Infrared Radiation:

Molecular Vibrations

Electromagnetic Radiation: Light, Energy, Heat

IR Tutor: http://chemistry.beloit.edu/Warming/pages/infrared.html

What do the sun’s energy, a molecule’s shape and a TV remote have to do with greenhouse gases?

http://chemistry.beloit.edu/Stars/EMSpectrum/index.html

Infrared Absorbance

Infrared Absorbance

RemoteTV

Infrared Absorbance

Sample

TV Remote

Infrared Absorbance & Molecular Vibrations for CCl4

Stretching & Bending

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 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

Infrared Absorption Frequencies

First examine the absorption bands in the vicinity of 4000-3000 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 (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

Carboxylic Acid

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

Aliphatic aldehyde

Mono-substituted aromatic aldehyde

Mono substituted aromatic ester

Para di-substituted aromatic ether & aldehyde

Infrared Spectroscopy

Common Functional Groups

Handout

Acetate Esters: Smell / Pheromones

Pheromone Synthesishttp://chemconnections.org/organic/chem226/Labs/acetate%20syn/Z-selective%20Insect%20Pheromones%20syn.pdf

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

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

[20:40-23:51]

Infrared Spectroscopy

Common Functional Groups

Questions

A) benzyl alcoholB) 2,4,6-cycloheptaheptatrien-1-one

C) acetophenoneD) benzaldehyde

E) phenylacetic acid

C7H6O

Question #1

Identify the compound from the IR above.

Question #2

C10H12O

Identify the compound from the IR above.

A) 2,4,5-trimethylbenzaldehydeB) p-allylanisole

C) 2-allyl-4methylphenolD) 1-phenyl-2-butanone

Question #3

C3H4O

A) cyclopropanoneB) propynol

C) acrylaldehydeD) propenoic acid

Identify the compound from the IR above.

Question #4

A) methylbenzoateB) o-hydroxyacetophenoneC) o-toluic acidD) p-anisaldehyde

(p-methoxybenzaldehyde)

Identify the compound from the IR above.

C8H8O2

Question #5

A) benzylformateB) o-hydroxyacetophenoneC) 2-methoxytroponeD) p-toluic acidE) o-anisaldehyde

(p-methoxybenzaldehyde)

Identify the compound from the IR above.

C8H8O2