Topic 19 Modern Analytical Techniques 2 Flashcards
What is the purpose of qualitative analysis?
To determine what elements are present in a compound.
Combustion analysis
Burn a known mass of compound, and the masses of CO2 and H2O formed are found by measuring the increase in mass of the absorbers in the U-tubes.
How can the empirical formula be found from the masses of the products of combustion?
Mass of CO2 x 12/44 = mass of C produced
Mass of H2O x 2/18 = mass of hydrogen produced
The difference between the sum of these two masses and the initial mass of the compound is the mass of oxygen.
Divide each of these masses by the Ar to find the ratio.
When calculating empirical formula from the % composition by mass, what does it mean when the given %s do not sum to 100%?
The remainder is oxygen.
What is needed to calculate the molecular formula from the empirical formula?
The molar mass
Test for the COOH functional group & positive result
Add sodium hydrogen carbonate. Effervescence.
Test for a carbonyl & positive result.
Add 2,4-dinitrophenylhydrazine solution. Orange-yellow precipitate forms.
Test for an aldehyde group (CHO) & positive result.
Add Tollen’s reagent, and warm. A silver mirror forms.
Test for an alcohol or aldehyde & positive result.
Heat with acidified potassium dichromate (VI). Colour change from orange to green.
High resolution mass spectrometry
A type of mass spectrometry that can produce Mr values with 4 or more decimal places.
When is HRMS suitable?
Only gives information about molecular formulae not structure. Structure can be deduced from the fragmentation patterns in mass spectrometry. If two compounds have the exact same molecular formula, HRMS cannot be used to distinguish between them.
How can we calculate accurate relative molecular mass?
Use a table of element Ars to 4 d.p.. Add them together as per the molecular formula. These may correspond to peaks on a high-resolution mass spectrum.
What are the 5 main differences between High Performance Liquid Chromatography & column chromatography?
- The solvent is forced through a metal tube under high pressure, rather than being allowed to pass through by gravity.
- The particle size of the stationary phase is much smaller leading to better separation of the components.
- The sample is injected into the column.
- The components are detected after passing through the column, usually by their absorption of UV radiation.
- The whole process is automated, and the results are quickly available on a computer display.
In HPLC, what is the retention time of a component?
The time taken from injection to detection.
Upon what 3 factors does retention time depend?
- The nature of solvent.
- The pressure used.
- The temperature inside the column.
What is the advantage of traditional column chromatography (e.g. using a burette) over GC or HPLC?
Reasonable amounts of the components can be collected. E.g., in GC, the components are often detected in a flame, so are destroyed in the detection process.
What are the 6 main differences between column chromatography and gas chromatography?
- The metal tube can be several metres long and is coiled to save space.
- The stationary phase is a solid or liquid coating the inside of the tube.
- The mobile phase is an inert gas carrier (often helium or nitrogen).
- The sample is injected into the column.
- The components passing through the column are detected.
- The whole process is automated, and the results are quickly available on a computer display.
Describe the graph produced by GC.
Absorption of the y-axis. Time (mins) on the x-axis. Each component has a peak. The area under the peak represents the relative concentration of components.
What happens after the sample is injected in GC?
The components vaporize and move through the tube with the carrier gas.
What determines the speed at which components move in GC?
How strongly they are attracted to the stationary phase. Weaker attractions move more quickly = shorter retention times.
In what areas do the results of HPLC & GC have to be exactly correct?
Providing forensic evidence.
Detecting banned drugs in athletes.
Other uses of HPLC & GC
Analysis of environmental pollutants.
Space probes.
Detecting explosives in airport baggage.
What is the main limitation of HPLC & GC?
They can separate small quantities of components but not identify them. This is because it’s hard to control all variables especially solvent, pressure & temperature. Also because different substances may have the same retention time.
Without mass spectrometry, what is used to identify substances in GC & HPLC?
A database of retention times.
Microgram
1 x 10^-6 g
Nuclear magnetic resonance spectroscopy
A technique used to find the structures of organic compounds. It depends on the ability of nuclei to resonate in a magnetic field.
Chemical shift
A number (units: ppm) that indicates the behaviour of a proton or group of protons in a magnetic field relative to tetramethylsilane. It can be used to identify the chemical environment of the carbon atoms or of the protons/hydrogen atoms attached to it.
The resonant frequency of an atom relative to TMS.
NMR spectrum
Absorption on the y-axis.
Chemical shift (ppm) on the x-axis.
Which atoms are detected by NMR?
Atoms with an odd number of nucleons: 13C, 1H, 15N & 31P.
What is the solvent used in NMR?
CDCl3
D is for deuterium, which contains a proton and a neutron, so their spins cancel out. This produces no signals that could interfere with the signals from the H atoms. It produces just one signal that could interfere with the signals from the carbon atoms, which can easily be removed from the spectra.
Describe the structure of tetramethylsilane.
Similar to methane, but with Si instead of C in the middle and 4 methyl groups attached (instead of H atoms).
Why is tetramethylsilane a suitable molecule for a reference standard in NMR?
It’s chemically unreactive, so will not react with most organic compounds.
It contains 12 H atoms in a symmetrical arrangement, so it produces a single, strong; easily-identifiable signal. The signal is strong, so only a small amount is needed. The peak doesn’t overlap with other peaks. Low boiling point, so it’s easily removed.
What is the chemical shift of TMS?
0 ppm
Chemical environments of carbon atoms in a molecule
Related to whether the carbon atoms are identically or differently positioned within a molecule.
Vertical lines in a 13C NMR spectrum
Signals or peaks
What does the number of signals in a 13C NMR spectrum indicate?
The number of different chemical environments of carbon atoms in a molecule– not necessarily the total number of carbons.
What does the position of signals in a 13C NMR spectrum on the horizontal scale indicate?
The chemical shifts that can be used to deduce the chemical environment.
The greater the height of a peak in 13C NMR…
…the more carbon atoms causing that peak.
What does a peak in a 1H NMR spectrum show?
The presence of H atoms/protons in a specific chemical environment.
Integration trace
Shows the relative numbers of equivalent protons (i.e., in the same chemical environment).
What is indicated on a low resolution 1H NMR spectrum?
- The number of peaks = the number of different chemical environments of the hydrogen atoms.
- The areas under the peaks indicate the relative numbers of protons in each chemical environment.
- The chemical shift of a peak indicates the functional group responsible.
The increases in height on an integration trace
Represent the relative areas under the peaks.
Equivalent protons
Hydrogen atoms in the same chemical environment.
Splitting pattern
The appearance of a peak as a small number of sub-peaks very close to one another.
Multiplets
The different splitting patterns observed (singlet, doublet, triplets, quartets etc.).
Where is the chemical shift taken for a multiplet?
The centre of the group of sub-peaks.
How can the relative peak areas be shown on high-res or low-res 1H NMR?
By an integration trace or with numbers next to the peaks. Remember: this is a ratio, not necessarily the actual numbers.
What causes splitting of the peaks?
Spin-spin coupling. The protons on the adjacent carbon influence the protons on the carbon.
Doublet ratio of peak areas
1: 1
Triplet ratio of peak areas
1:2:1
Quartet ratio of peak areas
1: 3: 3: 1
If a carbon has n protons…
…the peaks on the adjacent carbon will be split into n + 1 sub-peaks.
What causes a singlet?
Zero protons on the adjacent carbon atom.
In CH3 - CH2 - CH2 - Cl, into how many sub-peaks is the peak for the protons on carbon 2 split?
- There are 3 protons on the adjacent CH3 group and 2 protons on the adjacent CH2 group. Summed together = 5. n + 1= 6.
What happens, in terms of splitting, in completely symmetrical molecule?
There is no splitting as all the protons are equivalent.
Aromatic hydrogens…
… give similar peaks in 1H NMR.
2 things to always include when analysing 1H NMR spectra
Number of peaks/environments & ratio of relative peak areas.
What factors affect retention time in GC?
Higher boiling temperature compounds spend less time in the gas phase, so have longer retention times. Other factors: solubility, polarity, attraction & affinity.
How many significant figures should Rf values be given to?
2
