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Organic Spectroscopy 1 Michaelmas 2011

Lecture 6 Dr Rob Paton

robert.paton@chem.ox.ac.uk 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

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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

NH

H NH

Me

NH

H NH

Me

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IR Spectroscopy O-H bonds: Very broad absorption around 3300 cm-1 is characteristic

4-fluorophenol

Carboxylic acids display a characteristic V-shape in the O-H absorption

1-cyclopentene-1-carboxylic acid

OH

F

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

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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)

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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

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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.

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

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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

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

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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)

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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+

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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)