Structure Determination
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What 2 things can carbon-13 NMR tell us?
• Number of carbon environments • Type of each carbon environment
What is meant by equivalent carbon/hydrogen environments?
Two or more carbon/hydrogen atoms are in the same environment; they both have the same substituents either side
Why might a peak be shifted downfield in an NMR spectrum?
A more electronegative atom may be bonded to an adjacent carbon atom which reduces shielding
What is the position of a peak on an NMR spectrum called?
Chemical shift
What does the size of the peak in carbon-13 NMR represent?
Nothing - this is NOT proportional to the number of carbons causing it!!
What is TMS an acronym for in NMR spectroscopy?
Tetramethylsilane, Si(CH3)4
Why do we add TMS to a sample before we begin NMR spectroscopy?
TMS is a reference point in NMR, with a chemical shift defined as zero against which we measure all shifts against.
Why is TMS a good substance to use as a reference point for NMR - in chemical terms?
• It is a very inert compound; it is unlikely to react with the analyse. • It is also very volatile so evaporates readily once the analysis is complete, allowing us to retain the yield.
Give examples of suitable solvents for hydrogen NMR.
CCl4, CDCl3, D2O
Why must the solvent used for 1H NMR be free of hydrogen atoms?
The presence of hydrogen atoms would interfere with the NMR spectrum produced.
What can 1H NMR tell us about a compound?
• Number and type of hydrogen environments • RELATIVE number of hydrogen atoms in a given environment • Number of non-equivalent hydrogens on adjacent carbon atoms
How do we determine the relative number of hydrogen atoms causing a 1H NMR peak?
By integrating to find the area beneath the peak
How can splitting patterns in 1H NMR be used to determine the number of adjacent hydrogens?
• Determine the splitting pattern • Number of splits = number of adjacent hydrogens + 1
Why does peak splitting occur in 1H NMR?
Interaction of the proton’s spin and that of non-equivalent adjacent protons
Why do -OH peaks not undergo splitting in 1H NMR?
No adjacent carbon atoms; splitting requires hydrogens on adjacent carbons.
What is the use of proton exchange in 1H NMR?
Removal of -OH, -NH peaks through substitution of hydrogen with deuterium
What compound do we use in proton exchange for 1H NMR?
D2O, dideuterium monoxide This goes on to establish an equilibrium with the protonated and deuterated forms
What is the stationary phase in TLC?
Silica/aluminium oxide on a plastic sheet
What is the mobile phase in TLC?
The solvent which carries the substances to be analysed
How does TLC work?
• Different substances have different solubilities to the solvent and adsorb to the stationary phase differently • This affects the speed they travel up the plate compared to the solvent front
What metric is used to analyse TLC chromatograms?
Rf, or retention factor
How do we calculate retention factor in TLC?
distance moved by substance / solvent front
Why must the solvent be chosen carefully when conducting TLC?
• Since like dissolves like, polar compounds dissolve better in polar solvents and vice versa • This affects retention factor
What is the stationary phase in gas chromatography?
A highly viscous liquid adsorbed onto an inert solid support