Infrared Spectroscopy

Infrared Absorbance

IR- Empirical Comparisons

Identifying functional groups in organic molecules

Region of infrared that is most useful lies between2.5-16 mm (4000-625 cm-1)

depends on transitions between vibrational energy states

Stretching: higher energy / higher wave number (cm-

1)

Bending: lower energy / lower wave number (cm-1)

Infrared Spectroscopy

A bond must have a dipole or an induced dipole in order to have an absorbance in the IR spectrum.

When the bond stretches, the increasing distance between the atoms increases the dipole moment.Therefore, the greater the dipole, the more intense the absorption. (i.e., The greater the molar extinction coefficient () in Beer’s law, A = bc.

The energy (IR frequency/ wave number) and the intensity of the absorption band also depends on the concentration of solution from Beer’s law, A = bc.

It is easier to stretch an O–H bond if it is hydrogen bonded

The presence & absence of absorption bands must be considered in identifying a possible structure in IR spectroscopy. Empiricism is critical to successful identification.

NOTE: Bonds which lack dipole moments are notdetected.

Analyzing Structure:Functions & Infrared Spectra

The molecular formula is a critical piece of information, which limits the functional possibilities.

Structural/Functional Components

The peaks are quantized absorption bands corresponding to molecular stretching and

bending vibrations

An Infrared Spectrum

The fingerprint region

The functional groupstretching region

Structural unit Frequency, cm-1

Stretching vibrations (single bonds)

O—H (alcohols) 3200-3600

O—H (carboxylic acids) 3000-3100

N—H3350-3500

Infrared Absorption Frequencies

First examine the absorption bands in the vicinity of 4000-3000 cm–1

IR Spectrum of a Primary Amine(1o)

The N–H bending vibration occurs at ~1600 cm–1

Structural unit Frequency, cm-1

Stretching vibrations (single bonds)

sp C—H 3310-3320

sp2 C—H 3000-3100

sp3 C—H 2850-2950

sp2 C—O 1200

sp3 C—O 1025-1200

Infrared Absorption Frequencies

Structural unit Frequency, cm-1

Stretching vibrations (single bonds)

sp C—H 3310-3320

sp2 C—H 3000-3100

sp3 C—H 2850-2950

sp2 C—O 1200

sp3 C—O 1025-1200

Infrared Absorption Frequencies

Structural unit Frequency, cm-1

Stretching vibrations (multiple bonds)

Infrared Absorption Frequencies

C C 1620-1680

—C N

—C C— 2100-2200

2240-2280

Some hydrocarbon absorption bands

Structural Components & Functional Differences:

Therefore the amide has a longer (weaker) C=O bond (1680-1700 cm-1) and the ester (1730-1750 cm-1) is shorter (stronger).

The nitrogen of an amide is less electronegative than the oxygen of an ester.

Structural unit Frequency, cm-1

Stretching vibrations (carbonyl groups)

Aldehydes and ketones 1710-1750

Carboxylic acids 1700-1725

Acid anhydrides 1800-1850 and 1740-1790

Esters 1730-1750

Amides 1680-1700

Infrared Absorption Frequencies

C O

Cyclic aliphatic ketone

Mono substituted aromatic methyl ketone

Mono substituted aromatic ketone

Aliphatic ester I

Aliphatic ester II

Aliphatic ester III

Mono substituted aromatic ester

Mono substituted aromatic conjugated ester

Structural unit Frequency, cm-1

Stretching vibrations (single bonds)

sp2 C—O 1200

sp3 C—O 1025-1200

Infrared Absorption Frequencies

Dihexyl Ether

~1100 cm-1

1025-1200 cm-1

~1200 cm-1

Structural unit Frequency, cm-1

Bending vibrations of alkenes

Infrared Absorption Frequencies

CH2RCH

CH2R2C

CHR'cis-RCH

CHR'trans-RCH

CHR'R2C

910-990

890

665-730

960-980

790-840

wavenumber (cm–1) assignment

30752950

1650 and 890

???

cis- 665-730

trans- 960-980

Summary: C–H bond absorption and hybridizationof the carbon atom

Distinctive Stretch of C–H Bond in an Aldehyde (the “waggle” vibration)

Aliphatic aldehyde

Mono-substituted aromatic aldehyde

Mono-substituted aromatic conjugated aldehyde

Mono substituted aromatic ester

Para di-substituted aromatic ether & aldehyde

Infrared Spectroscopy

Common Functional Groups

Handout

http://chemconnections.org/general/chem121/Spectroscopy/IR-handout-11.htm

Structural unit Frequency, cm-1

Bending vibrations of derivatives of benzene

Monosubstituted 730-770 and 690-710

Ortho-disubstituted 735-770

Meta-disubstituted 750-810 and 680-730

Para-disubstituted 790-840

Aromatic Absorption Frequencies

20003500 3000 2500 10001500 500

Wave number, cm-1

Infrared Spectrum of tert-butylbenzene

H—C

Ar—H

Monsubstitutedbenzene

C6H5C(CH3)3

Chemical Communication: Smell / Pheromones

Pheromone Synthesis[20:40-23:51]

http://chemconnections.org/COT/COT-chemcomm-eg.html

http://www.learner.org/resources/series61.html

Infrared Spectroscopy

Common Functional Groups

QUIZ

Question

Is the following IR of cis or trans 2-pentene?

A) cis B) trans

A) benzyl alcoholB) 1,4,6-heptatrien-3-one

C) 2,4,6-heptatrienaldehydeD) benzaldehyde

C7H6O

Question

Identify the compound from the IR above.

Question

C10H12O

Identify the compound from the IR above.

A) 4-phenylbutanaldehydeB) phenylpropyl ketone

C) meta-isopropylbenzaldehydeD) 1-phenyl-2-butanone

Question

C3H4O

A) cyclopropanoneB) propynol

C) 2-cyclopropenolD) 1,2-propadienol

Identify the compound from the IR above.

Question

C3H7NO

Identify the compound from the IR above.

A) N-methylacetamideB) N,N-dimethylformamide

C) 3-aminopropanalD) N-methylamino-ethanal

Match the ortho, meta and para isomers of xylene.

I

III

II

ortho meta para

A) I II III

B) II I III

C) I III II

D) III II I

Question

Question

A) methylbenzoateB) phenylacetateC) p-anisaldehydeD) o-anisaldehyde

Identify the compound from the IR above.

C8H8O2