16.3 Infrared spectroscopy

Question 1

What is infrared spectroscopy used for

Answer 1

It is used by organic chemists to help identify compounds

Question 2

How does the vibrations of atoms affect infrared spectroscopy

Do stronger or weaker bonds have a higher frequency
How does the mass of the atom affect the frequency

Answer 2

. A pair of atoms joined by a chemical bond is always vibrating
. System behaves as two balls (atoms) joined by a spring (the bond)

. Stronger bonds vibrate faster (at a higher frequency)
. Heavier atoms make the bonds vibrate more slowly (at a lower frequency)

Every bond has its own unique natural frequency that is in the infrared region of electromagnetic spectrum

Question 3

How can you get a reading from the vibrating atoms

Answer 3

. Shine a beam of infrared radiation through the sample
. So the bonds absorb energy from the IR and vibrate more
. However, any particular bond can only absorb radiation that has the same frequency as the natural frequency of the bond
. Therefore the radiation that emerges from the sample will be missing the frequencies that correspond to the bonds in the sample

Question 4

What occurs in the infrared spectrometer

Answer 4

. Beam of infrared radiation containing a spread of frequencies is passed through the sample
. The radiation that emerges is missing the frequencies that correspond to the types of bonds found in the sample
. The instrument plots a graph of the intensity of the radiation emerging from the sample, called the transmittance, against the frequency of radiation

The frequency is expressed as a wavenumber, measured in cm^-1

Question 5

How can we use the infrared spectrum to help us identify functional groups in the compound

Answer 5

• The graph has peaks (which are actually dips in the graph) which represent particular bonds
• The OH bond produces a broad peak between 3230 and 3500cm-1
  This is found in alcohols ROH
• The OH bond produces a very broad peak between 2500 and 3000cm^-1 in carboxylic acids RCOOH

whilst the C=O bond produces a peak between 1680 and 1750cm^-1
This is found in aldehydes, ketones and carboxylic acids

So a table will tell you the frequencies, that will correspond to different bonds

Question 6

What is the fingerprint region of infrared spectrum

Answer 6

The area of infrared spectrum below 1500 has many peaks caused by complex vibrations of the whole molecule

This shape is unique for any particular substance, it can be used to identify the chemical
Eg how people can be identified by their fingerprints

Chemists use a computer to match the fingerprint region of a sample with those on a database of compounds
An exact match confirms identification of the sample

Question 7

How does the fingerprint region affect propan-1-ol and propan-2-ol

Answer 7

The molecules are very similar down until under 1500, and then they have different frequencies

Question 8

If we’re given an IR spectrum of propanal, how can we prove that it is

Answer 8

. Compare the fingerprint region to a database of the known propanal
. If there is an exact match, it is propanal

Question 9

How do you know if the ir spectrum is showing a carboxylic acid

Answer 9

. There is a peak where C=O is
. There is a very wide and broad band covering wide range of 2800-3500