[3.3.6] Organic Analysis

Question 1

What can high-resolution spectrometry be used for?

Answer 1

• To determine the molecular formula of a compound from the accurate mass of the molecular ion.
• It can measure up to 5 d.p. which can help to differentiate between compounds that appear to have similar Mr.

Question 2

ACCURATE MASSES OF ATOMS:
H = 1.0078
C= 12.0000
O = 15.9949
N = 14.0031

Answer 2

Question 3

What is a molecular ion?

Answer 3

• A molecular ion is a molecule with one electron knocked off.
• It is both an ion and a free radical.
• M -> [M]⁺· + e⁻

Question 4

In a mass spectrum, what does the peak with the highest mass/charge ratio represent?

Answer 4

• The peak with the highest mass/charge ratio will be normally due to the original molecule that hasn’t fragmented called the molecular ion.
• As the charge of the ion is +1, the mass/charge ratio is equal to Mr.

Question 5

What region should you NOT be looking at when analysing infrared spectroscopy?

Answer 5

• Fingerprint region i.e. below 1500 cm⁻¹.
• It is complicated and contains many signals - picking out functional groups is difficult.
• Above 1500 cm⁻¹ you’ll be able to identify functional groups.

Question 6

How does infrared spectroscopy work?

Answer 6

• Certain groups in a molecule absorb infrared radiation at characteristic frequencies.
• A spectra then can be obtained that provides information about the types of bonds present in a molecule.

Question 7

Identify the bonds and thus the functional group in this spectra and state the wavenumber range the bond/s is/are found.

Use the infrared absorption data to do this.

Answer 7

• FUNCTIONAL GROUP: C=O ∴ aldehyde.
  
  • (There is no OH functional group in this spectra, so it cannot be a carboxylic acid)
• WAVENUMBER RANGE: Betweeen 1680-1750.

Question 8

Identify the bonds and thus the functional group in this spectra and state the wavenumber range the bond(s) is/are found.

Use the infrared absorption data to do this.

Answer 8

• FUNCTIONAL GROUP: C=O & O-H (acid) ∴ carboxylic acid.
• WAVENUMBER RANGE FOR C=O: Betweeen 1680-1750.
• WAVENUMBER RANGE FOR OH (acid): Between 2500-3000.

Question 9

Explain the link between absorption of infrared radiation by bonds in CO₂. methane and water vapour and global warming.

Answer 9

• UV wavelength radiation passes through the atmosphere to the Earth’s surface and heats it up.
• The earth radiates out infrared long wavelength radiation.
• The C=O bonds in CO₂, C-H bonds in methane and the O-H bonds in water vapour absorb infrared radiation so the IR radiation does not escape from the atmosphere.
• This energy is transferred to other molecules in the atmosphere by collisions so the atmosphere is warmed.