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 108 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 Units are in wavenumbers (cm-1), (sometimes microns) E = hv = hc 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 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 to be IR active Large changes in dipole moment: very intense IR 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 sp3 C-H, just below 3000 cm-1 (to the right) sp2 C-H, just above 3000 cm-1 (to the left) sp C-H, at 3300 cm-1

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

Conclusions: Things to think about 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?

Overall, what did you learn about IR spectroscopy? 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: Which absorbs at a higher frequency: a C-H bond or a C-D bond? Explain. Why does H2 not have an IR spectrum? Explain why the C=C stretch for a trans-disubstituted alkene is weaker than for a cis-disubstituted alkene.