A. -2B. -1C. 0D. +1E. +2
C C
H
H
H
H
H
UICUniversity of
Illinois at Chicago
43
CHEM 494, Fall 2010 SlideLecture 2
Self Test Question
According to VSEPR, what is the bond angle between 2 H-atoms in methane (below)?
A. 105ºB. 109.5ºC.107ºD.180ºE. 120º
H
CH
HH?
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
Section: 1.8, 1.11You are responsible for Section 1.10.
Structure and Bonding: Resonance
UICUniversity of
Illinois at Chicago
45
CHEM 494, Fall 2010 SlideLecture 2
The Contradictory Case of Ozone
electrostatic potential map
red = negativeblue = positive
Microwave spectroscopy shows that ozone is symmetrical (C2v)
Lewis Structuresuggests ozone is non-symmetrical
OO
O
UICUniversity of
Illinois at Chicago
47
CHEM 494, Fall 2010 SlideLecture 2
Curious Case of Benzene
C–C bond length: 150 pmC=C bond length: 134 pm
H
H
H
H
H
H
Predicted Actual
All bonds = 140 pm
resonance
UICUniversity of
Illinois at Chicago
48
CHEM 494, Fall 2010 SlideLecture 2
Resonance Provides the SolutionProblems:
1.Lewis structures fail to describe actual atom electron densities for molecules with more than one possible electron distribution.
2.Lewis formulas show electrons as localized; they either belong to a single atom (lone pair) or are shared between two atoms (covalent bond).
3.Electrons are not always localized; delocalization over several nuclei leads to stabilization (lower energy).
H3C N
O
CH3
CH3
H3C N
O
CH3
CH3
H3C N
O
CH3
CH3
UICUniversity of
Illinois at Chicago
49
CHEM 494, Fall 2010 SlideLecture 2
Physical Proof of Resonance in Amides
C-N (amide) Bond Length = 133 pm
C-N (amine) Bond Length = 147 pm
C=N (imine) Bond Length = 129 pm
H3C N
O
CH3
CH3
H3C N
O
CH3
CH3
NH
HH
N
CH3H3C
H
UICUniversity of
Illinois at Chicago
50
CHEM 494, Fall 2010 SlideLecture 2
A Question of Arrows
Equilibrium between distinct species
Reaction from one species to another
“Movement” of electronsfrom donor to acceptorIndicates that 2 species are contributing resonance structures
UICUniversity of
Illinois at Chicago
51
CHEM 494, Fall 2010 SlideLecture 2
Resonance
Solutions:
1.Actual molecule is considered a resonance hybrid (weighted average) of contributing Lewis structures. (Note: double headed arrow does NOT indicate interconversion.)
2.Dashed-line notation is sometimes used to indicate partial bonds.
3.Not all structures contribute equally.
UICUniversity of
Illinois at Chicago
52
CHEM 494, Fall 2010 SlideLecture 2
Rules of Resonance
• How to draw resonance structures• How to distinguish between a resonance
structure and a unique electron configuration
• How to predict the major contributing structure
You are responsible for the “Rules of Resonance” (Pg. 27)
UICUniversity of
Illinois at Chicago
53
CHEM 494, Fall 2010 SlideLecture 2
Another Way to ThinkAbout Resonance Contributors
Dürer's Rhinoceros (1515 A.D.)
UICUniversity of
Illinois at Chicago
54
CHEM 494, Fall 2010 SlideLecture 2
Curved Arrow Notation
curved arrows show the
movement of electrons; never
atoms
electronsatoms
H3C N
O
CH3
CH3
resonance: electrons in a covalent bond
moving out to an atom
H3C N
O
CH3
CH3
resonance: lone pair of electrons
moving in between two
atoms to form a new covalent
bond
HO
H H H3O+
bond making: lone pair of
electrons forming a new bond to another atom
H3C O
OH
H3C O
O
+ H
bond breaking: electrons in a
bond leaving to most
electronegative atom
UICUniversity of
Illinois at Chicago
55
CHEM 494, Fall 2010 SlideLecture 2
Preview: Curved Arrows in Reaction Mechanisms
H3C O
O
HH
OH
+H3C O
O
+H
OH
H
bond making
bond breaking
UICUniversity of
Illinois at Chicago
56
CHEM 494, Fall 2010 SlideLecture 2
Another Way of Viewing Arrow-Pushing
Remember that curly arrows originate from e-donors (nucleophiles) and terminate at e-acceptors (electrophiles)
From an M.O. perspective curly arrows originate from filled orbitals (nucleophiles) and terminate at empty orbitals (electrophiles) - bonds are formed from the mixing of filled and empty orbitals
UICUniversity of
Illinois at Chicago
57
CHEM 494, Fall 2010 SlideLecture 2
Molecular Orbital Diagram
1s AO 1s AO
antibonding (σ*) MO
bonding (σ) MO
nodal planee- probability = 0
= no bond
H H
Rules Still Apply:1. Aufbau2. Pauli3. Hund
UICUniversity of
Illinois at Chicago
58
CHEM 494, Fall 2010 SlideLecture 2
Self Test Question
Which of the following is not descriptive of resonance as it pertains to organic chemistry?
A. stabilization of negative chargeB. movement of atomsC. delocalization of electronsD. involves bond-breakingE. involves bond-making
University of Illinois at Chicago
UICCHEM 494 Special Topics in Chemistry
How Structure Affects Physical Properties:
Acid Strength
UICUniversity of
Illinois at Chicago
60
CHEM 494, Fall 2010 SlideLecture 2
Arrhenius Acids and Bases
Acid: Dissociates to provide protons (H+) in water.
Base: Dissociates to provide hydroxide (OH–) in water.
strong acid: ionizes completely
weak acid: does not ionize completely
strong base: ionizes completely
weak base: does not ionize completely
UICUniversity of
Illinois at Chicago
61
CHEM 494, Fall 2010 SlideLecture 2
Brønsted-Lowry Acids & Bases
Acid: Proton (H+) donorBase: Proton (H+) acceptor
• definition does not depend on dissociation in water• broader definition• conjugate acid and base differ by only one proton• most often used definition in this course
B : + H A B + AH :
Base Acid Conjugate Acid Conjugate Base
UICUniversity of
Illinois at Chicago
62
CHEM 494, Fall 2010 SlideLecture 2
Molecular Orbital Picture
B H A B +H:
σ* (H-A)n (B) σ (B-H)
A :
n (A)
Anti-Bonding M.O. Bonding M.O.
H-A Bond Breaks!
Non-Bonding M.O. Non-Bonding M.O.
UICUniversity of
Illinois at Chicago
63
CHEM 494, Fall 2010 SlideLecture 2
Water Can Act as a Base or Acid Under Brønsted-Lowry Definition
OH
HClH+
HO
H
H
Cl–+
NH
HO
H
HH
NH
H
OH–+
water (base)
HCl (acid)
hydronium ion (conjugate acid)
chloride ion (conjugate base)
amide (base)
water (acid)
ammonia(conjugate acid)
hydroxide ion (conjugate base)
UICUniversity of
Illinois at Chicago
64
CHEM 494, Fall 2010 SlideLecture 2
Acid Strength is Measured by the Acid Dissociation Constant (pKa)
pKa = –log10 Ka
H A + AH :
Acid Proton Anion
Note that the value of K reflects the relativethermodyamic stability of acid and anion
UICUniversity of
Illinois at Chicago
65
CHEM 494, Fall 2010 SlideLecture 2
pKa Scale
HCl
H3O+
NH3
H3C
O
OH
OH
CH3CH2O H
H2O
acid pKa
hydrochloric
-3.9
hydronium -1.7
acetic acid 4.7
pyridinium 5.2
phenol 10
water 15.7
ethanol 16
ammonia 36
ethane 62
NH3
CH3CH3
Take home message: the larger the Ka value, the smaller the pKa value,the stronger the acid.
-log10(1) = 0-log10(10) = -1-log10(100) = -2
[H+][A-][HA]-log10pKa =
HA H A–+acid conjugate
base
dissociation
-log10(1/10) = 1-log10(1/100) = 2
UICUniversity of
Illinois at Chicago
66
CHEM 494, Fall 2010 SlideLecture 2
Structure Affects Acid Strength
1. Electronegativity of H-Donating Atom •dominant effect for same periods (rows)•more electronegative conjugate base = more stable conjugate base = Ka lies further to right•Alternative reasoning: H of conjugate acid (HA) becomes more positive with increasing electronegativity of A
conjugate base
acid
CH3 NH2 OH F
least stable (highest energy)
conjugate base =
weakest acid
most stable (highest energy)
conjugate base =
strongest acid
UICUniversity of
Illinois at Chicago
67
CHEM 494, Fall 2010 SlideLecture 2
Structure Affects Acid Strength
C C O
H
H
H
H
H
H C C O
H
H
H
H
H
+ H
C C O
H
H
F
F
F
H C C O
H
H
F
F
F
+ H
Ka = 1 x 10-16
pKa = 16
Ka = 5 x 10-12
pKa = 11.3
1. Electronegativity (cont.) •inductive effect of electronegative substituents also stabilizes conj. base•the more electronegative the group, the greater the stabilization•the closer the electronegative group, the greater the inductive effect •inductive effect = structural effects that are transmitted through bonds
UICUniversity of
Illinois at Chicago
68
CHEM 494, Fall 2010 SlideLecture 2
Self Test Question
Which of the following carboxylic acids (-CO2H) is the most acidic (lowest pKa)?
A.
B.
C.
D.
E.
CC
O
O
HCl
H H
CC
O
O
HH
H H
CC
O
O
HC
H HC
Cl
H H
H H
CC
O
O
HF
H H
CC
O
O
HCl
H H
R O
O
carboxylate anion(conjugate base)
UICUniversity of
Illinois at Chicago
69
CHEM 494, Fall 2010 SlideLecture 2
Structure Affects Acid Strength2. Bond Strength
•dominant effect for same groups (columns)•Similar sized bonding orbitals = better overlap = stronger bond•Stronger H–A bond = weaker acid
UICUniversity of
Illinois at Chicago
70
CHEM 494, Fall 2010 SlideLecture 2
Self Test Question
Which of the following is the strongest acid?
A. H2O
S
O
B. H2S
UICUniversity of
Illinois at Chicago
71
CHEM 494, Fall 2010 SlideLecture 2
Structure Affects Acid Strength3. Resonance (delocalization of e–s in conj. base): •delocalization of e–s in conj. base = increased stability (lower NRG) of conjugate base•more stable conj. base = larger Ka = smaller pKa = stronger acid
Ka = 1 x 10-16
pKa = 16
Ka = 2.0 x 10-5
pKa = 4.7
H3CC
OH
H H
H3CC
O
H H+ H
H3CC
O
O
H3CC
O
O
H3CC
OH + H
H3CC
O
OO
UICUniversity of
Illinois at Chicago
72
CHEM 494, Fall 2010 SlideLecture 2
Acid-Base Equilibria: Determining the Direction of Acid-Base Reactions
You must identify the ACID on each side of the equilibrium:
pKeq = pKa (acid left) - pKa (acid right)
Keq = 10-[pKa (acid left) - pKa (acid right)]
• remember: p = -log10• this equation works for any acid-base reaction; doesn’t
matter which way equilibrium is written
OH
C
O
OHO+
KeqO
C
O
OHHO+
Keq = 10-[10 - 6.4]
phenol(pKa = 10)
hydrogen carbonate(pKa = 10.2)
phenoxide anion(pKa = NA)
carbonic acid(pKa = 6.4)
= 10-[3.6]
= 2.5 x 10-4
since Keq <1, then equilibrium lies to the left
acid acidbasebase
Example:
UICUniversity of
Illinois at Chicago
73
CHEM 494, Fall 2010 SlideLecture 2
Finally, What About Strengths of Bases?
Question: Is ammonia or pyridine a stronger base?
Solution: 1. Determine which conjugate acid of each base is the weakest.2. The weaker the conjugate acid, the stronger the conjugate base.
UICUniversity of
Illinois at Chicago
74
CHEM 494, Fall 2010 SlideLecture 2
Molecule of the Week...α-AmanitinBe Careful What You Eat (and Who
You Marry)!
Amanita phalloides
α-Amanitinα-Amanitin (red) boundto RNA polymerase II
Read more about the death of Claudius......
Tiberius Claudius Caesar Augustus Germanicus (10 B.C.–A.D. 54)
Amanita caesarea