lec6 handout completed- analytical chem
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Organic Spectroscopy 1 Michaelmas 2011
Lecture 6 Dr Rob Paton
[email protected] http://paton.chem.ox.ac.uk
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Recap of Lecture 5 UV-vis Spectroscopy Measures the gaps between electronic energy levels Most useful for conjugated double bonds since the HOMO-LUMO energy gap is small enough to promote an electron in this spectral region Increasing conjugation leads to greater absorption, and a shift to absorption at longer wavelengths (λmax) Characteristic absorption for certain classes of organic compounds may be predicted (albeit relatively crudely) using Woodward’s rules Steric effects and geometric strain may prevent efficient conjugation, and therefore will affect UV-vis absorption wavelengths Infrared (IR) Spectroscopy Measures molecular vibrational energy levels Molecules vibrate in many ways simultaneously, however, fundamental normal modes give rise to characteristic IR absorptions. The strength of absorption depends on the change in dipole-moment of the vibrating group.
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IR Spectroscopy The X-H region: 2500-4000 cm-1 For example: C-H bonds:
1-ethynyl-1-cyclohexene Typical C-H stretching frequencies: sp3 C-H 2800-3000 cm-1 usually –CH3 and –CH2 symmetric and antisymmetric stretches are seen sp2 C-H 3040-3125 cm-1 exact form depends on the number of alkene substituents sp C-H 3270-3340 cm-1 usually appears as a single sharp peak
ring CH single bond: 2800-300 cm-1
alkene CH single bond: 3040 cm-1
alkyne CN triple bond: 3300 cm-1
H
H
HH
H
H
HH
ring CH bond: 2800-3000
alkene CH bond: 3040
terminal alkyne CH bond: 3300
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IR Spectroscopy Some C-H stretches are diagnostically useful (Bohlmann bands):
N-H bonds: Slightly higher than C-H stretches (3330-3450 cm-1):
N-methylaniline & aniline
OR
H
NH
H NH
Me
NH
H NH
MeN
HHN
HH
symmetric antisymmetric3400 cm-1 3500 cm-1
NHH
NHH
symmetric antisymmetric3400 cm-1 3500 cm-1
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IR Spectroscopy O-H bonds: Very broad absorption around 3300 cm-1 is characterstic
4-fluorophenol
Carboxylic acids display a characteristic V-shape in the O-H absoprtion
1-cyclopentene-1-carboxylic acid
ROHOR
HOR
HOHR
OH
F
O
O
H
RO
O
HR
O
O
H
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IR Spectroscopy Hydrogen bonding affects the strength of an O-H bond:
pka: -2.1 5.2 10.8
Steric factors inhibit H-bonding:
2-tbutyl-6-methylphenol
OH
Me
tBu
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IR Spectroscopy Example Problem Using NMR, UV-vis and IR:
A + B (C7H6O2)
λmax 285 (ε 16,000) λmax 255 (ε 10,000)
δ 7.0 (d, 2H), 7.8 (d, 2H), 9.8 (s, 1H), 10.4 (brs, 1H)* δ 7.0-7.4 (m, 4H), 9.8 (s, 1H), 10.9 (s, 1H)*
ν 3600 (dil. sol.) 3100-3400 (conc. sol.), 1690 ν 3500-3000 (no change on dilution.), 1660
*exchanges in D2O
C6H6O CHCl3/OH-/H2O
A B
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IR Spectroscopy The triple bond region: 2000-2500 cm-1 C≡N and C≡C bonds:
1-Pentyne IR Spectrum
propionitrile
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IR Spectroscopy
ethylisocyanoacetate Cumulenes
3-methyl-1,2-butadiene (an allene)
allene C=C 2160 cm-1
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IR Spectroscopy The double bond region: 1500-2000 cm-1 C=C bonds: Alkenes, aromatics
oleyl alcohol
E-4-nonene
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IR Spectroscopy
methyl crotonate Nitro groups are also diagnostic:
Nitrocyclohexane
0540060080001002100410061008100020022004200620082000300230043006300830004WAVENUMBERS
0
10
20
30
40
50
60
70
80
90
100 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.80.91.0
2.0
.05
%TRANSMITTANCE
ABSORBANCE
NICOLET 20SX FT-IR
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IR Spectroscopy Carbonyl Groups – Learn these numbers! IR spectroscopy is particularly useful in identifying which of the several different kinds of carbonyl group is present in a molecule. C=O double bonds show a strong absorption band since they have a large dipole moment. The position of the absorption is governed by the electronic structure.
R R'
O
R O
O
R'
O
R Cl
O
R H
O
R O
OR'
O
O
R NR'2
O
R SiR'3
O
R OH
O
anhydrides
acid chlorides
esters
aldehydes
ketones
acids
amides
acylsilanes
carboxylates
incr
easin
g st
retc
hing
freq
uenc
y(s
tron
ger C
=O b
ond)
decr
easin
g st
retc
hing
freq
uenc
y(w
eake
r C=O
bon
d)
1.18 Å
1.21 Å
1.22 Å
1.25 Å
1.20 Å
stretching frequency(approx.)
carbonyl group
R Cl
O
R
OCl
OH H
R NR'2
O O
R NR'2
R O
O O
R O
H
1820 & 1760 cm-1
1800 cm-1
1740 cm-1
1730 cm-1
1715 cm-1
1710 cm-1
1660 cm-1
1640 cm-1
1580 cm-1
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IR Spectroscopy
Electron Donating Groups weaken the C=O bond – shifting IR frequency down Electron Withdrawing Groups strengthen the C=O bond – shifting the IR frequency up
R R'
O
R O
O
R'
O
R Cl
O
R H
O
R O
OR'
O
O
R NR'2
O
R SiR'3
O
R OH
O
anhydrides
acid chlorides
esters
aldehydes
ketones
acids
amides
acylsilanes
carboxylates
incr
easin
g st
retc
hing
freq
uenc
y(s
tron
ger C
=O b
ond)
decr
easin
g st
retc
hing
freq
uenc
y(w
eake
r C=O
bon
d)
1.18 Å
1.21 Å
1.22 Å
1.25 Å
1.20 Å
stretching frequency(approx.)
carbonyl group
R Cl
O
R
OCl
OH H
R NR'2
O O
R NR'2
R O
O O
R O
H
1820 & 1760 cm-1
1800 cm-1
1740 cm-1
1730 cm-1
1715 cm-1
1710 cm-1
1660 cm-1
1640 cm-1
1580 cm-1
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IR Spectroscopy Conjugation:
Conjugated ketones Non-conjugated ketones
C=O stretch ca. 1690 cm-1 C=O stretch ca. 1715 cm-1
Conjugation lowers the stretching frequency, typically by around 30 cm-1. Ring Strain: When a carbonyl is part of a ring, the C=O stretching frequency depends on ring size: as ring size decreases, the carbonyl stretching frequency increases.
Conjugation weakens C=O and C=C
X
O
X
O
X
O O
1715 cm-1 1750 cm-1 1790 cm-1 1815 cm-1
+40 cm-1 +40 cm-1 +25 cm-1X
O
1710 cm-1X = CH21750 cm-1 1774 cm-1 1841 cm-11727 cm-1X = O1673 cm-1 1717 cm-1 1750 cm-11669 cm-1X = NH2
O O
O O O O
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IR Spectroscopy
Kirby et al. Angewandte Chemie International Edition 1998, 37, 785.
N
O
MeN
OMe
Me
Me
pka = 5.3Hydrolyses in H2O/H+ in 45s
pka = -0.5Stable to hydrolysis
C=O stretch 1650 cm-1 C=O stretch 1732 cm-1CN 1.475 Å CO 1.196 ÅCN 1.352 Å CO 1.233 Å
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IR Spectroscopy
Example: Six derivatives of the steroid cholestane: match up the structures with the IR data
OO
OO
O
OO
O
OOEt
OO
OOEt
O
A B C
D E F
2 x C=O (conjugated):1715 - 30 = 1685 cm-1 (s)1 x C=C (conjugated):1650 - 30 = 1620 cm-1 (w)
41 x C=O (ketone):1715 cm-1 (s)1 x C=O (ester):1730 cm-1 (s)
21 x C=O (ester):1730 cm-1 (s)1 x C=O (conjugated ketone):1715 - 30 = 1685 cm-1 (s)1 x C=C (conjugated):1650 - 30 = 1620 cm-1 (w)
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1 x C=O (conj. ketone):1730 - 60 = 1670 cm-1 (s)1 x C=C (enol ether):1630 cm-1
51 x C=O (ketone):1730 cm-1 (s) 1
1 x C=O (ester):1730 cm-1 (s)1 x C=C:1650 cm-1 (w)1 x C=C (enol ether):1630 cm-1
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Selected IR absorptions (s=strong, w=weak):
Spectrum ν (cm-1)123
1715 (s)1724 (s), 1712 (s)1730 (s), 1695 (s), 1642 (w)
Spectrum ν (cm-1)456
1695 (s), 1686 (s), 1608 (w)1653 (s), 16261730 (s), 1658 (w), 1626
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IR Spectroscopy
Example: The molecular mass of X has been determined by high-resolution mass spectrometry as 68.02621. What is the formula and structure of X? (masses H: 1.0078 C: 12.0000 O: 15.9949)
IR spectrum of X
4 x 12.0000 + 1 x 15.9949 + 4 x 1.0078 = 68.0261 C4H4O
1700O
conjugated
2100 strength suggests conjugated
O
CH3
O
HH3C
or3300 strong terminal acetylene C-H
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IR Spectroscopy
Example: It was envisaged that the condensation of two equivalents of ethyl acetoacetate with formaldehyde would produce Hagemanns’ ester. Is the product’s IR spectrum consistent with this proposal?
Product IR spectrum
O
HH EtO
OO piperidine (cat.)then heat
O
CO2Et
"Hagemann's ester"
(2 equiv)
1685 (s)O
conjugated
1620 (w) conjugated
1730 (s)O
OR
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IR Spectroscopy Example: Identify the product from the reaction of cyclohexenone and dimethylcopper lithium
Reactant IR spectrum
Product IR spectrum
Oi) Me2CuLi
C7H12Oii) H+
O
Nuc
O
Me
O
NucHO Me
C=O 1690C=C 1630C(sp2)-H 3100
C=O 1710C=C goneC(sp2)-H goneno broad OH!
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IR Spectroscopy Example: Use the following IR and 1H NMR spectra to assign the structures of two isomers of C6H12
Overlay of X & Y IR spectra
1H NMR spectrum of X (ppm) 1H NMR spectrum of Y (ppm)
H
H
CH2
CH3
H2C
CH3
4.7 d
2.0 t
1.8 s1.6 sextet
0.9 t
4.7 d
H
H
CH2
H
CCH3
5.0 dd
5.8 m
5.0 dd
CH3
1.95 dd
1.7 nonet
0.9 d
H
both:C=C 1650C(sp2)-H 3090