8.11 - Infrared Spectroscopy
Infrared spectroscopy and the bonds it identifies
- Infrared spectroscopy identifies which bonds are present in an organic molecule, and from those bonds which class of compound it is.
- Bonds absorb specific frequencies of infrared radiation. The machine records these absorptions as wavenumbers (in cm-1) on a spectrum graph.
Reading the graph:
- The horizontal axis is wavenumber, which runs backwards - highest wavenumber on the left, lowest on the right.
- The peaks point downwards as sharp dips in the line, because the vertical axis shows transmittance: how much radiation passes through the sample.
The bonds and their wavenumbers
| Bond | Where it is found | Wavenumber (cm-1) |
|---|---|---|
| O-H (alcohols) | alcohols | 3230 to 3550 |
| C-H | most organic molecules | 2850 to 3300 |
| O-H (acids) | carboxylic acids | 2500 to 3000 |
| C=O | carboxylic acids | 1680 to 1750 |
| C=C | alkenes | 1620 to 1680 |
| C-O | alcohols, carboxylic acids | 1000 to 1300 |
| C-C | most organic molecules | 750 to 1100 |
Reading a spectrum
- Read the wavenumber of each peak off the horizontal axis.
- Find which range in the table each wavenumber falls inside to identify the bond.
- Set aside C-H and C-C bonds - almost every organic molecule contains both, so neither narrows down the class.
- Use the remaining functional group bonds to determine the class of compound.
- The O-H range tells alcohols and carboxylic acids apart: alcohol O-H absorbs at 3230 to 3550 cm-1, while acid O-H absorbs at 2500 to 3000 cm-1.
- An alkane has only C-H and C-C bonds, so it shows an absence of functional group peaks.
Worked example - Class of compound from an IR spectrum
An infrared spectrum is recorded for an unknown organic compound, with its strongest peaks at 3350 cm-1, 3050 cm-1 and 1150 cm-1. Using the table below, work out which class of compound it is.
| Bond | Where it is found | Wavenumber (cm-1) |
|---|---|---|
| O-H (alcohols) | alcohols | 3230 to 3550 |
| C-H | most organic molecules | 2850 to 3300 |
| O-H (acids) | carboxylic acids | 2500 to 3000 |
| C=O | carboxylic acids | 1680 to 1750 |
| C=C | alkenes | 1620 to 1680 |
| C-O | alcohols, carboxylic acids | 1000 to 1300 |
| C-C | most organic molecules | 750 to 1100 |
Step 1 - Read the wavenumber of each peak
The three strongest peaks are at 3350 cm-1, 3050 cm-1 and 1150 cm-1.
Step 2 - Match each peak to a bond
- 3350 cm-1 falls inside 3230 to 3550 cm-1, so it's an O-H bond in an alcohol.
- 3050 cm-1 falls inside 2850 to 3300 cm-1, so it's a C-H bond.
- 1150 cm-1 falls inside 1000 to 1300 cm-1, so it's a C-O bond.
Step 3 - Set aside C-H and C-C
- Set aside the C-H bond at 3050 cm-1.
- That leaves the O-H (alcohols) bond and the C-O bond.
Step 4 - Read off the class the remaining bonds share
- O-H (alcohols) is found in alcohols; C-O is found in alcohols and carboxylic acids.
- Both bonds are found in alcohols, so the compound is an alcohol.
TermDefinition
Infrared spectroscopyAn instrumental method that identifies the bonds in an organic molecule, which shows which class of compound it is.