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
Why can’t you use regular solvents in proton NMR
NMR spectra are recorded with the molecule that is being analysed in solution.
Can’t use an ordinary solvent like water or ethanol as the hydrogen nuclei in the solvent would add peaks to the spectrum and interfere with results
Instead the hydrogen nuclei in the solvent are replaced with deuterium (D) (an isotope of hydrogen)
Why can deuterium be used as a solvent
Deuterium nuclei don’t absorb the radio wave energy, so they don’t add peaks to the spectrum.
What is the solvent commonly used in proton NMR
A commonly used example of a ‘deuterated solvent’ is deuterated chloroform, CDCI3
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
Test for alkene
Tests for the double bond functional group
Add 2cm3 of substance to test tube
Add 2cm3 bromine water
Shake
Alkene: solution goes from orange to colourless
Test for haloalkanes
Add five drops of the unknown substance to a test tube.
Add 1 cm3 of ethanol and 1 cm3 of aqueous silver nitrate.
Place the test tube in a water bath to warm it.
Watch for a precipitate and observe the colour.
Chloroalkane - white precipitate.
Bromoalkane - pale cream precipitate.
Iodoalkane - pale yellow precipitate
How to test for phenols
Tests for the acidic properties of phenol
1cm3 of substance
Add 2cm3 of NaOH
Acidic(Phenol): solid will dissolve and colourless solution of sodium salt forms
Not acidic: nothing happens
Then add same sample of substance to test tube and add 2cm3 of Na2CO3
Phenol (base): nothing happens
Strong acid: effervescence
Why use the two tests for phenol
Phenol is a weak acid so add a strong base (NaOH) that reacts with any acid
Carbonates are weaker bases which can only react with strong acid so will not react with phenol
How to test for carboxylic acids
Carboxylic acids react with carbonates to form a salt, CO2 and H2O
Add 2cm3 of substance to test tube
Add 2cm3 of carbonate solution
Bubble any gas that is produced through limewater in a second test tube
Carboxylic acid: solution will effervesce and CO2 produced turns limewater cloudy
How to test for carbonyl group
Dissolve 0.2g of 2,4-DNP in 1cm3 of H2SO4, 2cm3 water and 5cm3 of methanol
In different test tube add 5 drops of substance to 2cm3 of 2,4-DNP solution
Shake test tube and watch for ppt
Aldehyde/ketone - bright orange ppt forms
How to test for aldehydes
Used to tell aldehydes and ketones apart
2cm3 of 0.1moldm-3 silver nitrate solution in test tube
Add 5 drops of 0.8moldm-3 NaOH and light brown ppt should form
Add drops of dilute ammonia solution until brown ppt dissolves completely
Place test tube in hot water bath
Add 10 drops of aldehyde or ketone
Aldehyde: silver mirror on walls of test tube
Ketone/not aldehyde: nothing
How to test for primary and secondary alcohol
Add 10drops of alcohol to 2cm3 of acidified potassium dichromate solution in test tube
Warm mixture gently in hot water bath
Watch for colour change
Primary: orange solution turns green slowly as aldehyde forms
Secondary: orange solution turns green slowly as a ketone forms
Tertiary: nothing happens
Different types of chromatography
Thin layer chromatography (TLC)
Gas chromatography (GC)
How does thin layer chromatography separate components
Adsorption
What happens in TLC
A solvent (ethanol) moves over glass or plastic plate which is covered in thin layer of solid
E.g. silica gel or aluminium powder
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 to identify what was in the thin layer chromatography mixture
Calculate Rf value and compare them to a table of known values
Why do substances travel different distances on the TLC plate
How far a substance travels depends on how strongly it’s attracted to layer of solid on surface of the plate
This attraction is called adsorption
A substance that is strongly adsorbed will move slowly and won’t travel as far compared to one that is weakly adsorbed
This creates different Rf values
What affects how strongly a substance is adsorbed to the plate
Polarity can affect this
Why do the conditions have to be kept the same in TLC
The distance the substance moves up the plate (Rf value) is affected by
Solid coating on the plate
Solvent used
Temperature
Means that small changes in the setup can affect the Rf value
How to increase the reliability of the chromatography experiment
Once the unknown substance is identified you can check by running a pure sample of the known substance alongside the unknown compound on a TLC plate
If they are the same they should have the same Rf value
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)
Why use an external calibration curve for GC
Area under peak says relative amount of substance
External calibration curve is used to find exact concentration
How to create an external calibration curve
Create series of standard solutions of different concentrations of analyte (substance chosen to detect for calibration)
Inject standard solutions into a gas chromatography instrument and record result
Calculate area under peak of each response for each standard solution
Plot these on graph of area over concentration
Join up points to create external calibration curve
Why might you run a blank when making a calibration curve
A blank is a solution containing all the solvents and reagents used when making standard solutions
But without analyte
Subtract the response of blank from responses of each standard solution to find corrected peak value
What factors effect retention times
Solubility
Boiling point
Temperature of GC instrument
How does solubility affect retention time
Determines how long each component of the mixture spends dissolved in the oil or on the solid and how long they spend moving along the tube in the gas.
A highly soluble substance will spend more time dissolved, so takes longer to travel through the tube to the detector than one with a lower solubility.
How does boiling point affect retention time
A substance with a high boiling point will spend more time condensed as a liquid in the tube than as a gas.
This means it will take longer to travel through the tube than one with a lower boiling point.
How does temperature of GC instrument affect retention time
A high temperature means the substance will spend more time evaporated in the gas and so will move along the tube quickly.
It shortens the retention time for all the substances in the tube
