Chapter 29 Flashcards
How does TLC work?
- The stationary phase is silica or alumina (this is then on a glass/ plastic plate)
- The mobile phase is the solvent
- Separation occurs due to relative adsorption to the stationary phase, as the components in the mixture would each adsorb differently to the stationary phase
How would you carry out TLC?
- Draw a base line near the bottom in pencil
- Using a capillary tube, spot a small sample of the solution to be tested onto the baseline of the plate
- Pour some solvent into the chromatography tank, ensuring the depth is lower than the pencil line
- Place the TLC plate into the solvent, ensuring the solvent does not touch the spot, and that the edges of the TLC plate do not touch the sides of the tank
- Place a lid on the solvent tank
- Allow the solvent to rise until it is about 1 cm below the top of the plate
- Remove the plate, and mark the solvent level immediately with pencil
- Allow the plate to dry, and use a dye if necessary to show the position of the spots
How are TLC plates analysed?
- The Rf (retention factor) is calculated for each component
- Rf values are compared to a database to identify components
How are Rf values calculated?
- Distance moved by the component / distance moved by the solvent front
How can you use chromatography to tell if a substance is impure?
- If the substance is impure, it will have a spot where the pure substance has a spot
- It would also have a (smaller) spot where the starting material (or other contaminant) was
How is chromatography used to monitor the course of a reaction?
- Take samples from the reaction mixture at regular intervals
- Run on a TLC plate, comparing against a control of the starting material
What is gas chromatography used for?
- To analyse compounds with volatile components or low boiling points
How does gas chromatography work?
- The stationary phase is a high boiling liquid adsorbed onto an inert solid support
- The mobile phase is an inert carrier gas (e.g. nitrogen)
- Separation is by relative solubility with the stationary phase
How are gas chromatograms analysed?
- The retention time (the time taken for each component to travel through the column) is compared to a database and is used to identify the component
- The peak integration (area under the peak) tells you the relative concentrations of the components
How can you find the the concentration of a component using gas chromatography?
- You prepare standard solutions of the component and run gas chromatograms for each
- Plot a calibration curve of peak area against concentration
- Run a gas chromatogram for your sample, and find the peak integration of the component
- Use your calibration curve to find the concentration of the component
How are alkenes tested for?
- Decolourise bromine water
How are haloalkanes tested for?
- Heat with aqueous silver nitrate in ethanol
- Chloroalkanes will form a white precipitate
- Bromoalkanes will form a cream precipitate
- Iodoalkanes will form a yellow precipitate
How are carbonyls tested for?
- Add 2,4-DNP
- Orange precipitate formed
How are aldehydes tested for?
- Add Tollens’ reagent
- Silver mirror formed
Which 2 functional groups are tested for in the same way? How are they tested for?
- Primary alcohols
- Secondary alcohols
- Aldehydes
- Warmed with acidified potassium dichromate, colour change from orange to green
How are phenols tested for?
- As weak acids, they can react with strong bases (e.g. NaOH) but not weak bases, such as Na2CO3
How are carboxylic acids tested for?
- They can react with weak bases like Na2CO3
- Effervescence would be given off
How does NMR spectroscopy work?
- In nuclei where there is an odd number of nucleons, the nucleus is either ‘spin up’ or ‘spin down’ (when there is an even number of nucleons equal numbers are spin up and spin down)
- In the presence of a strong magnetic field and radio waves of the correct frequency, the nucleus can absorb energy and rapidly flip between the 2 states
- NMR spectrometers measure what frequency of radio waves interact with the sample
What 2 substances are needed to run NMR?
- TMS
- A deuterated solvent
What is TMS, and why is it used in NMR spectroscopy?
- Si(CH3)4
- It is used as the standard reference chemical that all chemical shifts are measured against (it has a chemical shift of 0ppm)
What are deuterated solvents, and why are they needed in NMR spectroscopy? Give an example of one.
- Most solvents contain carbon and hydrogen atoms, so would produce a peak in both 13C and 1H NMR spectra
- A deuterated solvent is used instead as all of the 1H atoms are replaced with 2H atoms, so any hydrogen atoms would not produce a peak
- CDCl3
What 2 pieces of information can be obtained from a 13C NMR spectrum, and how?
- The number of carbon environments from the number of peaks
- The types of carbon environments present from their chemical shift
Why do carbon atoms in the same environment have the same chemical shift?
- They would absorb radiation of the same frequency
What are some shortcuts to tell if 2 carbon atoms are in the same environment?
- Line of symmetry
- Rotation
- If there is only one substituent, counting on from it
What limitation is there when finding the type of carbon environment from an NMR spectrum?
- Only the bond with the highest shift value is shown on the spectrum
If there are 2 or more peaks in the same region of an NMR spectrum, how can their environments be told apart? What exception is there to this?
- The one with a higher shift value would be closer to the bonds in the molecule that would have a higher shift value
- This is not true for aromatic compounds
What other type of NMR is there, and why does it work?
- Proton NMR
- Hydrogen atoms are also spin active
What similarity is there in the way C-13 and H-1 NMR are run?
- Both use TMS as their standard reference chemical
What 4 pieces of information can be found from a proton NMR spectrum, and how?
- The number of hydrogen environments from the number of peaks
- The type of proton environment from the chemical shift
- The (relative) number of hydrogen atoms in each environment from the integration
- The number of non-equivalent protons attached to an adjacent carbon atom from the peak’s splitting pattern
How can you tell if hydrogen atoms are in the same environment?
- If they are bonded to the same carbon atom, they are in the same environment
- If they are bonded to carbon atoms that are in the same environment, the hydrogen atoms are also in the same environment
What is the integration of a peak in proton NMR? What do you need to be aware of when using it?
- The area under the peak
- Peaks come labelled with these values, but sometimes this can be a ratio
What causes peaks to split?
- The n+1 rule; for a proton peak with n non-equivalent protons attached to an adjacent carbon atom, the number of sub-peaks in a splitting pattern is n+1
What is meant by an equivalent proton?
- A proton in the same environment
How are splitting patterns named, and what does each mean?
- Singlet
- Doublet- 1 adjacent hydrogen atom
- Triplet- 2 adjacent hydrogen atoms
- Quartet- 3 adjacent hydrogen atoms
- Multiplet (5 or more peaks) - 4 or more adjacent hydrogen atoms
How can you tell if a peak is one peak, or 2 close together?
- Peaks that are split have a ratio of heights, with the middle the highest, and the ones on either side decreasing in height
What causes a singlet?
- When there are no non-equivalent protons on the adjacent carbons
- There is an OH or NH group, as their protons do not have an adjacent carbon, so their peaks are not split
What can make OH and NH peaks stand out?
- They can have broad peaks (V-shaped)
How else can OH and NH peaks be identified?
- You run an NMR sample
- You then add a small volume of D2O (deuterium oxide), which replaces all the protons in any NH or OH groups with deuterium atoms
- You run the NMR sample again, and any peaks that disappear or from NH or OH groups
How can you use a proton NMR spectrum to identify a molecule?
- Draw a table and fill in these columns for each peak:
- shift
- functional group
- integration
- splitting
- number of adjacent hydrogens
- identity (including neighbouring atoms)
- Make sure to underline the protons responsible for the peaks
Which peaks do not always appear as they’re meant to?
- Peaks from hydrogen atoms bonded to a benzene ring
