Organic Analysis: Infrared Spectroscopy

Question 1

What is infrared spectroscopy?

Answer 1

• Analytical technique that identifies the functional groups present in organic compounds

Question 2

How does infrared spectroscopy work?

Answer 2

• It measures wavenumbers (frequencies) of IR radiation beams that pass through a chemical sample
• (Covalent) bonds in a molecule absorb infrared radiation at characteristic wavenumbers (frequencies)

Question 3

What factors affect the frequency absorbed by a certain bond?

Answer 3

• The bond enthalpy (strength of bond)

- The masses of atoms

Question 4

How does bond enthalpy affect the frequency absorbed by a certain bond?

Answer 4

• Stronger bonds vibrate faster so absorb at higher frequencies

Question 5

How does the masses of atoms affect the frequency absorbed by a certain bond?

Answer 5

• Heavier atoms vibrate more slowly so absorb at lower frequencies
• E.g heavier guitar string = lower frequency

Question 6

Name different types of bond vibrations

Answer 6

• Bending
• Stretching
• Symmetric stretching
• Asymmetric stretch

Question 7

Explain how radiation affects bond vibrations

Answer 7

• IR radiation is absorbed by covalent bonds increasing their vibrational energy
• Different covalent bonds have different strengths due to masses of different atoms at either end of bond
• So, bonds vibrate at different frequencies
• There are different types of bond vibrations

Question 8

What does an infrared spectrometer look like?

Answer 8

• Wavenumber on x-axis; transmittance on y-axis
• Shows what frequencies of radiation the bond in molecules are absorbing
• The peaks show you where radiation is being absorbed
• The peaks are upside down

Question 9

How do you interpret an infrared spectrometer?

Answer 9

• Look at wavenumber of a particular peak

- Look at data sheet and identify group that includes the peak’s wavenumber

Question 10

Why are some peaks broader than others?

Answer 10

• Some organic compounds form hydrogen bonds between molecules which makes peak broader

Question 11

What is the fingerprint region?

Answer 11

• Region between 1500-400cm^-1 is known as ‘fingerprint’ region
• Each compound has a unique fingerprint region
• Compound can be identified by comparing spectrum with sample spectra stored in a database
• Spectrum - the entire range of wavelengths of electromagnetic radiation

Question 12

How do greenhouse gases contribute to global warming?

Answer 12

• The Sun emits UV radiation which is absorbed by the Earth’s surface and re-emitted as IR radiation
• Molecules of greenhouse gases have bonds that are good at absorbing infrared energy
• If amount of greenhouse gases in atmosphere increase, more IR radiation is absorbed leading to global warming

Question 13

Name examples of greenhouse gases

Answer 13

• Carbon dioxide
• Methane
• Water vapour