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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