Experimental Determination of Structure

Question 1

mass spectroscopy

Answer 1

technique used in determining the accurate molecular mass of an organic compound and identifying structural features

Question 2

how is a mass spectrum produced?

Answer 2

a vaporised sample is injected into the mass spectrometer where it is broken up by high energy electrons into positively charged ion fragments, positive fragments are detected as they leave the mass spectrometer and a mass spectrum is produced

Question 3

mass spectrum

Answer 3

graph of relative intensity vs mass/ charge (m/z) ratio of fragments, the peak with the highest m/z value is often the molecular ion (heaviest) and will give the molecular mass and formula of the compound

Question 4

infrared spectroscopy

Answer 4

can be used to identify certain bonds and functional groups in organic molecules

Question 5

how are bonds identified in IR spectroscopy?

Answer 5

the range of wavelengths within the IR part of EMS is passed through the sample and depending on bonds present, specific wavelengths will be absorbed causing them to vibrate. By measuring the intensity of what passes through the sample, specific bonds within the molecule can be determined.

Question 6

IR spectrum

Answer 6

transmission vs wave number

Question 7

Proton nuclear magnetic resonance (NMR) spectroscopy

Answer 7

used to determine:

• how many different chemical environments the protons in an organic molecule can exist in
• how many protons are in each of these environments
• the type of proton environment

Question 8

how is NMR carried out?

Answer 8

The protons that are aligned with the direction of a magnetic field have slightly lower energy than those aligned against and the energy difference between the two corresponds to the radiofrequency region of the electromagnetic spectrum. When the protons are exposed to radio waves, the absorption of energy promotes those in the lower energy states to the higher energy states. This effectively flips those aligned with the magnetic field to being aligned against it. The protons then fall back to the lower energy state and the same radio frequency that was absorbed is emitted which can be measured with a radio receiver.

Question 9

high resolution NMR spectra

Answer 9

obtained using a higher radio frequency than used for low resolution spectra so more detailed spectra are produced

Question 10

how to read high resolution NMR spectra?

Answer 10

many of the single peaks in low resoltuion are split into clusters of peaks, number of sub peaks in a cluster is one more than the number of hydrogens attached to the next door carbon (s)

Question 11

n+1 rule

Answer 11

n= no of hydrogens attached to the neighbouring carbon atom