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University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Prof. Duncan WardropSeptember 17, 2012
CHEM 494 - Lecture 2
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Course Website
2
http://www.chem.uic.edu/chem494
SyllabusCourse PoliciesOther handoutsAnnouncements (Course News)Course Calendar
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Periodicity
3
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Periodicity
4
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Lecture Summary
5
Revision & Proof of Resonance
Introduction to Hydrocarbons
Classi!cation
Structure & Bonding
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Revision of Resonance
6
Do not confuse equilibrium with resonance: double-headed arrows represent resonance contributors; two separate arrows represent the equilibrium of distinct chemical species....
O
MeO
O
MeO
Equilibrium(atoms move)
O
H
OH
Resonance(electrons move)
HH
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Prioritizing Resonance Contributors(How much Unicorn?!)
7
Priority 1No !rst-row elements (B, C, N, O) can have more than eight electrons in its valence shell.....
NO
ONO
ONO
O
EqualContributor
EqualContributor
Non-PermissableContributor
(N has 10 elect.)
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Prioritizing Resonance Contributors
8
Priority 2Resonance structures in which all atoms are surrounded by an octet of electrons are almost always lower in energy than those resonance structures in which one or more atoms are electron de!cient.
OH
OH8 electrons
8 electrons
6 electrons
MoreStable
OH
120 pm (C=O)128 pm (calculated)143 pm (C-O)
C-O Bond Length
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Prioritizing Resonance Contributors
9
Priority 3Resonance structures with charge separation are typically higher in energy than those without. If charge separation exists, then electropositive atoms (C, B) are better able to bear + charge...
H HH H
H H
UnimportantContributor
MajorContributor
PhysicalEvidence
0.66 D
Small DipoleMoment
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Prioritizing Resonance Contributors
10
Priority 4If charge separation exists in a resonance structure, then the electronegative atoms should gain the formal negative charge and vice versa:
4.02 D
O O
MajorContributor
Minor(but significant)
Contributor
O
UnimportantContributor
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Self Test Question
11
Rank the following acids in order of increasing acidity.
A. a, c, b, e, dB. e, c, a, b, dC. c, e, a, d, bD. b, a, c, d, eE. d, b, a, c, e
OH
O
OH
OH
O
O+ H
O + H
O + H
SH
O
S
O+ H
HN N + H
a.
b.
c.
d.
e.
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Self Text Question
12
Which is the most acidic proton in the molecule below?
A. aB. bC. cD. dE. e
O
OH
H HO
H
HH
HH
a
b
c
d
O He
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Classes, Bonding & Structural Features of
Hydrocarbons
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Hydrocarbons
Aliphatic
Alkanes
Classes of Hydrocarbons
14
• aleiphar Greek for “fat”• sources were fats and oils
• only single bonds• nomenclature: -ane suffix
Alkenes Alkynes
• contain a double bond• nomenclature: -ene suffix
• contain a triple bond• nomenclature: -yne suffix
Aromatic
• only C & H atoms• framework for funct. groups• generally, non-reactive
• aka: arenes• many isolated from plants;
impurities can be fragrant• aromaticity de"nition: Ch. 12
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Alkanes
Examples of Aliphatic Hydrocarbons
15
• only single bonds• CnH2n+2
• nomenclature: -ane suffix
Alkenes Alkynes• contain a double bond•CnH2n (1 double bond)•nomenclature: -ene
suffix
• contain a triple bond•CnH2n-2 (1 triple bond)•nomenclature: -yne
suffix
C CH
HC
H
HC H
H
HH
H
H
CH3CH2CH2CH3
C CH
CH
HCHH
H H
H
CH3CH2CHCH2
C C CH
HCHH
HH
CH3CH2CCH
butane 1-butene 1-butyne
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Examples of Aromatic Hydrocarbons
16
Aromatic• aka: arenes• many isolated from plants;
impurities were fragrant• aromaticity de"nition: Ch.
12
CCCCC
C
CH3 N O
HH
HH
H
H
benzene benzene
toluene napthalene pyridine furan
• many are derivatives of benzene
• resonance: delocalized electrons
• adjacent double bonds• planar (120º bond angles)• polyaromatic hydrocarbons
(PAHs) contain two or more aromatic rings; highly carcinogenic and environmental toxin
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Structural Features of Simple Alkanes
17
• 4 bonds to carbon, 1 bond to H• C–H bond length ~ 110 pm• C–C bond length ~ 153 pm• all bond angles are nearly tetrahedral
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Initial Valence Bond Model of Carbon
18
2s2p 2p 2p
1s 1s
C
H H
CH H
• carbon only has two half-!lled orbitals• should, then, form two bonds• actually forms 4 bonds!?!?
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Valence Bond Model: Hybridization
19
2s
2p 2p 2p
sp3 sp3 sp3 sp3
mix s & p orbitals
• hybrids: mixture of s & p; sp3 lower in energy than p• 25% s-character and 75% p-character• 4 degenerate orbitals = equal in energy• each half-"lled, can form 4 bonds• VSEPR: tetrahedral (bond angles = 109.5º)
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
+ –+
Understanding the Shape of sp3 Orbitals
20
+
+ –
+ –
destructiveinterference
constructiveinterference
• constructive interference: reinforce electron wave where sign is the same• destructive interference: cancel out wave where sign is opposite• higher probability of "nding an electron on one side of the nucleus
x3
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Sigma Bonds (σ-bond) in Methane
21
• σ-bond: head-to-head overlap of orbitals; strongest bond type• hybridization increases bond strength of σ-bonds since electrons are
concentrated on one side of nucleus• only larger lobe of sp3 involved in bonding• typically ignore (don’t draw) smaller lobe
109.5º
C H
H
HH
= 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
C(2sp3)––H(1s)σ-bond
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
The C–C σ-bond in Ethane
22
• in-phase overlap of an sp3-hybridized orbital from each carbon atom• overlap is along internuclear axis (σ-bond)• VSEPR: tetrahedral around each carbon atom
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Nomenclature of Hydrocarbons
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
What’s In a Name?
24
N
O
CH3
CH3H3C
IUPAC: ethyl (3R,4R,5S)-5-amino-4-acetamido-3-(pentan-3-yloxy)cyclohex-1-ene-1-carboxylateGeneric: oseltamivirTrade: Tamiflu
IUPAC: N,N-diethyltoluamideInitialism: DEETTrade: OFF
IUPAC: sodium dodecyl sulfateCommon: sodium lauryl sulfateTrade: SDS
IUPAC: Sodium (2S)-2-amino-5-hydroxy-5-oxo-pentanoateCommon: mono sodium glutamateInitialism: MSG
O
O
OClHO
HOCl
HO
ClOH
OH
IUPAC: 1,6-Dichloro-1,6-dideoxy-β-D-fructofuranosyl-4-chloro-4-deoxy-α-D-galactopyranosideCommon: sucraloseTrade: Splenda
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
IUPAC: Alkanes with NO Substituents
25
IUPAC: International Union of Pure and Applied Chemistry
common name
retained
Greekprefixes
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Ide
nti
cal
Constitutional (Structural) Isomers
26
C9H20
C9H20 C9H20
C9H20
constitutional isomers: same molecular formula; different connectivity
connectivity: order in which the atoms are bonded
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Common Names for Branched Isomers of Simple Alkanes
27
n-butane n-pentane n-hexane
iso-butane iso-pentane iso-hexane
neo-pentane neo-hexane
“normal”straight chain
isoCH3
CHH3C
n neoCH3
CH3C CH3
homologous series: successive members differ by a –CH2– group
methyl: CH3
methylene: CH2
methine: CH
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Common Names Become Too Complex:Systematic Method of Naming is Needed
28
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
IUPAC: Naming Alkyl Substituent Groups
29
H3CCCCC
H H
H3C H
H HCH3
H H
substituent: atom or group other than H bonded to carbonalkyl group: one H atom less than an alkane
Steps:1. Identify longest continuous chain of carbon atoms.2. Locate substituents attached to the longest continuous chain of carbon atoms.3. Name alkane substituent by replacing -ane of parent alkane with -yl
CH3CH2CHCH2CH2CH3CH3
CH3CH3Ethane
CH3(CH2)5CH3Heptane
CH3(CH2)16CH3Octadecane
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Common & IUPAC Names of 3-Carbon Alkyl Groups
30
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Common & IUPAC Names of 4-Carbon Alkyl Groups
31
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Carbon Classification
32
University of Illinois at ChicagoUICCHEM 494
Special Topics in Chemistry
Physical Properties of Alkanes
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Crude Oil - Source of Alkanes
34
cracking: (chemical change)converts high molecular weight hydrocarbons to more useful, lower molecular weight fragments
reforming: (chemical change)increases branching of the hydrocarbon chains; branched hydrocarbons have better burning characteristics
distillation: (physical separation)increases branching of the hydrocarbon chains; branched hydrocarbons have better burning characteristics
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Oil Refining
35
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Boiling Points Increase with # of Carbons
36
• boiling point increases with increasing number of carbon atoms
• branched alkanes have lower boiling points than unbranched isomers
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Intermolecular Attractive Forces
37
van der Waals forces (VWF): intermolecular attraction between neutral species1. dipole-dipole (including hydrogen bonding)2. dipole/induced-dipole3. induced-dipole/induced-dipole (London dispersion)
• alkanes are non-polar; dipole-dipole & dipole/induced-dipole forces absent
X
X
✓
• only forces of attraction between non-polar compounds are induced-dipole/induced-dipole
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Induced-Dipole/Induced-Dipole(London Dispersion Forces)
38
• green = electric !eld of surrounding two covalently bonded nuclei
• a.k.a. cumulative electron cloud
initial picture: centers of positive and negative charge in each individual molecule are identical
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Induced-Dipole/Induced-Dipole(London Dispersion Forces)
39
induced dipole in left molecule: movement of electrons creates instantaneous dipole in left molecule
• constant motion of electrons in orbitals• blue/red: separation of positive and negative areas of
charge within the electric !eld of molecule• caused by asymmetric distribution of electrons
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Induced-Dipole/Induced-Dipole(London Dispersion Forces)
40
left dipole induces right dipole: electron cloud in right molecule spontaneously adjusts to complement left
• electric attraction between two molecules• both electric "elds $uctuate, but always in the direction
that produces weak attraction• more atoms = more electrons = more induced dipoles =
more attractive forces = higher boiling points
attractive force
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Induced-Dipole/Induced-Dipole(London Dispersion Forces)
41
HC
CC
CC
H H H H H H
H
HHHH
HC
CC
CC
H H H H H H
H
HHHH
more atoms =
more electrons =
more induced dipoles =
more attractive forces =
higher boiling point
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
London VWFs in Branched Alkanes
42
Increasing London Dispersion Forces
Increasing Surface Area
unbranched highly branched
Gas at RTLiquid at RT
UICUniversity of Illinois at Chicago CHEM 494, Fall 2012
SlideLecture 2
Contribution of London Forces toIntermolecular Interaction Energy
43
Molecule Pair % of the Total Interaction Energy
H2O-H2O 24
NH3-NH3 57
HCl-HCl 86
HBr-HBr 96
HI-HI 99
Ne-Ne 100
CH4-CH4 100