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Organic Chemistry Laboratory
Infrared Spectroscopy

Theory of Infrared Spectroscopy
Introduction
Infrared (IR), Nuclear Magnetic Resonance (NMR) and Ultraviolet (UV) Spectroscopy are three analytical techniques that are commonly used to elucidate the structure of organic molecules. The principles of each of these analytical methods are derived from the interaction of electromagnetic radiation with molecules. Each type of spectroscopy uses different frequencies of electromagnetic radiation, and different kinds of structural information about the molecules can be obtained from each type of spectral analysis. Table 6.2 lists three different types of spectroscopy, the frequencies of the electromagnetic spectrum utilized, and the kind of structural information each spectral analysis provides.

Type of Spectroscopy	Frequency of EM Spectrum (Hertz (Hz))	Wavelength (meters (m))	Structural Information
IR Spectroscopy	5 X 10¹⁴ - 3 X 10¹² ^{(infrared region)}	1 X 10^-6 - 1 X 10^-4	Bond types, Functional Groups
NMR Spectroscopy	1 X 10¹¹ - 1 X 10⁹ ^{(radio wave region)}	1 X 10^-2 - 1.0	Carbon Types, Carbon Skeleton
UV Spectyroscopy	2.5 X 10¹⁶ - 2 X 10¹⁵ ^{(ultraviolet region)}	1 X 10^-8 - 3.8 X 10^-7	Pi and Conjugated System

Table 6.2: Regions of the Electromagnetic Spectrum Used in Spectroscopy

In IR spectroscopy, a sample molecule is exposed to radiation with a frequency in the infrared region of the electromagnetic spectrum. Only a portion of the infrared region of the electromagnetioc spectrum(2.5 X 10^-6- 2.5 X 10^-5m) is used in conventional IR spectroscopy. Specific frequencies of radiation within the infrared region of the spectrum are absorbed by individual bonds in the molecule, causing the bonds to vibrate. The frequency of the radiation that is absorbed by the molecule is recorded as a peak or absorbance in the spectrum. These peaks are characteristic of certain types of bonds. Table 6.3 lists various functional group classes and specific bonds associated with that functional group, and the characteristic position of the peak corresponding to that bond. Click on the link to view a sample spectrum containing peaks corresponding to the bond type indicated.

Absorbance Range	FunctionalGroup/Bond Type	Intensity of Absorption
2850-2960 cm^-1	C_sp3-H of alkanes and alkyl groups	medium-strong
3020-3100 cm^-1	C_sp2-H of alkenes and alkenyl groups	medium-strong
1640-1680 cm^-1	C_sp2=C_sp2 of alkene	medium
3300 cm^-1	C_sp-H of alkynes	strong
2100-2260 cm^-1	CºC of alkynes CºC of nitriles	weak-medium
500-800 cm^-1	C_sp3-X, X = halogen of alkyl halide	strong
3200-3650 cm^-1	O-H of alcohol, phenol, carboxylic acid	strong, broad
1660-2000 cm^-1 1450-1600 cm^-1	C_sp2=C_sp2 of aromatic	weak medium
1680-1850 cm^-1	C=O of carbonyls ketones, aldehydes, esters, carboxylic acids, amides See more specific ranges	strong
3300-3500 cm^-1 1030-1230 cm^-1	N-H of amines C-N of amines	medium medium
1540 cm^-1	N=O of nitro group (-NO₂)	strong

Table 6.3: Characterisic IR Absorbances Ranges for Various Bond Types

Sample Preparation and the Infrared Spectrophotometer
There are a variety of methods for preparing samples for IR analysis. The most common methods of sample prepartion for IR analysis are listed in Table 6.4. Liquid samples will typically be run in this course using sodium chloride plates (i.e. salt plates). The sample is prepared by placing a few drops of the liquid compound on one salt plate and placing a second salt plate on top of the first. Solid samples will be prepared for IR analysis using potassium bromide pellets. Pellets are prepared by mixing the solid sample with KBr, placing the mixture in a mini-press and compressing the mixture for form a small disc or pellets. Other sampel prepartion methods are available but will not be used in this course. These include mulls and solutions. A mull is prepared by grinding a solid sample with mineral oil to form a paste which is then placed between salt plates for analysis. The solutions of samples, either liquids or solids can analyzed using special solution cells.

Sample Prep Method	Compound State
Neat (undiluted, salt plates)	Liquids
Mull	Solid
KBr Pellet	Solid
Solution	Liquid or Solid

Table 6.4: Sample Preparation Methods for IR Spectroscopy

Interpretation of the IR Spectrum
Interpretation of IR spectra involved correlating peaks in an experimentally generated spectrum with known ranges for specific bond or functional group types. There are some general and more specific guidelines that can be used to interpret an IR spectrum. Refer to Table 6.3 (click on links) to view spectra of common organic functional groups/bonds.

General Guidelines for Simple Organic Functional Groups

Focus on Peaks between 1500-3600cm^-1. Peaks below 1500cm^-1 generally cannot be specifcally interpreted.

Focus on the major peaks in the spectrum with absorbances of 50% of scale or greater. Weaker absorbances generally cannot be specifically interpreted.

The most characteristic regions of the IR spectrum are 3000-3500 cm^-1where OH (very broad and intense) and NH (sharper than OH) peaks appear and 1650-1850cm^-1where carbonyl peaks appear.

The absence of peaks can be just as characteristic as the presence of peaks. For example, a peak at 1750cm^-1 indicates a carbonyl containing functional group is present in the molecule. If no peak appears between 1650-1850cm^-1, then no carbonyl containing functionl groups are present.

Specific Guidelines for Simple Organic Functinal Groups

Step 1	Identify all the major peaks in the spectrum between 3600cm^-1 and 1500cm^-1 (+/- 100cm^-1). Label the peaks 1,2,3 etc and prepare a table with four columns with the following headings: Peak Number, Absorbance Range, Absorbance Intensity, Bond Type. Enter all peak numbers in the "Peak Number" column, their corresponding absorbance range in the "Absorbance Range column, and the absorbance intensity (Strong =50% of scale or more; Medium 25-505 of scale; Weak = less than 25% of scale) in the "Absorbance Intensity" column.
Step 2	Is there a peak between 1650-1850cm^-1 AND 3600-3200cm^-1? If yes go to Step 3. If no go to Step 5.
Step 3	Is the peak between 3600-3200cm^-1 broad and intense? If no, go to Step 4. If yes, the compound probably contains one of the following functional groups or functional group combinations. 1) Carboxylic acid 2) Ketone and alcohol or phenol 3) Aldehyde and alcohol or phenol 4) Ester and alcohol or phenol 4) Alkene and Alcohol or phenol 5) Phenol List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table.
Step 4	Is the peak between 3600-3200cm^-1sharp and medium to weak in intensity? If yes, the compound probably contains one of the following functional groups or functional group combinations: 1) Secondary amide (if only one peak between 3600-3200cm^-1) primary amide (if two peaks between 3600-3200cm^-1) or 2) Ketone and amine or 3) Aldehyde and Amine List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table.
Step 5	Is there a peak between 1650-1850cm^-1 ? If no, go to Step 6. If yes, the compound contains one of the following carbonyl or C=C containing functional groups: 1) ester 2) ketone 3) aldehyde 4) tertiary amide 5) alkene (~1660cm^-1) 6) aromatic (~1450 and 1600cm^-1) List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table.
Step 6	Is there a peak between 3600-3200cm^-1 ? If no, go to Step 7. If yes, the compound contains one of the following functional groups: 1) Alcohol (if the peak is broad and strong in intensity) 2) Amine (if the peak is sharp and medium in intensity) List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table.
Step 7	The compound contains one or more of the following functional groups: 1) Alkane 2) Ether 3) Alkyl Halide List these possible functional groups in the table under the "Bond Type" column next their corresponding peaks listed in the table.