University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Lecture 13Organic Chemistry 1
Professor Duncan Wardrop
February 23, 2010
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University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Chapter 13
Spectroscopy &Spectrometry
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University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Sections: 13.1-13.2
Introduction toAnalytical Methods
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Spectroscopy vs. Spectrometry
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Spectroscopystudy of the interaction of electromagnetic radiation with matter; typically involves the absorption of electromagnetic radiation
Spectrometryevaluation of molecular identity and/or properties that does not involve interaction with electromagnetic radiation
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Spectroscopic Methods
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Method Measurement/Application
InfraredSpectroscopy
• 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
MassSpectrometry
• 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 !eld
• determine structural groups and connectivity
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Absorption/Transmission Spectroscopy: Simpli!ed Principles
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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)
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Electromagnetic Spectrum
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shorter wavelength (λ)higher frequency (ν)
higher energy (E)
longer wavelength (λ)lower frequency (ν)
lower energy (E)
Electromagnetic Radiation
• propagated at the speed of light (3 x108 m/s)
• has properties of particles and waves
• energy is directly proportional to frequency
• energy is indirectly proportional to wavelength
E = hν c = νλ
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Quantized Energy States
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Types of States
EnergyRange (λ)
Spectroscopic Method
nuclear spin radiofrequency1-10 m
NMR
rotational microwave10-100 cm
Microwave
vibrational infrared0.78-1000 μm
IR
electronic ultraviolet800-200 nm
UV-visIncreasing Energy
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University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Sections: 13.20-13.22
Infrared Spectroscopy
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Principles of Infrared Spectroscopy
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Stretching
Bending
IR: Measures the vibrational energy associated with stretching or bending bonds that contain a dipole moment (µ).
δ−δ− δ−δ+ δ+ δ+
δ+
δ−
δ+
δ−
δ+
δ−
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Stretching & Bending Vibrations
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Dipole Moment
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In order to measure the stretching or bending frequency of a covalent bond, it must have a dipole moment (μ).
moreelectronegative
atom
lesselectronegative
atomdipole arrow
δ δ
covalent 2electron bond
(partially negatively charged) (partially positively charged)
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Hooke’s Law: Bonds are Like Springs
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Vibrational Energy Depends both on bond strength (spring force constant) and the mass of atoms (objects) attached
(m1 + m2)
(m1 * m2)f *√ν~ = k
ν = vibrational “frequency” in wavenumbers (cm-1)~
k = constant (1/2πc)
f = force constant; strength of bond (spring)
m1, m2 = masses (not molecular weights) of attached atoms
Trends:
↑ bond strength = ↑ frequency
↑ mass = ↓ frequency
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Spring Analogy
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smaller mass =higher frequency =
higher energy
stronger spring (bond) =higher frequency =
higher energy
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Wavenumber (ῡ) and Infrared Scale
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3400 3000 2200 1000
N-H C NC CCO2
(2380)
C O
C NC C
fingerprintregion
wavenumber (cm-1)2600
O-HC(sp)-H
C(sp2)-HC(sp3)-H
14001800
C O
higher wavenumber (ῡ) =higher frequency (υ) =lower wavelength (λ) =higher energy (E)
lower wavenumber (ῡ) =lower frequency (υ) =
longer wavelength (λ) =lower energy (E)
wavenumber = reciprocal of the wavelength measured in centimeters (cm); directly proportional to frequency
ῡ (cm-1) = 1λ (cm)
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Infrared Spectrum
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5001000150020002500300035004000Wavenumbers
20
40
60
80
100
Transmittance: amount of light that passes through sample; not absorbed by molecular vibrations
% T
rans
mis
sion
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
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Characteristic Stretches - Alkanes
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CH2
CC
CC
CH3
H H
H HH H
HH1
2
3
hexane
H
5001000150020002500300035004000Wavenumbers
20
40
60
80
100
2 31
• 2 = sp3 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 (-CH3); 1370-1350 cm-1
• 3 = scissor motion of -CH3 hydrogen atoms; 1470-1450 cm-1
• 1300-900 cm-1 = !ngerprint region for organic molecules; typically complex and unhelpful
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Characteristic Stretches - Alkenes
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H3CC
CC
CC
H H
H H
HH
4
1-hexene
H
H
H5
5001000150020002500300035004000Wavenumbers
20
40
60
80
100
5 4• 5: notice sp2 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
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Characteristic Stretches - Alkynes
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CC
H
H3C 7
6
1-hexyne
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• 7: notice sp C-H (~3300 cm-1) at higher frequency than sp2 C-H (~3100 cm-1), which was higher than sp3 C-H (~2950 cm-1)
• 6: C≡C stretch is very weak because carbons have almost identical electronegativities = small dipole moment
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Characteristic Stretches - Alcohols
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5001000150020002500300035004000Wavenumbers
20
40
60
80
100
5
9
4
HO
CC
C
H H
4H
H
H5
prop-2-en-1-ol(allyl alcohol)
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• 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
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Characteristic Stretches - NItriles
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OC
NH
8
9
3-hydroxy-propionitrile
5001000150020002500300035004000Wavenumbers
20
40
60
80
100
8
9
• 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
• size of the dipole does NOT affect frequency of vibration
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Example: Ester, Amine, Benzene
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5001000150020002500300035004000Wavenumbers
20
40
60
80
100
11
104
C
C
CO
O
NH
H
10
11
4
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 (-NH2) gives two bands
• 4: several alkene bands ~1600 cm-1 for benzene ring C=C double bonds
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Characteristic Stretches - Carboxylic Acids
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• 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
CC
CO
H
O
H
H5 4
10
9
cyclohex-2-enecarboxylic acid
5001000150020002500300035004000Wavenumbers
20
40
60
80
1009
10
4
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Characteristic Stretches - Aldehydes
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5001000150020002500300035004000Wavenumbers
20
40
60
80
100
10
4
12
CC
CH
O 10
12
H
H4
hept-2-enal
• 12: usually two bands for C-H of aldehydes; may overlap with sp3 C-H bands
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Self Test Questions
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Which molecule is represented by the IR below?
A. aB. bC.cD.d
OH
H3C CH3
OH2C O CH3
H2C
CH3
OHH
O
cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal
a. b. c. d. e.
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Self Test Questions
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Which molecule is represented by the IR below?
A. aB. bC.cD.d
OH
H3C CH3
OH2C O CH3
H2C
CH3
OHH
O
cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal
a. b. c. d.
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Self Test Questions
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Which molecule is represented by the IR below?
A. aB. bC.cD.d
OH
H3C CH3
OH2C O CH3
H2C
CH3
OHH
O
cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal
a. b. c. d. e.
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Self Test Questions
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Which molecule is represented by the IR below?
A. aB. bC.cD.d
OH
H3C CH3
OH2C O CH3
H2C
CH3
OHH
O
cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal
a. b. c. d. e.
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Self Test Questions
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Which molecule is represented by the IR below?
A. aB. bC.cD.d
OH
H3C CH3
OH2C O CH3
H2C
CH3
OHH
O
cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal
a. b. c. d. e.
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Self Test Questions
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Which molecule is represented by the IR below?
A. aB. bC.cD.d
OH
H3C CH3
OH2C O CH3
H2C
CH3
OHH
O
cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal
a. b. c. d. e.
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UICUniversity of Illinois at Chicago CHEM 232, Spring 2010 Slide
Lecture 13: February 23
Example
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A. aB. bC.cD.d
a. b. c. d. e.
Greater s character =
stronger, shorter bonds = higher
frequency
OH
H3C CH3
OH2C O CH3
H2C
CH3
OHH
O
cyclobutanol 2-butanone ethyl vinyl ether 2-methyl-2-propen-1-ol 2-methylpropanal
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University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Section: 13.23This topic will be covered in Chapter 10.
Ultraviolet-VisibleSpectroscopy
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University of Illinois at ChicagoUICCHEM 232
Organic Chemistry I
Chapter 13: Sections 13.23,13.24, 13.25
Next Lecture. . .
Quiz This Week. . .Chapter 5 & 6
Problem Set 1 has been posted
Quiz This Week. . .Chapter 5 & 6
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