2.5. Experimental Determination of Structure Flashcards
How do you calculate the empirical formula of a substance?
First find out the mass of carbon and hydrogen in the compound by taking the weight of carbon dioxide/ water produced in the end and multiplying it by either 12/44 for carbon or 2/18 for water (this is the mass of the element wanted over the mass of the molecule itself).
Then you take these masses and subtract them from the mass of the original compound to give you the mass of oxygen in the original compound. If the masses of hydrogen and carbon add up to the whole mass of the compound, oxygen is not in the original substance.
Then find the number of moles of each element. The smallest number of moles will be the element with the smallest number of atoms in the compound.
This is called elemental microanalysis.
How do you do mass spectrometry?
You take a very small sample of the unknown compound. This is vapourised and then injected into the mass spectrometer. Then it is bombarded by high energy electrons. These electrons then knock other electrons out of the molecule and these break into smaller ion fragments. You then accelerate these with a high voltage electric field into a strong magnetic field which deflects them into a series of paths according to their mass/charge (m/z) ratio. Each path is then detected and a spectrum is recorded.
Mass spectrometry can be used alongside elemental microanalysis.
What does the highest peak in a mass spectrometer symbolise?
The GFM of the substance.
Where is Infrared found on the electromagnetic spectrum?
Between microwaves and visible light.
How is Infrared spectroscopy carried out?
When infrared radiation is absorbed by organic compounds, the bonds vibrate but don’t break. The wavelength of the light absorbed depends on the type of atom and its bonds. Light atoms absorb shorter wavelengths due to the stiff bonds, vice versa for heavy atoms.
Infrared radiation is passed through the sample. Some of the wavelengths are absorbed which causes the bonds in the molecule to vibrate. The transmitted radiation then passes to a detector where the intensity at different wavelengths is measured. The absorption of a wavelength corresponds to a peak pointing downwards.
Explain Proton Nuclear Magnetic Resonance.
As protons can spin in one of two directions, they behave as tiny magnets. So when placed between between to poles of a powerful magnet, some align themselves with the magnetic field while some align against it. The ones aligned against it are of higher energy.
When protons are exposed to radio waves they can jump from the lower energy level to the higher energy level. So the protons flip from being with the magnetic field to against it. As the protons lose energy they emit this energy in the form of radio waves. This can be measured with a radio receiver.
What do peaks in low resolution H NMR spectroscopy symbolise?
The different proton environments
What do the number of small peaks in high resolution H NMR spectroscopy symbolise?
The number of hydrogen atoms on neighbouring atoms.
