Flashcards in Structure Determination Deck (13):
How do we use data from a mass spectrum to identify a molecule?
Use the M peak to find the molecular mass, and see if there is an M+2 peak, which will indicate if the molecule contains a halogen. We can look at the fragmented peaks to see which groups are in the molecule, too.
Give the equation for the fragmentation of a molecular ion.
M+ --> free radical + ion
How can we calculate the number of carbon atoms in a molecule from its mass spectrum?
height of M+1 peak/height of M peak x100
Why are some peaks on a mass spectrum higher than others?
The most abundant peaks occur with more stable ions. Examples of these are acylium ions, and carbocations.
What is the process of infrared spectroscopy?
A beam of IR radiation is projected through a sample. The IR energy is absorbed in the bonds - different bonds absorb different wavelengths. A detector then records how much of each wavelength is getting through the sample, to see which bonds are present.
Why does carbon-13 NMR give a much simpler spectrum than proton NMR?
They have fewer, sharper peaks, as there are generally fewer environments to be recorded.
What does chemical shift depend on?
The environment of the carbon or proton. They absorb different amounts of energy depending on what environment they are in.
What does an integration trace show?
The ratio of protons in each environment.
What do we dissolve a substance in before obtaining a proton NMR spectrum?
A dueterated solvent - the hydrogen atoms have been replaced with deuterium atoms so it does not have a spin any more, because there equal numbers of protons and neutrons.
CCl4 (tetrachloromethane) can also be used.
Why do we use TMS (tetramethylsilane) as a standard in NMR?
Because all the hydrogen atoms are in identical environments, so it produces a single absorption peak. It is given a chemical shift value of 0.
How can we deduce the splitting pattern in proton NMR?
Look at how many hydrogens are in the adjacent group, and add one to it. For example, if the adjacent group it a CH2 group, the splitting pattern will be a triplet.
What is gas-liquid chromatography used for?
To separate mixtures of volatile liquids.