Module 6 Section 3: Analysis Flashcards
What does NMR stand for
Nuclear magnetic resonance
How does NMR work
A sample of a compound is placed in a strong magnetic field and exposed to a range of different frequencies of radio waves.
The nuclei of certain atoms within the molecule absorb different amounts of energy from the radio waves.
Absorption patterns gives information about: types of atoms, position of atoms, number of certain type of atoms in a molecule
Can use this info to work out structure of the molecule
What does Carbon-13 NMR tell you
Gives you information about the number of carbon atoms that are in a molecule, and the environments that they are in.
What does high resolution proton NMR tell you
Gives you information about the number of hydrogen atoms that are in a molecule, and the environments that they’re in
What creates a nucleus’ environment in NMR
A nucleus is partly shielded from the effects of external magnetic fields by its surrounding electrons or any other atoms and groups of atoms
E.g. If a carbon atom bonds to a more electronegative atom (like oxygen) the amount of electron shielding around its nucleus will decrease.
An environment of an atom depends on all the groups that it’s connected to going along the molecule (not just atoms it’s directly bonded to)
Why do different nuclei of atoms in a molecule absorb different amounts of energy
Nuclei in a molecule feel different magnetic fields depending on the amount of electron shielding from atoms around it (environment)
Nuclei in different environments will absorb different amounts of energy at different frequencies
How will two atoms be in the same environment
They must be bonded to the exact same things so the electron shielding is the same
Environments for chloroethane
Environments for 2-chloropropane
Environments for 1-chlorobutane
What does each peak of an NMR spectrum represent
Each peak represents an environmental
What does the single absorption peak at 0 represent in NMR
Represents TMS peak (tetramethylsilane) or (Si(CH3)4)
Single peak because all the carbon and hydrogen nuclei are in the same environment
Why use TMS in NMR
It’s chosen as a standard because the absorption peak is at a lower frequency than everything else.
This peak is given a value of 0 and all the peaks in other substances are measured as chemical shifts relative to this.
A small amount of TMS is added to samples to give reference peak on spectrum
What is a chemical shift
Difference in the radio frequency absorbed by the nuclei of molecule being analysed and that absorbed by the same nuclei in TMS (relates to C or H)
Given symbol of δ
Measured in parts per million (ppm)
Steps of how to interpret C13 NMR
Count number of carbon environments:
Count number of peaks in spectrum to get this
Don’t count peak at δ0 as this is reference peak from TMS
Look up chemical shifts in diagram
Use diagram in data sheet
E.g. a peak at δ≈10 could be C-C bond as the range is 0-50
E.g. a peak at δ≈210 is C=O as the range is 160-220
Then try possible structures
Estimate structure of this molecule
Spectrum has 3 peak meaning 3 different carbon environments (doesn’t mean 3 carbons as may be multiple in one environment)
Using data sheet: peaks at δ≈10 and δ≈25 are C-C bonds (at different positions as they have different chemical shifts due to molecules around the like Oxygen)
Peak at δ≈210 is C=O so ketone or aldehyde
Try out structures:
Structure is pentan-3-one
Find structure
What can the area under the peaks in proton NMR represent
Indicates the relative number of H atoms in each environment
E.g. if the area under the 2 peaks is in the ratio 2:1 there will be 2 H atoms for every 1 in the other
How can the areas of the peaks be shown in an NMR peak
Can be shown in a number above the peaks or with an integration trace
What is spin-spin coupling on an H NMR spectrum
Peaks that represent a hydrogen environment can be split
This is due to hydrogen atoms which are bonded to neighbouring carbons (1 along the chain from the carbon the hydrogen is attached to)
Only hydrogen nuclei in adjacent carbon atoms effect eachother
Splitting pattern on proton NMR peaks
Peaks split into one more peak than the number of hydrogen on neighbouring carbon atoms
This is the N+1 rule
E.g. if there are 2 hydrogens on the adjacent carbon atoms, the peak will be split into 2+1=3
What difference numbers of peaks are called and how many hydrogens are bonded to the neighbouring carbon
Single peak: singlet, 0 hydrogen on neighbouring carbon atoms
Split into 2: doublet, 1 hydrogen on neighbohring carbon atoms
Split into 3: triplet, 2 hydrogen on neighbohring carbon atoms
Split into 4: quartet, 3 hydrogen on neighbohring carbon atoms
Draw out each peaks from 0-4+ neighbouring hydrogens
Explain the peaks on the proton NMR spectrum of 1,1,2-trichloroethane
What is the problem with OH and NH groups in proton NMR
The chemical shift due to protons attached to oxygen (OH) or nitrogen (NH) is very variable
Shown as huge ranges given in the table on data sheet
They make quite a broad peak that isn’t usually split
How do chemists identify OH and NH protons given the difficulties
Run two NMR spectra of the molecule, one with deuterium oxide (D2O)
If an OH or NH proton is present it will swap with the deuterium which makes the peak disappear
This is because deuterium doesn’t absorb the radio wave energy
Splitting patterns of ethyl, CH2-CH2, CH3-CH, methyl and OH groups with ratios of intensity
Match structures to peaks using ratios of H
How to carry out TLC
Draw pencil line near bottom of the plate and put a spot of the mixture to be separated on the line.
Dip the bottom of the plate (not the spot) into a solvent.
As the solvent spreads up the plate, the different substances in the mixture move with it, but different distances — so they separate out.
When the solvent’s nearly reached the top of the plate, take the plate out and mark the distance that the solvent has moved (solvent front) in pencil.
How to calculate Rf values
How does gas chromatography work
The sample to be analysed is injected into a stream of gas which carries it through a coiled tube coated with a viscous liquid (an oil) or a solid
Components of the mixture constantly dissolve in the oil or onto the solid, evaporate back into the gas and the redissolve as they travel through the tube
The time taken for the substance to pass through the coiled tube and reach the detector is called the retention time
This can be used to identify substances
What does a gas chromatogram show
Shows series of peaks at time when detector senses something other than the carrier gas leaving the tube
Each peak corresponds to a substance with a particular retention time
Retention times are measured from 0 to centre of peak (on Y axis)
Can be looked up a reference table to identify the substances present
Area under peaks are proportional to the relative amount of each substance in the original mixture (not height)
