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University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Prof. Duncan WardropSeptember 24, 2012
CHEM 494 - Lecture 3
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Course Website
2
http://www.chem.uic.edu/chem494
SyllabusCourse PoliciesOther handoutsAnnouncements (Course News)Course Calendar
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Chemical Properties of Alkanes
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Hydrocarbons are Weak Acids(Carbanions are Strong Bases)
4
pKa = 26
pKa = 43
pKa = 45
pKa = 62
Incr
eas
ing
Aci
d S
tre
ngt
h
C CH H C CH + H
C C C C + HH
H
H
H
H
H H
CCCC C
C H
HH
H
H HCC
CC C
C
HH
H
H H
+ H
C CHHH
HH
HC C H
HHH
H+ H
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Combustion is Exothermic
5
CH4 + 2O2
CO2 + 2H2O
CH4 + 2O2 ➞ CO2 + 2H2O
ΔHº (enthalpy of rxn)
Ea (activation energy)
ΔHº = Hº(products) - Hº (reactants)
exothermic ΔHº = negative
higher potential energy of reactant hydrocarbon = larger enthalpy of combustion
endothermic ΔHº = positive
higher energy of reactants =larger heat of combustion =
more exothermic -ΔHº (heat of combustion)
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Enthalpy of Reaction & Heat of Combustion
6
heat of combustion = -ΔHº = 890 kJ (212.8 kcal)
heat of combustion = –ΔHº
heat of combustion = -ΔHº = 3529 kJ (843.4 kcal)
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Heats of Combustion of Unbranched Alkanes
7
increase number of carbon atoms = increased heat of combustion (-ΔHº)
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Heats of Combustion of Unbranched Alkanes
8
increased branching of isomers = increased intramolecular VWF = lower energy (more stable) =
decreased heat of combustion (-ΔHº)intramolecular forces:same electronic forces previously described
more nuclear attractions & more intramolecular VWF =
more stable =
lower energy =
smaller heat of combustion
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Octane Rating
9
100: iso-octane (2,2,4-trimethylpentane)
0: heptane
The burning qualities (knocking) of gasoline are compared to the burning qualities of iso-octane and heptane mixtures. This does NOT mean that gasoline contains iso-octane and heptane.lower heat of combustion
slower burning
higher heat of combustionfaster burning
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Internal-Combustion Engine: Knocking
10
engine-knocking or detonation: spontaneous combustion of the remaining fuel/air mixture left in the combustion chamber after normal combustion burn initiated by spark-plug
pre-ignition: spontaneous combustion of the fuel/air mixture before the spark plug "res
http://www.streetrodstuff.com/Articles/Engine/Detonation/
More highly branched alkanes produce less energy, but burn more effectively in an internal combustion engine by reducing knocking.
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Combustion is an Oxidation Reaction
11
CH4 + 2O2 ➞ CO2 + H2O
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Definitions of Oxidation and Reduction
12
a. lose electrons (LEO GER)b. gain bonds to oxygenc. lose bonds to Hd. oxidation # increases
a. gain electrons (LEO GER)b. lose bonds to oxygenc. gain bonds to Hd. oxidation # decreases
All de"nitions above describe the amount of electron density centered on an atom. In summary, any process that decreases electron density, whether formal or informal, is termed oxidation. Likewise, any process that increases electron density, whether formal or informal, is termed reduction.
Oxidation(sometimes represented as [O])
Reduction(sometimes represented as [H] or [R])
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Determining Oxidation Numbers on Carbon
13
Three Simple Rules:
1. For each bond to an atom less electronegative than carbon (i.e. H) add (-1).
2. For each bond to an atom more electronegative than carbon (i.e. O) add (+1).
3. For each bond to another carbon atom add (+0).
H3CC
OH
H H
H3CC
O
H
H3CC
O
OHoxidation
reduction
oxidation
reduction
C–H: (-1)C–H: +(-1)C–O: +(+1)C–C: +(0)
Ox. #: –1
C–H: (-1)C–O: +(+1)C–O: +(+1)C–C: +(0)
Ox. #: +1
C–O: (+1)C–O: +(+1)C–O: +(+1)C–C: +(0)
Ox. #: +3
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Oxidation States of Carbon
14
-3 -2 -1
smaller number (more negative) = more electron density
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Oxidation States of Carbon
15
increasing oxidation state of carbon(increasing number of bonds to oxygen)
-3, -3 -3, -1 -3, +1 -3,+3
CH
HCH HH
HCH
HCH HOH
HCH
HCH HO
CH
HCH OHO
each carbon atom can have a different oxidation number
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Alkenes and Alkynes: sp2 & sp Hybridization
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Alkenes
17
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
sp2 Hybridization
18
One p-orbital is Reserved–Not Hybridized
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Valence Model of Bonding in Ethylene(with Hybridization)
19
2p
2sp2 2sp2 2sp2
2p
2sp2 2sp2 2sp2
C C
H H H HC CH H
HH1s 1s 1s 1s
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Orbitals on sp2-Hybridized Carbons
20
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Double Bond: 1 Pi-Bond & 1 Sigma-Bond
21
A double bond is formed by two orbital overlaps
1 pi (π) bond: side-to-side overlap of two p-orbitals; 2
π-electrons
1 sigma (σ) bond: head-to-head overlap of two
sp2-orbitals (not shown in "gure on left)
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Acetylenes
22
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Self Test Question
23
If the carbon atoms in acetylene are sp hybridized, what set of valence orbitals does each carbon atom contain?
A. one 2s, three 2pB. one 2s, two 2p, one spC. two sp, two p, one sp2
D. three sp, one pE. two sp, two p
C CH H
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Hybridization in Acetylene
24
Two p-orbitals are Reserved–Not Hybridized
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Valence Model of Bonding in Acetylene(with Hybridization)
25
2p
2sp 2sp
2p 2p 2p
2sp 2sp
C C
H H1s 1s
C C HH
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Orbitals on sp-Hybridized Carbons
26
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Triple Bond: 2 pi-bonds & 1 sigma-bond
27
A triple bond is formed by three orbital overlaps
2 pi (π) bonds: side-to-side overlap of two sets of
p-orbitals; 4 π-electrons
1 sigma (σ) bond: head-to-head overlap of two
sp2-orbitals
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Acetylene As a Fuel
28
Moweaqua, IL Mine Disaster 1932
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Hybridization and Acidity
29
increased s-character =increased electronegativity of carbon =
electrons closer to the nucleus =stronger acid
pKa = 26
pKa = 45
pKa = 62Incr
eas
ing
Aci
d S
tre
ngt
h
sp
sp2
sp3
C CH H C CH + H
C C C C + HH
H
H
H
H
H H
C CHHH
HH
HC C H
HHH
H+ H
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Molecule of the Week...2,4,6-Tribromoanisole The Smell of Unintended Consequences
30
Read more about tribromoanisole...
Paecilomyces variotii
Br
OH
Br
OMePaecilomyces
variotii
2,4,6-Tribromophenol 2,4,6-Tribromoanisole
BrBr Br Br
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Self Test Question
31
Which of the following best depicts a π-bond?
A. aB. bC. cD. dE. e
a.
b.
c.
d.
e.
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Summary of Bond Types
32
π-bond
C C C C
σ-bond
bond σ-bonds(head-to-head)
π-bonds(side-to-side)
single 1 0double 1 1triple 1 2
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Self Test Question
33
Rank the following hydrocarbons in order of increasing acidity.
A. ethane, ethylene, ethyne
B. ethane, ethyne, ethylene
C. ethyne, ethylene, ethane
D. ethyne, ethane, ethylene
E. none of the above
ethane ethylene ethyne
C CH
H H
HC C HHC C
HHHH
HH
four 2sp3 one 2pthree 2sp2
two 2ptwo 2sp
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Hybridization and Acidity
34
increased s-character =increased electronegativity of carbon =
electrons closer to the nucleus =stronger acid
pKa = 26
pKa = 45
pKa = 62Incr
eas
ing
Aci
d S
tre
ngt
h
sp
sp2
sp3
C CH H C CH + H
C C C C + HH
H
H
H
H
H H
C CHHH
HH
HC C H
HHH
H+ H
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Conformational Isomers of Alkanes
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Isomer Classification
36
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Classification of Isomers
37
stereoisomers
Can the molecules be interconverted by
rotation around single bonds?
noyes
conformational con$gurationalWe will continue the
isomer tree in Chapters 5 & 7 where we’ll
encounter subdivisions of con"gurational
isomers
HH
Me
Me
H
H
H
Me
H
Me
HH
anti butane gauche butane
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Model Activity
38
1. Make a model of butane.
2. Make a separate model of isobutane.
3. Using a minimum number of changes, convert your model of isobutane into butane.
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Self Test Question
39
What action did you have to perform to convert isobutane to butane?
A. rotate around C2-C3 bondB. remove methyl group from C-2C. add one methyl group to C-1D. add one methyl group to C-2E. rotate around C1-C2
If you have to break bonds to interconvert isomers, they are constitutional
(structural) isomers
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Rotation Around Single Bonds
40
conformations: different spatial arrangements of atoms generated by rotation around single bonds
conformational analysis: comparison of the relative energies of different conformational isomers and how they in&uence properties and reactivity
lowest energy
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Measuring Relative Positions of Atoms
41
dihedral angle: angle between two intersecting planes;also called the torsion angle
plane can be de$ned by:
• 3 non-collinear points
• a line & a point not on that line
• two intersecting lines
• two parallel lines
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Eclipsed Conformation of Ethane
42
• C–H bonds on adjacent carbons are parallel (same plane)• H–C–C–H angle (dihedral angle) = 0º• highest energy conformation
H
H
0º
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Staggered Conformation of Ethane
43
• C–H bond bisects (cuts in half ) H–C–H angle on adjacent carbon• H–C–C–H angle (dihedral angle) = 60º• lowest energy conformation for ethane
HH
60º
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Drawing Conformations: Wedge & Dash
44
= group is pointing toward you, in front of the plane of paper
= group is pointing away from you, behind the plane of paper
= group is either toward or away from you, usually denotes mixtures
= group lies in the plane of the drawing surface
rarely followed convention:
thickest part of wedge or dash is always closest to
viewer
H
HHH
HH
H H
HH
HH
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Drawing Conformations: Sawhorse
45
• one C–C bond viewed “head-on” from oblique angle (acute or obtuse, but not 0º, 90º or 180º); skewed
• all atoms on central C–C bond are shown
H3CCH3
H3C HH CH3
H
H
H HH
H
H
CH3H
H H
H
HH3C
H3CCH3 H3C
CH3
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Why a Sawhorse?
46
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Drawing Conformations: Newman Projection
47
• one C–C bond viewed “head-on” at 0º angle• all atoms on central C–C bond are shown• circle represents back carbon atom
H3CCH3 H3C
CH3 H3CCH3
HH
H
H
HCH3
HH3C
H
H
CH3H
H
H
H
HHH3C
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Spatial Relationships in Staggered Conformations: Anti & Gauche
48
•anti: dihedral angle (torsion angle) = 180º•gauche: dihedral angle (torsion angle) = 60º•these relationships apply to any groups on adjacent carbon atoms
H
X
H
Y
H
HX
H
Y
HH
HH
X
Y
HHH
0 °60 °
180 °
Torsion Angle = 0°Eclipsed
Torsion Angle = 60°Gauche
Torsion Angle = 180°Anti-Periplannar
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Comparison of Conformational Drawings of Eclipsed and Staggered Ethane
49
Ball & Stick Newman Sawhorse Dash & Wedge
HH
H
H
HH
H
H
H
H
HH
H HH H
H
H
H
H
H H
H
H
H H
HH
HH
H
HHH
HH
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Self Test Question
50
What is the IUPAC name for molecule below?
A. 1,2,2,4,4-pentametnylhexaneB. 3,3,5,5-tetramethylheptaneC. 2-ethyl-2,4,4-trimethylhexaneD. 1,2,4,4-tetramethylhexaneE. 3,3-dimethyl-5,5-dimethylheptane
1
23
45
67
HH3C
H
CH3
H3C
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 3
Self Test Question
51
Which set of molecules are conformational isomers?
A. aB. bC. cD. d
a.
b.
c.
d.
HH3C
H
CH3
CH3H3C
HH
CH3
CH3
HH3C
HCl
H
H
HH
ClH
H
H
HH
HH3C
CH3
H
H
HH3C
CH3
H
H
H
BrH
O
HHO
CH3 H
BrHO
H O
H
CH3
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