6.3 - Analysis
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What can NMR spectroscopy tell us
Types of carbon environment Types of environment (chemical shift) Relative no. of these diff types of carbon (integration/strength)
Integration of carbon environment
Depends on height Ratio of height matches ratio of carbon environment
Chemical environment
Diff bonds and atoms surrounding a nucleus
Why isn’t C-12 used in NMR
Has an even mass no. so there’s no overall spin Not NMR active
When is the chemical shift greater
More electron deficient the carbon More deshielded the nucleus
Which solvent is used in NMR
Liquid CDCl3 as deuterium (2-H) has an even mass no. and no overall spin so the C-13 peak is easily identified and removed
What is used as the reference peak
Tetramethylsilane (TMS)
Why is TMS used as the reference peak
Produces strong single peak Chemical shift defined at zero Peak found at far right of spectrum Scale increases from right to left
Aromatic environment
How many of the carbon environments are part of the aromatic ring
H NMR
Uneven mass no. so NMR active Requires solvents w/out H or w/ D
Why can’t organic solvents be used in H NMR
Contain C and H atoms which will produce NMR signal
Relative peak areas in H NMR
Shows relative no. of protons on each environment
Stationary phase in TLC
Silica gel plate
Mobile phase in TLC
Solvent
What may give diff compounds sim Rf values
Same functional group
What affects the Rf values in TLC
Solubility Strength of adsorption
The more soluble the compound is in the mobile phase …
Higher Rf value
The stronger the adsorption onto the stationary phase …
Lower Rf value
Stationary phase in gas chromatography
Liquid w/ high bp that’s adsorbed onto the inside of a tube
Mobile phase in gas chromatography
Inert gas e.g. argon or nitrogen
What affects Rf values in gas chromatography
The solubility in the stationary phase Easier it dissolves, longer retention time
How do you find out how much there is of each compound during gas chromatography
Prepare standard solutions of known concentration and measure their peak integration Plot a graph and use the integration of your sample to read off the conc
What does the height of the peaks tell you on a gas chromatogram
Relative ratios of each compound