PL 8 - Modern Analytical Techniques

Question 1

How do you calculate the molecular formula from the M+ peak on a mass spectrum?

Answer 1

• The M+ peak will tell you the Mr of the compound
• Find the number of empirical formula units per molecule:
• Mr / empirical mass
• Multiply this number by the empirical formula to get the molecular formula

Question 2

What is the difference in mass between peaks due to?

Answer 2

• The loss of groups of atoms
• These fragments can be analysed to help deduce the structure of the molecule

Question 3

What is carbon-13 NMR?

Answer 3

• A C13 spectrum will tell you the number of carbon environments (number of peaks = number of carbon environments)
• It will also tell you the types of each carbon environment present. Each carbon environment will have a chemical shift value.
• Compare this to chemical shift values found in your data sheet to identify the type of environment

Question 4

What is proton NMR?

Answer 4

A proton NMR spectrum provides 4 important bits of information:
- The number of different proton environments
- The types of proton environments present
- The relative number of protons present in each environments
- The number of protons present in different environments adjacent to a given proton environment

Question 5

How can you use a proton NMR spectrum to determine the number of different proton environments in a molecule?

Answer 5

The number of peaks in the spectrum = the number of different proton environments

Question 6

How can you use a proton NMR spectrum to determine the types of proton environments present in a molecule?

Answer 6

Look at the chemical shift values and find the corresponding groups/proton environments on the data sheet

Question 7

How can you use a proton NMR spectrum to determine the relative number of protons present in each environment?

Answer 7

This is normally done using an integration trace and so the relative peak areas will be provided and are usually written on the spectrum

Question 8

What can you determine from the splitting pattern of a proton NMR spectrum?

Answer 8

• Using the ‘n+1’ rule you can determine the number of protons present in the different environment adjacent to a given proton environment
• For a proton with n protons bonded to an adjacent carbon atom, the peak will split into n+1

Question 9

How do you use a combination of spectroscopic techniques to determine the structure of organic molecules?

Answer 9

1. Mass spectrum (to determine molecular formula)
2. Infrared spectrum (to determine type of molecule/functional group)
3. C13 NMR spectrum and H+ NMR spectrum (to determine the exact structure of the molecule)