Transcript Exercise M1 Infrared Spectroscopy Part I M1: Recording the IR
Infrared Spectroscopy
Dr. Milkevitch Organic Chem II Lab Spring 2010 Feb 11 & 13, 2010
Introduction
The purpose of this experiment
To introduce the student to spectroscopy Discuss the specific technique of Infrared Spectroscopy
Which is used to acquire structural information on organic molecules Use this technique in the laboratory
First: In order to Understand Spectroscopy
Must understand electromagnetic radiation (EMR)
EMR is a form of energy
Has a particle and wave nature
Examples: Light, microwaves, radiowaves
We Use Symbols to Designate Properties of Waves
λ is the wavelength of the waves
ν is the frequency of the waves
c is the speed of light
of all EMR actually
Relationships Between These Variables
Speed = wavelength x frequency
Therefore:
c = λν λ = c/ν ν = c/λ
For electromagnetic waves, the speed (c) is constant
3 x 10 8 m/s
What This Means
Wavelength has a direct, inverse relationship with frequency:
λ
∝
1/ν
The higher the frequency, the shorter the wavelength
The longer the wavelength, the smaller the frequency
Summary of Relationships
Wavenumbers
When the wavelength is measured in centimeters:
the reciprocal of the wavelength (1/cm) Is directly proportional to the frequency
1/cm Is called the wavenumber and is a commonly used term in spectroscopy
The Electromagnetic Spectrum
Spectroscopy Is the Study of the Interaction of Matter and Electromagnetic Radiation
In Organic Chemistry, the common techniques include:
Infrared Spectroscopy
Nuclear Magnetic Resonance Spectroscopy UV/Visible Spectroscopy
What Spectroscopy Tells Us
Specific information on the structural features of the molecules being studied
The presence or absence of specific patterns of chemical bonding in a molecule
Infrared Spectroscopy: The use of infrared radiation to determine the presence or absence of specific patterns of bonding in a molecule (i.e., functional groups)
The Infrared Region
When IR Radiation is Applied to a Molecule Some passes through it, but some does not Some of it is absorbed All bonds in a molecule have a vibrational frequency If the frequency of the IR energy matches the specific vibrational frequency of a bond in a molecule
The molecule will absorb the IR radiation at that frequency The bond is excited from a lower to a higher vibrational state
Amplitude of vibration increases dramatically We can measure this absorbance of IR radiation We can come up with a graph of absorbance intensity vs. Wavelength
What an IR Spectrum Looks Like
Graph of absorption intensity vs. radiation frequency
Given as % transmittance E = hv = hc
Units are in wavenumbers (cm -1 ), (sometimes microns
)
λ
Now, Organic Molecules are Quite Diverse
Millions of organic compounds exist
Remember the 12 families of organic compounds?
Structurally different molecules can have different functional groups
Do not absorb exactly the same frequencies of IR radiation
Therefore, give different patterns of absorption Specific bonds and functional groups in a molecule
Have specific vibrational frequencies Therefore, will absorb characteristic frequency ranges of IR radiation
This means:
IR spectroscopy is a valuable tool for identifying different functional groups
Also, a valuable tool for helping identify the structure of an organic compound
Ways Molecules Vibrate: Vibrational Modes
Vibrational Modes:
Fancy way to describe the ways a molecule can vibrate 2 most important vibrational modes in IR spectroscopy:
Stretching: involves a change in interatomic distance Bending: involves a change in bond angles Change in interatomic distance Change in bond angles
IR-Active and Inactive Bonds
Stretching and bending must:
Change the molecule’s dipole moment in order
absorption This is really important
Polar bonds will absorb strongly
Does a polar bond have a dipole moment?
A nonpolar bond will absorb weakly or not at all
Does a nonpolar bond have a dipole moment?
Some Trends in Vibrational Frequency
The smaller the atoms in a bond, frequency increases
Larger the atoms in a bond, frequency decreases
Bond strength also effects frequency of absorption
Stronger bonds, higher frequency of absorption Alkyne 2250 – 2100 cm -1 Alkene 1680 – 1600 cm -1 Alkane 1200 – 800 cm -1
Trends: Carbon-Hydrogen Stretching
Bonds with more s character absorb at a higher frequency
More s character, shorter and stronger bond
sp 3 sp 2
C-H, just below 3000 cm C-H, just above 3000 cm sp C-H, at 3300 cm -1 -1 -1 (to the right) (to the left)
The Three Most Important Regions of the IR Spectrum
3600 – 3100 cm -1
Where OH and NH stretching occur
Region around 1700 cm -1
Where C=O stretching occurs
Region around 1650 cm -1
Where C=C stretching occurs
Many of the important functional classes are identified by the presence (or absence) of absorptions in these regions
An Infrared Spectrophotometer
=>
FT-IR Spectrometer
“The modern IR spectrometer”
Small and compact
Computer controlled
Has better sensitivity than dispersive instruments
Irradiate the sample with all IR frequencies at the same time
Does multiple scans quickly
Averages the results
An Alkane IR Spectrum: Notable Peaks (or absorbances)
An Alkene IR Spectrum: Notable Peaks
An Alkyne IR Spectrum: Notable Peaks
=>
Correlation Table:
Summary of Notable IR absorbances for the functional Groups
Very important For the organic Chemist
Procedure
Each group should choose one of the following compounds: Chlorooctane Dodecane Chlorodecane Cyclohexane Cyclohexene 3,3-dimethyl-1-butene Heptane Hexane 1-heptyne 1-hexene
Procedure (2)
Obtain an IR spectrum
See Dr. M in the instrument lab
Interpret major absorption frequencies
Using tables in this handout
Annotate the spectrum with your interpretations
Your Report
Your introduction should include a discussion of IR spectroscopy
Your textbook also has a chapter on IR Spectroscopy, use it if necessary No reaction mechanism or balanced equation in this experiment Physical properties section should be the relevant physical properties of your chosen compound
Your results section should include the spectrum of your chosen compound
Annotated
What do the peaks correspond to?
If it’s not annotated, it is meaningless
Make a table of relevant absorbances, along with their identity
Conclusions: Things to think about
Did you successfully obtain your IR spectrum?
Is the spectrum clean? Crappy? Easy to obtain?
What does your IR spectrum look like?
What major peaks do you have?
What stretches do these peaks correspond to?
Do the peaks correspond to what type of compound you have (alkane, alkene or alkyne)?
Prove this: correlate your peaks to what peaks these compounds should have in an IR spectrum
Prove it further: find an IR spectrum of the compound and compare it to your spectrum
Overall, what did you learn about IR spectroscopy?
Additional Questions to answer: 1.
Which absorbs at a higher frequency: a C-H bond or a C-D bond? Explain.
2.
Why does H 2 not have an IR spectrum?
3.
Explain why the C=C stretch for a trans-disubstituted alkene is weaker than for a cis-disubstituted alkene.