Studied by 0 people
Looks like no one added any tags here yet for you.
What is chromatography?
An analytical method to separate individual components from a mixture of substances
What is the stationary phase of chromatography?
A solid or liquid that doesn't move and allows for the mobile phase to travel on/through
What is the mobile phase of chromatography?
A liquid or gas that moves to carry the molecules being separated to further separate the molecules
What is Thin Layer chromatography (TLC)?
Quick and inexpensive analytical technique that indicates the components of a mixture
What is the stationary phase in TLC?
Thin layer of plastic or glass with a thin layer of solid adsorbent
What is a common solid adsorbent substance used in TLC?
Silica
What is adsorption?
The process by which the stationary phase (solid silica) holds the analytes from the mixture to its surface
How is separation of molecule achieved in adsorption (TLC)?
Different components of a mixture will have different affinities for the adsorbent, binding with different strengths
What is a common chromatography tank?
Small beaker with a watch glass on top
What are the steps to setting up a TLC?
1. Take a TLC plate, drawing a line across the plate as your base line, 1cm above the bottom
2. Using a capillary tube, spot a small volume of the solution of the sample onto the base line of the plate
3. Prep a chromatography tank for the TLC plate
4. Pour some solvent into your chromatography tank (usually 0.5 cm^3)
5. Place the TLC plate into the chromatography tank making sure the solvent doesn't cover the spots
What are the final steps in using a TLC?
6. Allow the solvent to rise up the plate until it's 1cm^3 below the top of the plate
7. Remove the TLC plate from the chromatography tank and mark the solvent front with a pencil
8. Circle visible spots or hold the TLC plate below a UV lamp, or use iodine as a locating agent
How do you calculate Rf values in TLC?
What is Gas Chromatography?
A technique used to separate volatile compounds (mostly organic) in a mixture
Show the schematic of a gas chromatography
What is the stationary phase in gas chromatography?
Extreme high boiling liquid adsorbed onto an inert solid support
What is the mobile phase in gas chromatography?
"Inert" carrier gases including;
- Hydrogen
- Helium
- Argon
- Nitrogen
- Argon/Methane
What are the steps of how gas chromatography works?
1. Small volume of the volatile mixture in liquid or gas form is injected into the injection port of the gas chromatograph
2. The mobile phase (carrier gas) carries the components in the sample through a capillary column
3. Components slow down as they interact with the liquid stationary phase inside the column
4. The components (analytes) of the mixture are separated based on their solubility and volatility in the liquid stationary phase
5. Analytes reach the detector at different times (retention times) depending on their interactions with the stationary phase in the column
What is taken as a measurement in gas chromatography?
Retention time for each analyte that travelled through the column
What are the two pieces of information that can be taken from a gas chromatogram?
- Retention times can be used to identify the components present in the sample
- Peak integrations (areas under each peak) can be used to determine the concentration of components in the sample
What does comparison of retention times in gas chromatography allow for?
- To detect the presence of certain organic compounds
- If you have a similar matching retention time peak to your known peak you can known if that sample is present
How is the concentration of certain components determined , outline the procedure.
1) Prepare standard solutions of known concentrations of the compound being investigated
2) Obtain gas chromatograms for each standard solution a plot a calibration curve of peak area against concentration (external calibration)
3) Obtain a gas chromatogram of the compound being investigated under the same conditions
4) Use the calibration curve to measure the concentration of the compound
Example of gas chromatogram peak area relative to finding concentration
What method can you use to analyse the presence of any organic compound?
Distill the compound relative to it's boiling point
What are the five organic functional groups you MUST ALREADY known how to analyse qualitatively?
- Alkene
- Haloalkane
- Carbonyl (aldehydes, ketones)
- Primary, secondary alcohols
- Carboxylic acids
How do you test for an alkene?
- Add the alkene with bromine water drop-wise
- Orange to colourless as bromoalkane will form from bromine
How do you test for a haloalkane?
- Add aqueous silver nitrate and ethanol and warm to 50˚C in a water bath
- white, cream or yellow precipitate will form depending on the haloalkane present
How do you test for a carbonyl group?
- Add 2,4-DNP (Brady's reagent)
- An orange-precipiate will form
How do you test for an aldehyde between multiple carbonyl groups?
- First do Brady's reagent to test for a carbonyl group
- Add Tollen's reagent and warm
- A silver mirror will form
How do you test for a primary, secondary alcohol or aldehyde?
- Add acidified potassium dichromate (VI) and sulphuric acid and reflux and warm in a water bath
- Colour change from orange to green
How do you test for a carboxylic acid?
- Add aqueous sodium carbonate
- Effervescence of carbon dioxide gas will occur
What is nuclear spin?
Electrons spinning
When is nuclear spin significant?
When there is an odd number of nucleons (protons and neutrons)
Why is NMR centred around H-1 and C-12?
Nearly all organic molecules contain carbon and hydrogen
What can NMR spectroscopy be used for?
Detect isotopes of other elements with an odd number of nucleons
What is H-1 NMR also known as?
Proton NMR
How many spin states does an electron have?
An electron has two different spin states
How man spin states does the nucleus of an atom have?
A nuclei also has two different spin states with different energy
What is resonance?
When the right combination of a strong magnetic field and radius frequency radiation causes the nuclei to absorb energy and rapidly flip between two spin states
Is nuclear magnetic resonance interchangeable with resonance?
Yes
What is the radio frequency radiation required for resonance proportional to?
Magnetic field strength
When will a small quantity of energy released actually be detected in NMR? What type of magnetic field is needed?
When the radio frequency radiation, proportional to the magnetic field strength is in strong and uniform magnetic fields
What is TMS?
Tetramethylsilane (CH3)4Si
What is tetramethylsilane needed for?
TMS is needed as the standard reference chemical to which all chemical shifts are measured against
What is chemical shift?
The difference between absorption frequency of a carbon or proton in TMS and a carbon or proton in a sample
What is chemical shift measured in?
ppm (parts per million) relative to ppm of TMS
What does the absorption of nuclear magnetic resonance at a certain energy and frequency do?
Allows detection of absorption peaks to be identified
What is the amount of chemical shift dependent on? What would this determine?
- Dependent of the chemical environment
- Determines the nuclear magnetic resonance at a certain energy and frequency
What are the benefits of using NMR compared to other analytical techniques?
- NMR can determine number of carbons and hydrogens without needing to carry out conventional tests
- NMR can determine the number of carbons and hydrogens without needing to destroy the organic compound
What are the steps of running an NMR spectrometer?
1) Dissolve the sample in a solvent and place the narrow NMR sample tube with a small amount of TMS
2) Place the NMR sample tube inside the NMR spectrometer
3) Allow the NMR to spin the sample tube to even out any imperfections in the magnetic field within the sample
4) Calibrate the NMR spectrometer against the TMS standard
5) Send a pulse of radiation containing radiation frequencies with a constant magnetic field to the sample
What occurs after a pulse of radiation with radiation frequencies is sent to the sample?
Any absorption's of energy from resonance is detected and displayed on a computer screen
What happens after a sample has undergone NMR analysis?
Sample can be recovered through evaporation of the solvent
What is a deuterated solvent?
Where H-1 has been replaced by H-2 (deuterium)
Why should we use a deuterated solvent ?
As a deuterated solvent will produce no NMR signals in the frequency ranges with H1 and C-13 NMR spectroscopy
What three factors can move a peak outside the normal NMR peaks? Relative to TMS
- Solvent
- Concentration
- Substituents
When using a deuterated solvent in H-1 and C-13 NMR, what must also be used as a solvent?
Deuterated trichloromethane
When deuterated trichloromethane is used as a solvent, what peak will the computer filter out before displaying the spectrum?
A peak in a carbon-13 NMR spectrum
What two important pieces of information does carbon-13 give?
- Number of different carbon environments from the number of peaks
- The types of carbon environments present from the chemical shift (ppm ~ ∂)
What are the four main types of carbon environments?
- Carbon part of a carbonyl group or carboxylic acid
- Carbon part of a C=C or aromatic ring
- Carbon bonded to an electronegative atom
- Carbon bonded to a carbon
What occurs with the chemical environments when two carbons positioned are symmetrical?
The chemical environments are classed as equivalent,
- Therefore the carbon atoms will absorb the same radiation and give the same chemical shifts
- Therefore the only addition to your spectrum would be an increase in peak height
Show the carbon environments of propanal
Show and draw the spectrum for propanal (you will need a data sheet)
Show the carbon environments for propanone
Show and draw the spectrum for propanone (you will need a data sheet)
Remember the relative peak heights will be twice as more
Isomer of alcohols NMR data interpretation with carbon-NMR
Aromatic isomers, NMR data interpretation with carbon NMR
What are four pieces of information proton (H) NMR can give?
- Number of different proton environments from the number of peaks
- Types of proton environments present from the chemical shift
- Relative numbers of each type of proton; relative peak areas
- Number of non-equivalent adjacent protons from the spin-spin splitting pattern
What are the principles for proton equivalency and non-equivalency?
- If two or more protons are equivalent, they will absorb at the same chemical shift, increasing the height of the peak
- Protons of different types will have different chemical environments and will absorb at different chemical shifts
Show the proton environments in butanoic acid
Show the proton environments in butandioic acid
Remember symmetry
In proton-NMR is the ratio of the number of relative areas under each peak proportional to the number of protons responsible to each peak?
Yes!
- Understood through an integration trace which provides number of peak areas
Show the integration trace of methyl chloroethanoate
What is spin-spin coupling? What does it allow?
- When one spin state of a proton interacts with another protons spin state in a different environment
- This will cause splitting of the main peak into sub-peaks (spin-spin coupling)
- Allows information to be given about the number of protons on adjacent carbon atoms
What is the n+1 rule in proton NMR?
How the number of sub peaks is one greater than the number of protons that cause the splitting of the main peak (spin-spin coupling)
How many peaks will an adjacent molecule with only 3 hydrogens cause?
4 peaks
How many peaks will an adjacent molecule with only one hydrogen cause?
2 Peaks
How many peaks will an adjacent molecule with no hydrogens cause?
1 peak (0+1 = 1)
Show the spin-spin splitting patterns, include;
- n
- n+1
- Splitting pattern name
- Number of peaks
- Pattern itself
- An extremely possible feature (using carbon as the atom hydrogens are bonded to)
What is a multiplet?
A splitting pattern more than four
When could a multiplet occur?
1) When there are two carbons on either side of the carbon bonded to your hydrogen
2) The combined hydrogens of both adjacent carbon is 4 ≤ (number of adjacent hydrogen)
What will spin spin coupling of aromatic compounds be?
Multiplets
In splitting patterns, will there always be a single or pair of peaks?
There will ALWAYS be a pair of peaks causing the splitting
When will there be a broad peak in proton NMR?
When the hydrogen is attached to an oxygen (i.e. alcohol) or nitrogen
What is proton exchange?
A technique after a broad peak has been seen
How is proton exchange to identify -OH or -NH groups carried out? Explain all of the steps
1. A proton NMR spectrum is run as normal
2. A small volume of deuterium oxide is added
3. The mixture is shaken and a second spectrum is run
What occurs when proton exchange is carried out?
- The hydrogens attached to an oxygen or nitrogen is replaced by Deuterium (H-2)
- Deuterium doesn't absorb in the chemical shift range and no peaks are run in the second spectrum
Do tons of practice paper questions of Carbon-13 NMR and Proton NMR. Seriously. Its very tough and is usually combined with other analysis techniques.
You're a few flashcards from the end.
What are the four analytical techniques to determine the structure of an organic compound?
- Elemental analysis
- Mass spectra
- Infrared spectra
- NMR spectra
What is the use of elemental analysis?
Useful in giving percentage composition by mass to determine the empirical formulae of a compound
What is the use of mass spectra?
- Use of the molecular ion peak to determine the molecular mass and use of fragment ions to determine parts of the molecule.
- Molecular formulae can be determined from the empirical formula and molecular mass
What is the use of infrared spectra?
Use of absorption peaks to identify bonds present and the functional groups of a compound
What is the use of NMR spectra?
Determination of the number and type of carbon and hydrogen atoms from the chemical shift of peaks and the order of atoms within the molecules from splitting patterns
