CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC
Chapter 19

Addition of Electrophiles to Alkene
C C
C C
C C
C C
C C
+
H H hydrogenation hydrogen halide addition
H C C H
+
H X H C C X
+
H Br free radical bromine addition
+ H
O
O S
O
OH sulfuric acid addition hydration
+ H OH
H C C Br
H C C OSO
3
H
H C C OH
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 2
Lecture 9: November 19

C C
Electrophilic Addition of HX hydrogen halide addition
+
δ + δ –
H X H C C X nucleophile electrophile
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 3
Lecture 9: November 19

H
H
Reaction Conditions
H Br
CHCl
3
, -30 ºC
H
Br
H H
University of
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UIC
• hydrogen halide: HXcommon solvents: chloroform (CHCl
3
) ,dichloromethane
(CH
2
Cl
2
), pentane, acetic acidgenerally performed at low temperature (below 0
°C)generally a fast reaction
CHEM 494, Fall 2012
Slide 4
Lecture 9: November 19

Electrophilic Addition (Ad
E
) Mechanism
H
H Br
Protonation
(slow)
H
Br
H
Br
H
H H
Cation
Capture
(fast)
H H
• electrophilic addition: AdERDS = protonation of carbonrate = k[alkene][hydrogen halide]unlike oxygen and nitrogen, protonation of carbon is slowproceeds through carbocation intermediate
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 5
Lecture 9: November 19

HX Addition is Regioselective
Regioselectivity
Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction
CHEM 232 Definition, 2010
H X
R
H
H
H
R
H
X
H
H
H
R rather than
H
H
H
X
H
R
R
H
H
H X R
R
X
H
H
H rather than
R
R
H
H
X
H
R
R
University of
Illinois at Chicago
UIC
R
H H X R
R
X
H
H
R rather than
R
R
H
H
X
R
CHEM 494, Fall 2012
Slide 6
Lecture 9: November 19

3º
Markovnikov’s Rule
CH
3
H
2º
H Br
CH
2
Cl
-40 ºC
2
H
3
C Br
H
H addition of HX to an unsymmetrically substituted alkene proceeds so that hydrogen (H) adds to the least substituted carbon and the halide (X) adds to the most substituted carbon atom
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 7
Lecture 9: November 19

Self Test Question
Predict the product when
2,4-dimethyl-2-pentene is treated with HCl?
A. 3-chloro-2,4-dimethylpentane
HCl
Cl
B. 2-chloroohexane
C. 2,3-dichloro-2,4dimethylpentane
D. 2-chloro-2,4-dimethylpentane
E. 1-chloro-2,4-dimethylpentane
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 8
Lecture 9: November 19

Mechanistic Basis for Markovnikov’s Rule
X
X H X
H
3º carbocation
H
3º alkyl halide
H X H
2º carbocation
X
+
H
X
2º alkyl halide curved arrows do not indicate which carbon is protonated fastest protonation leads to more stable
(more substituted) carbocation
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012 more substituted carbocation = more substituted alkyl halide
Slide 9
Lecture 9: November 19

Mechanistic Basis for Markovnikov’s Rule
Hammond Postulate: transition state structure resembles closest energy intermediate
University of
Illinois at Chicago
UIC
• transition state resembles carbocation for endothermic
RDS (late transition state)
• what stabilizes carbocation also stabilizes transition state
• lowest energy transition state leads to more substituted carbocation
• more substituted carbocation provides more substituted alkyl halide
CHEM 494, Fall 2012
Slide 10
Lecture 9: November 19

Rate of Alcohol Dehydration
Mirrors Ease of Carbocation Formation rate of dehydration = 3º > 2º > 1º alcohol
OH tertiary alcohol (3º) tertiary cation (3º) secondary alcohol (2º)
OH primary alcohol (1º)
University of
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OH
CHEM 494, Fall 2012
H secondary cation (2 °)
H
H primary cation (1 °)
Slide 11
Lecture 9: November 19

Self Test Question
Predict the product for the following reaction scheme.
OH
H
2
SO
4
140 ºC
?
A.
B.
-H


H

H

H
O
H
H

H

-H
2
O
H

H

H

H

C.
D.
E. no reaction
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 12
Lecture 9: November 19

Self Test Question
Predict the product for the following reaction scheme.
OH
H
2
SO
4
140 ºC
?
A.
B.
H
H
O
-H
2
O
-H

 or -H


H

H

H

H

H

H

H

C.
D.
E. no reaction
University of
Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 13
Lecture 9: November 19

Dehydration can be “Coupled” with Other
Chemical Transformation
N
O
Cl
HF
Cl
HF
Cl
N
OH
N
N
Me
N
Me
N
Me
Loratidine (Claritin
 
Two-step, one-pot transformation involves a Friedel-Crafts reaction (see, Chapter 12) and dehydration of the resulting 3 ° alcohol
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 14
Lecture 9: November 19

CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC
Chapter 19

Self Test Question
What is the product(s) of the following reaction?
University of
Illinois at Chicago
HO
UIC
H
2
SO
4
80 ºC
?
CHEM 494, Spring 2012
A.
B.
C.
D.
E.
O
2
S
Slide 16
Lecture 9: November 19

Types of Selectivity in Organic Chemistry
There are three forms of selectivity to consider . . .
.
Chemoselectivity: which functional group will react
Regioselectivity: where it will react
Stereoselectivity: how it will react with regards to stereochemical outcome
. . . for each transformation, always question which of these are factors are at play.
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 17
Lecture 9: November 19

Regioselectivity of Elimination
Regioselectivity: Where Will It React?
Preferential reaction at one site of a single functional group over other sites that could undergo the same reaction
CHEM 232 Definition, 2009
HO
H
2
SO
4
80 ºC
+ +
10% (identical) 90%
H H
H
3
C
HO
C H
3
CH
3
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 18
Lecture 9: November 19

Rearrangement Can Precede Addition
H
H Br
H
H
H
H
H hydride shift
CH
2
H
2º carbocation
Br
H
Br
CH
2
H
3º carbocation
Br
Br
CH
2
H
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 19
Lecture 9: November 19

Stability of Carbocations (Lecture 8)
2. Hyperconjugation stabilizing interaction that results from the interaction of the electrons in a σbond (C –H or C–C bond ) with an adjacent empty (or partially filled) orbital.
Leads to the formation of an extended molecular orbital that inc reases the stability of the system filled

orbital • stabilization results from σ-donation to empty p orbital of planar carbocation
H empty p orbital
• electron donation through σ-bonds toward carbocation delocalizes charge
(spreads out)
C C
H
H
H
H
• methyl cations cannot be stabilized by hyperconjugation since σ-bonds are perpendicular to the empty p orbital
1º cation
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 20
Lecture 9: November 19

Stability of Carbocations (Lecture 8)
2. Hyperconjugation filled

orbital
H
H
H
C C
H empty p orbital
H
C-H bonding
(filled)
E n e r g y
 empty
2p
Z
orbital
 p
Z
Stabilization resulting from hyperconjugation
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 21
Lecture 9: November 19

Rearrangement Can Precede Addition
H Br
CH
3 1,2-methyl shift
H
2º carbocation
H
3º carbocation
Br Br
University of
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UIC
Normal
Addition
Br
Br
H
CHEM 494, Fall 2012
Abnormal
Addition
H
Slide 22
Lecture 9: November 19

Reversal of Addition Regioslectivty
The Peroxide Effect
1-butene
H Br no peroxide
CH
2
Cl
2
Br
2-bromobutane
(90% yield)
1-butene
H Br
H
2
O
2
CH
2
Cl
2
1-bromobutane
(95% yield)
R
O
O
R peroxide
R
O
O
H
H
O
O
H hydroperoxide hydrogen peroxide
University of
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UIC
• alkyl peroxides easily fromed from alkanes/alkenes by reaction with O
2 in the air
• presence of peroxides leads to anti-
Markovnikov product (least substituted alkyl bromide)
• peroxide effect only operates when HBr
CHEM 494, Fall 2012
Slide 23
Lecture 9: November 19

Mechanistic Rationale for Peroxide Effect peroxides are radical initiators: they undergo homolysis to generate alkoxy radicals, which begin the chain mechanism
R
O
O
R initiation heat (² )
or light (h

2 R O alkoxy radical
R O
University of
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H Br initiation
CHEM 494, Fall 2012
ROH + Br bromine radical
Slide 24
Lecture 9: November 19

Mechanistic Rationale for Peroxide Effect
H H
Br
H H
Br propagation via alkene addition
H
H
H
H Br
Br
H H
University of
Illinois at Chicago
UIC
• bromine radical adds to the least substituted carbon of alkene
• this generates the most substituted and most stable alkyl radical
• alkyl radical undergoes hydrogen abstraction from
HBr to generate a new bromine radical (chain mechanism)
CHEM 494, Fall 2012
Slide 25
Lecture 9: November 19

Addition of Sulfuric Acid to Alkenes
C C alkene
+ H
O
O S
O
OH sulfuric acid addition
H C C OSO
3
H alkyl hydrogen sulfate
(ester of sulfuric acid) hydrolysis
H
2
O
H C C OH alcohol compare to:
HO alcohol
H
2
SO
4
80 ºC elimination
+ alkene(s)
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 26
Lecture 9: November 19

Sulfuric Acid Addition: Ad
E
Mechanism
H H
H
3
C CH
3
H O
O
S
O
OH
Protonation
(slow)
H
H
H
CH
3
CH
3
O
O
S
O
OH
Cation
Capture
(fast)
H
H
H
C C
CH
3
OSO
3
H
CH
3
• Markovnikov’s rule appliesprotonation occurs to provide most stable (most substituted) carbocationleads to formation of most substituted alkyl hydrogen sulfate
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 27
Lecture 9: November 19

Hydrolysis of Alkyl Hydrogen Sulfates
H C C O
O
S
O
OH alkyl hydrogen sulfate
(ester of sulfuric acid) hydrolysis
H
2
O
H C C O H alcohol
H
O
O S
O
OH sufuric acid
• don’t worry about mechanism for hydrolysisonly requires hot watercleavage of the O-S bondsubstitution of S with H
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 28
Lecture 9: November 19

Examples of Alkene Hydration
University of
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CHEM 494, Fall 2012
Slide 29
Lecture 9: November 19

Hydration of Alkenes (Addition of Water) hydration
C C alkene
+ H OH compare to:
HO alcohol
H
2
SO
4
80 ºC elimination
+ alkene(s)
H C C OH alcohol
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 30
Lecture 9: November 19

Hydration: Ad
E
Mechanism
H
H
Protonation
CH
3
CH
3 H
H
O
H
(slow)
H
H
H
CH
3
CH
3
H
O
H
H
2
O + HCl
H
H
H
HO
CH
3
CH
3 Protonation
(fast)
H
H
H
Cation
Capture
(fast)
H
O
H
CH
3
CH
3
H
O
H
Principle of Microscopic Reversibility in an equilibrium, the forward mechanism is identical to the reverse mechanism
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 31
Lecture 9: November 19

Hydration: Ad
E
Mechanism
O
H
H O
O
S
O
OH fast & reversible
O
H
H
O
O
S
O
OH slow & reversible
Me
H
CH
2
O
O
S
O
OH fast & reversible
Me
Me
H
2
O
Principle of Microscopic Reversibility in an equilibrium, the forward mechanism is identical to the reverse mechanism
Me
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 32
Lecture 9: November 19

CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC

Analytical Chemistry
Separation of MixturesandIdentification of
Components
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 34
Lecture 9: November 19

High Performance Liquid Chromatography
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 35
Lecture 9: November 19

Gas Chromatography
University of
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UIC
CHEM 494, Fall 2012
Slide 36
Lecture 9: November 19

Structural Determination
University of
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CHEM 494, Fall 2012
Slide 37
Lecture 9: November 19

Spectroscopy vs. Spectrometry
Spectroscopy study of the interaction of electromagnetic radiation with matter; typically involves the absorption of electromagnetic radiation
Spectrometry evaluation of molecular identity and/or properties that does not involve interaction with electromagnetic radiation
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 38
Lecture 9: November 19

Spectroscopic Methods
Method
Infrared
Spectroscopy
Measurement/Application
• vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment
• functional group determination
Ultraviolet-Visible
(UV-vis)
Spectroscopy
• electronic states: energy associated with promotion of an electron in a ground state to an exited state
• chromophore determination
Mass
Spectrometry
• molecular weight: of parent molecule and fragments produced by bombardment with “free” electrons
• fragment and isotope determination
Nuclear Magnetic
Resonance
Spectroscopy
• nuclear spin states: energy associated with spin states of nuclei in the presence of a magnetic field
• determine structural groups and connectivity
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 39
Lecture 9: November 19

Absorption/Transmission Spectroscopy:
Simplified Principles
University of
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• sample absorbs different frequencies of light corresponding to molecular vibrations (IR) or electronic transitions (UV-vis)
• detector determines what frequencies of light passed through (transmittance) and what frequencies of light were absorbed
(absorbance)
CHEM 494, Fall 2012
Slide 40
Lecture 9: November 19

shorter wavelength
(λ) higher frequency (ν) higher energy (E)
Electromagnetic Spectrum longer wavelength (λ) lower frequency (ν) lower energy (E)
Electromagnetic Radiation
• propagated at the speed of light (3 x10 8 m/s)
• has properties of particles and waves
• energy is directly proportional to frequency
• energy is indirectly proportional to wavelength
E = hν c = νλ
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 41
Lecture 9: November 19

University of
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UIC
Quantized Energy States
Types of
States
Energy
Range (λ)
Spectroscopic
Method nuclear spin radiofrequency
1-10 m
NMR rotational microwave
10-100 cm vibrational infrared
0.781000 μm electronic ultraviolet
800-200 nm
Microwave
IR
UV-vis
CHEM 494, Fall 2012
Slide 42
Lecture 9: November 19

CHEM 494
Special Topics in Chemistry
University of
Illinois at Chicago
UIC

Principles of Infrared Spectroscopy
IR: Measures the vibrational energy associated with stretching or bending bonds that contain a dipole moment ( µ).
Stretching
         
 
University of
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 
 
Bending
 
 
CHEM 494, Fall 2012
 
 
Slide 44
Lecture 9: November 19

Stretching & Bending Vibrations
University of
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UIC
CHEM 494, Fall 2012
Slide 45
Lecture 9: November 19

Dipole Moment more electronegative atom

(partially negatively charged) covalent 2 electron bond dipole arrow less electronegative atom

(partially positively charged)
In order to measure the stretching or bending frequency of a covalent bond, it must have a dipole moment (μ).
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 46
Lecture 9: November 19

Hooke’s Law: Bonds are Like Springs
Vibrational Energy Depends both on bond strength (spring force constant) and the mass of atoms (objects) attached
ν~ = k
f *
( m
1
+ m
2
)
( m
1
* m
2
)
Trends :
ν = vibrational “frequency” in wavenumbers (cm -1 ) k = constant (1/2πc) f = force constant; strength of bond (spring) m
1
, m
2
= masses (not molecular weights) of attached atoms
University of
Illinois at Chicago
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CHEM 494, Fall 2012
↑ bond strength =
↑ frequency
↑ mass =
↓ frequency
Slide 47
Lecture 9: November 19

Spring Analogy smaller mass = higher frequency = higher energy
University of
Illinois at Chicago
UIC stronger spring (bond) = higher frequency = higher energy
CHEM 494, Fall 2012
Slide 48
Lecture 9: November 19

Wavenumber ( ῡ ) and Infrared Scale ῡ (cm -1 ) =
1
λ (cm) higher wavenumber (ῡ)
= higher frequency (υ) = lower wavelength (λ) = higher energy (E)
N-H O-H
C(sp)-H
C(sp2)-H
C(sp3)-H
3400 3000
CO
2
(2380)
C N
C C
2600 2200 wavenumber (cm
-1
)
C O
C C
C N lower wavenumber (ῡ) = lower frequency (υ) = longer wavelength (λ) = lower energy (E) fingerprint region
C O
1800 1400 1000 wavenumber = reciprocal of the wavelength measured in centimeters (cm); directly proportional to frequency
University of
Illinois at Chicago
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CHEM 494, Fall 2012
Slide 49
Lecture 9: November 19

Infrared Spectrum
100
80
60
40
20
Transmittance: amount of light that passes through sample; not absorbed by molecular vibrations
Frequency: typically measured in wavenumbers; higher wavenumber = higher frequency = higher energy vibration
Bands: frequency of vibration absorbed by molecules; can be broad or narrow; number of bands does not equal number of bonds
4000
University of
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3500 3000 2500 2000
Wavenumbers
CHEM 494, Fall 2012
1500 1000 500
Slide 50
Lecture 9: November 19

100
80
60
40
20
Characteristic Stretches - Alkanes
2
4000 3500 3000
University of
Illinois at Chicago
UIC
3
1
2500 2000
Wavenumbers
1500 1000
CHEM 494, Fall 2012
500
3
1
H H H H
H C C CH
3
C
H
2
C C
H H H H
2 hexane
• 2 = sp 3 C-H bond stretching motion; general absorb around 2850-2950 cm -1
• 1 = C-H rocking motion when C atom is part of a methyl group (-CH
3
); 1370-
1350 cm -1
• 3 = scissor motion of -CH
3 hydrogen atoms; 1470-1450 cm -1
• 1300-900 cm -1 = fingerprint region for organic molecules; typically complex and unhelpful
Slide 51
Lecture 9: November 19

100
80
60
40
Characteristic Stretches - Alkenes
5 4
H
3
C
H
C
H H H
C
C C
H H H
5
H
C
H
4
1-hexene
• 5: notice sp 2 C-H (~3100 cm -1 ) at higher frequency than sp3 C-H (~2950 cm -1 )
• more s-character = stronger bond = higher frequency
• 4: also, C=C bond at higher frequency than C-C bond;
~1600 cm -1
20
4000 3500
University of
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3000 2500 2000
Wavenumbers
1500 1000
CHEM 494, Fall 2012
500
Slide 52
Lecture 9: November 19

Characteristic Stretches - Alkynes
7
6
H
3
C
6
C
C
H
7
1-hexyne
• 7: notice sp C-H (~3300 cm -
1 ) at higher frequency than sp 2 C-H (~3100 cm -1 ), which was higher than sp 3
(~2950 cm -1 )
C-H
• 6: C ≡C stretch is very weak because carbons have almost identical electronegativities = small dipole moment
University of
Illinois at Chicago
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CHEM 494, Fall 2012
Slide 53
Lecture 9: November 19

100
80
60
40
20
Characteristic Stretches - Alcohols
9
5
4
4000 3500
University of
Illinois at Chicago
UIC
3000 2500 2000
Wavenumbers
1500 1000
CHEM 494, Fall 2012
500
H
9
5
O
H H
C
C
H
H
C
4
H prop-2-en-1-ol
(allyl alcohol)
• 9: hydroxyl groups (-OH) exhibit strong broad bands;
~3300 cm -1
• broad peak is a result of hydrogen bonding; width depends on solution concentration
• lower concentration = less hydrogen bonding = more narrow -OH band
Slide 54
Lecture 9: November 19

Characteristic Stretches - NItriles
100
80
60
40
20
9
8
4000 3500
University of
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UIC
3000 2500 2000
Wavenumbers
1500 1000
CHEM 494, Fall 2012
H
9
O
C
N
8
3-hydroxy-propionitrile
• 8: nitriles ~2200 cm -1
• nitriles (C≡N) absorb a greater magnitude of energy than alkynes (C≡C) because they have a larger dipole moment
• larger dipole moment = more intense peak
500
• size of the dipole does NOT affect frequency of vibration
Slide 55
Lecture 9: November 19

100
80
60
40
20
Example: Ester, Amine, Benzene
11
10
4
4
C
C
O
10
C
O
N
H
11
H
2-amino-benzoic acid butyl ester
• 10: strong carbonyl (C=O) band ~1700 cm -1
• 11: amines; secondary amines (-NH) give one band; primary amines (-
NH
2
) gives two bands
• 4: several alkene bands
~1600 cm -1 for benzene ring C=C double bonds
4000 3500 3000
University of
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UIC
2500 2000
Wavenumbers
1500 1000
CHEM 494, Fall 2012
500
Slide 56
Lecture 9: November 19

100
80
60
40
Characteristic Stretches - Carboxylic Acids
9
10
4
5
C
H
C
O
10
C
O
H
9
H
4 cyclohex-2-enecarboxylic acid
• 10: strong carbonyl (C=O) band ~1700 cm -1
• 9: hydroxyl band (-OH) can be less intense and sharper in carboxylic acids
• 4: weak alkene band (C=C) since small dipole moment
20
4000 3500 3000
University of
Illinois at Chicago
UIC
2500 2000
Wavenumbers
1500 1000
CHEM 494, Fall 2012
500
Slide 57
Lecture 9: November 19

100
80
60
40
Characteristic Stretches - Aldehydes
4
O
10
H
C
4
C
C
H 12
H hept-2-enal
12
• 12: usually two bands for C-
H of aldehydes; may overlap with sp 3 C-H bands
10
20
4000 3500 3000
University of
Illinois at Chicago
UIC
2500 2000
Wavenumbers
1500 1000
CHEM 494, Fall 2012
500
Slide 58
Lecture 9: November 19

University of
Illinois at Chicago
UIC
Self Test Questions
Which molecule is represented by the
IR below?
O
OH O H
2
C
OH
CH
3
H
2
C O CH
3 H
H
3
C CH
3 cyclobutanol a.
2-butanone b.
ethyl vinyl ether c.
2-methyl-2-propen-1-ol 2-methylpropanal d.
e.
CHEM 494, Fall 2012
A.a
B.b
C.c
D.d
Slide 59
Lecture 9: November 19

University of
Illinois at Chicago
UIC
Self Test Questions
Which molecule is represented by the
IR below?
O
OH O H
2
C
OH
CH
3
H
2
C O CH
3 H
H
3
C CH
3 cyclobutanol a.
2-butanone b.
ethyl vinyl ether c.
2-methyl-2-propen-1-ol 2-methylpropanal d.
CHEM 494, Fall 2012
A.a
B.b
C.c
D.d
Slide 60
Lecture 9: November 19

University of
Illinois at Chicago
UIC
Self Test Questions
Which molecule is represented by the
IR below?
O
OH O H
2
C
OH
CH
3
H
2
C O CH
3 H
H
3
C CH
3 cyclobutanol a.
2-butanone b.
ethyl vinyl ether c.
2-methyl-2-propen-1-ol 2-methylpropanal d.
e.
CHEM 494, Fall 2012
A.a
B.b
C.c
D.d
Slide 61
Lecture 9: November 19

University of
Illinois at Chicago
UIC
Self Test Questions
Which molecule is represented by the
IR below?
O
OH O H
2
C
OH
CH
3
H
2
C O CH
3 H
H
3
C CH
3 cyclobutanol a.
2-butanone b.
ethyl vinyl ether c.
2-methyl-2-propen-1-ol 2-methylpropanal d.
e.
CHEM 494, Fall 2012
A.a
B.b
C.c
D.d
Slide 62
Lecture 9: November 19

University of
Illinois at Chicago
UIC
Self Test Questions
Which molecule is represented by the
IR below?
O
OH O H
2
C
OH
CH
3
H
2
C O CH
3 H
H
3
C CH
3 cyclobutanol a.
2-butanone b.
ethyl vinyl ether c.
2-methyl-2-propen-1-ol 2-methylpropanal d.
e.
CHEM 494, Fall 2012
A.a
B.b
C.c
D.d
Slide 63
Lecture 9: November 19

University of
Illinois at Chicago
UIC
Self Test Questions
Which molecule is represented by the
IR below?
O
OH O H
2
C
OH
CH
3
H
2
C O CH
3 H
H
3
C CH
3 cyclobutanol a.
2-butanone b.
ethyl vinyl ether c.
2-methyl-2-propen-1-ol 2-methylpropanal d.
e.
CHEM 494, Fall 2012
A.a
B.b
C.c
D.d
Slide 64
Lecture 9: November 19

OH cyclobutanol
O
CH
H
3
C
2-butanone
3
Example
H
2
C O CH
3 ethyl vinyl ether
H
2
C
O
OH
CH
3
H
2-methyl-2-propen-1-ol 2-methylpropanal a.
b.
c.
Greater s character = stronger, shorter bonds = higher frequency
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012 d.
e.
A.a
B.b
C.c
D.d
Slide 65
Lecture 9: November 19

CHEM 494
University of
Illinois at Chicago
UIC

Spectroscopy vs. Spectrometry
Spectroscopy study of the interaction of electromagnetic radiation with matter; typically involves the absorption of electromagnetic radiation
Spectrometry evaluation of molecular identity and/or properties that does not involve interaction with electromagnetic radiation
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 67
Lecture 9: November 19

Self Test Question
H
3
C a
Which molecule corresponds to the IR spectrum below?
O
O
O
O NH
2
OH
OH b c d e
A.a
B.b
C.c
D.d
E.e
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 68
Lecture 9: November 19

Self Test Question
O C O
A
Which covalent bond, highlighted in bold (red) in the molecules below, would not be expected to exhibit an IR stretching band?
H
3
C C N
B
H
3
C C N
C
H
H C
H
D
H H
3
C C C CH
3
E
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 69
Lecture 9: November 19

CHEM 494
University of
Illinois at Chicago
UIC
Section 13.24
You are responsible for section 13.25!

Mass Spectrometry
Primary Applications:
1.Determine molecular mass.
2.Establish fragmentation patterns, which can be indexed in a database.
3.Determine presence of some heteroatoms.
4.Determine the exact mass of molecules.
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 71
Lecture 9: November 19

Mass Spectrometer Schematic creates charged molecules called radical cations radical cations are accelerated by negatively charged plates
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012 magnetic fields exert forces on moving charges
Slide 72
Lecture 9: November 19

Formation of Radical Cations
• organic molecules are bombarded with 70-eV electrons
• causes organic molecule to lose one electron from a covalent bond
• organic molecule is then charged
• the mass of charged species is determined by a mass spectrometer
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 73
Lecture 9: November 19

Molecular Ion Peaks
C: 6 x 12 = 72
H: 6 x 1 = 6
Total = 78
• molecular ion peak = highest m/z (mass/charge) peak
• since charge (z) is usually 1, molecular ion peak = molecular mass
• molecular ion peak does not have to have relative intensity of
100%
• most intense peak = base peak
• relative intensity = height of peak ÷ base peak at Chicago
UIC
CHEM 494, Spring 2012
Slide 74
Lecture 9: November 19

Fragmentation heterolysis
• radical cation fragments
(heterolysis) to give a neutral radical species and a cation
• fragment contains less mass than parent ion
• relative intensity of the fragment depends on its concentration (likelihood of occurring)
• more stable cations are more likely; give more intense peaks
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 75
Lecture 9: November 19

Fragmentation
Common Fragmentation Pattern for
Alkanes
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 76
Lecture 9: November 19

Fragmentation
Common Fragmentation Pattern for Alkyl
Benzenes benzylic carbon
H
C
H benzylic carbocation
University of Illinois at Chicago
UIC
H
C
H
H
C
H
H
C
H
CHEM 494, Spring 2012
H
C
H benzylic carbocation stabilized by resonance
= common fragment in
MS
Slide 77
Lecture 9: November 19

Fragmentation
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 78
Lecture 9: November 19

Fragmentation
Since fragmentation patterns should be the same for identical molecules, they can be saved in a database and matched to unknowns later. CSI anyone?
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 79
Lecture 9: November 19

Isotopic Clusters: Carbon and Hydrogen
M + 1 M + 1
University of Illinois at Chicago
UIC
CHEM 494, Spring 2012
Slide 80
Lecture 9: November 19

Isotopic Clusters: Carbon and Hydrogen
M + 1
Natural Abundance of Isotopes
Isotope
13 C
12 C
2 H (D)
1 H
Abundance
1.10%
98.90%
0.015%
99.985%
University of Illinois at Chicago
UIC
Probability of M+1
6 x 1.1% = 6.6% of 13 C
6 x 0.015% = 0.1% of 2 H
Total Probablility = 6.7%
M + 1
• mass spectrometry is sensitive enough to resolve exact masses of isotopes
• intensity of the peaks corresponds to natural abundance of each isotope
• probability = number of atoms in molecule x natural abundance
CHEM 494, Spring 2012
Slide 81
Lecture 9: November 19

Isotopic Clusters: Chlorine & Bromine
Natural Abundance of Isotopes
Isotope
35 Cl
37 Cl
79 Br
81 Br
Abundance
75.77%
24.23%
50.69%
49.31%
University of Illinois at Chicago
UIC
•
Chlorine: M:(M+2) ~ 3:1
•
Bromine: M:(M+2) ~ 1:1
• probability = number of atoms in molecule x natural abundance
CHEM 494, Spring 2012
Slide 82
Lecture 9: November 19

Spectroscopic Methods
Method Measurement/Application
Infrared
Spectroscopy
• vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment
• functional group determination
Ultraviolet-Visible
• electronic states: energy associated with promotion of an electron in a ground state to an
(UV-vis exited state
)Spectroscopy
• chromophore determination
Mass
Spectrometry
Nuclear
Magnetic
University of Illinois
UIC
Spectroscopy
• molecular weight: of parent molecule and fragments produced by bombardment with “free” electrons
• fragment and isotope determination
• nuclear spin states: energy associated with spin states of nuclei in the prescence of a magnetic
Slide 83 field
CHEM 494, Spring 2012
Lecture 9: November 19
• determine structural groups and connectivity

Spectroscopic Methods
Method Measurement/Application
Infrared
Spectroscopy
• vibrational states: stretching and bending frequencies of covalent bonds that contain a dipole moment
• functional group determination
Ultraviolet-Visible
• electronic states: energy associated with promotion of an electron in a ground state to an
(UV-vis exited state
)Spectroscopy
• chromophore determination
Mass
Spectrometry
Nuclear
Magnetic
University of Illinois
UIC
Spectroscopy
• molecular weight: of parent molecule and fragments produced by bombardment with “free” electrons
• fragment and isotope determination
• nuclear spin states: energy associated with spin states of nuclei in the prescence of a magnetic
Slide 84 field
CHEM 494, Spring 2012
Lecture 9: November 19
• determine structural groups and connectivity

Mass Spectrometry: Mass and Molecular Formula
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 85
Lecture 9: November 19

Mass Spectrometer - General Layout
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 86
Lecture 9: November 19

Fragmentation of Toluene Parent Ion
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 87
Lecture 9: November 19

Different Molecules Can Have the Same
Molecular Weight!
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 88
Lecture 9: November 19

Accurate Mass Measurement is the Solution!
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 89
Lecture 9: November 19

Parent Ions Undergo Fragmentation
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 90
Lecture 9: November 19

Mass Spectrometer - Location of
Fragmentation
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 91
Lecture 9: November 19

The Course of Fragmentation is
Directed by Daughter Ion Stability
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 92
Lecture 9: November 19

Fragmentation Patterns - Elimination of Water
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 93
Lecture 9: November 19

Fragmentation Patterns - McLafferty
Rearrangement
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 94
Lecture 9: November 19

McLafferty Rearrangement of Butyraldehyde
University of
Illinois at Chicago
UIC
CHEM 494, Fall 2012
Slide 95
Lecture 9: November 19