Chemical analysis Flashcards
Why do chemicals in a mixture separate in chromatography?
They differ in the extent to which they are soluble in the mobile phase (and the stationary phase if it’s a liquid)
They differ in the extent to which they adsorb/stick to the stationary phase
How does thin layer chromatography work?
Thin layer of Al2O3 or SiO2 (stationary phase) supported on a glass plate is inserted into a container of solvent with a lid to keep the solvent evaporating. The height that each substance reached is recorded, and used to calculate the Rf (distance moved by spot divided by distance moved by solvent). UV light can be used to view invisible compounds
What is gas chromatography?
A sensitive technique used for analysing complex mixtures which vaporise when heating. This method also tells you how much of each compound is present
How does gas chromatography work?
A sample is vapourised and injected into the head of the chromatographic column. The column is full of an unreactive gas (the carrier gas), and a solid or liquid adsorbed into an inert solid. If the stationary phase is a liquid, separation depends on the relative solubility of the tested compound in the stationary and mobile phases. If the stationary phase is a solid, it depends on the adsorption of the components into the stationary phase. The components reach the end of the column one by one and pass into a detector, which plots a graph of time and recorded chemical response. The area under each peak gives an indication of the relative amount of that component
What is retention time?
Time taken between the injection of a sample and emergence from the column
Disadvantages of gas chromatography
Similar compounds such as methanal and ethanal often have overlapping peaks. Identification of unknown compounds is hard because reference times vary based on rate of flow of carrier gas and temperature. These limitations are overcome by pairing it with mass spectrometry
How does NMR spectroscopy work?
A sample of compound is placed in a strong magnetic field and exposed to a range of different frequencies of radio waves. The nuclei of certain atoms in the molecule absorb energy from the waves. The amount of energy a nucleus absorbs tells you what environment it’s in
What are the two types of NMR spectroscopy?
Carbon-13 NMR (gives information about the number of carbon atoms and their environments), and proton NMR (number of hydrogen atoms)
Why do different carbon atoms absorb different amounts of energy?
They are shielded from the effects of external magnetic fields by surrounding electrons. The amount of shielding is also affected by which atoms the C or H is bonded to
How is tetramethylsilane used in NMR?
TMS produces a single peak at a lower frequency than almost everything else. This peak is given the value of 0, and peaks are measured as chemical shifts relative to this
What is chemical shift?
Difference in the radio frequency absorbed by TMS and that absorbed by the atoms in the molecules being analysed. It’s measured in ppm
How do you interpret a Carbon-13 NMR diagram?
Count the number of peaks that aren’t 0. This tells you how many different carbon environments there are. Look up chemical shifts in the data sheet to determine which bonds are present, and then try out possible structures that match the chemical formula given.
What does the relative area under each peak tell you in a proton NMR diagram?
The number of H atoms in each environment. If two peaks have areas in the ratio 2:1, the number of H atoms in the second peak is 2x the number of H atoms in the first
What is spin-spin coupling?
The number of mini-peaks in a peak is one more than the number of hydrogens on all the surrounding carbons not in the environment definition
What solvents are used for proton NMR?
Deuterated solvents, because deuterium doesn’t absorb radio wave energy like hydrogen does. This solvent is often CDCl3
How can OH and NH protons be identified? (since their peaks are so wide)
Run two spectra, one with D2O added. The deuterium should turn the OH and NH into OD and ND, causing the peak to disappear
How do you test for an unsaturated hydrocarbon such as an alkene?
It will decolourise bromine water
How do you test for a haloalkane?
Add silver nitrate, ethanol and water. White precipitate = chloro-, cream = bromo-, yellow = iodo-
How do you test for a carbonyl functional group?
Acidified K2Cr2O7: Aldehydes turn it green
Fehling’s solution: Aldehydes cause dark red precipitate
Tollens’ reagent: Aldehydes cause silver mirror
Ketones produce no change
How do you test for carboxylic acid?
Add universal indicator, and the pH should be that of a weak acid
What happens to covalent bonds when they absorb infrared radiation?
They stretch or bend
How does infrared spectroscopy work?
All bonds vibrate at their own unique frequency, usually between 300 and 4000 cm^-1. The IR spectrometer fires a beam of radiation containing the full range of IR frequencies. The bonds in the molecules absorb certain frequencies, and the emerging beam is analysed to plot a graph of frequency vs transmittence.
What happens in a mass spectrometer?
Some molecules lose an electron and are ionised. The resulting ion is called the molecular ion and has the symbol M+. Excess energy from the ionisation process is transferred to the molecular ion, causing it to vibrate and weakening its bonds. This causes random fragmentation, producing neutral species and smaller ions
What is m/z?
Mass/charge ratio
