Q. What is chromatography?
A process used to separate substances in a mixture. Separation of the substance depends on the distribution between a mobile phase and a stationary phase.
Q. In thin-layer chromatography (TLC), what are the different phases?
Mobile phase - solvent, stationary phase - layer of silica gel or alumina on a piece of glass.
Q. Why are substances separated during TLC?
Substances separate because of different solubilities in the mobile phase and different attractions to the stationary phase.
Q. What are the different phases in gas-liquid chromatography (GC)?
Mobile phase - inert carrier gas.
Stationary phase - liquid with a high boiling point that is adsorbed into a solid.
Q. What does the term retention time mean in relation to gas chromatography?
The amount of time taken for a sample to travel from the injector to the detector.
Q. How does boiling point affect the retention time of a compound in gas chromatography?
Compounds with higher boiling points will condense sooner in the column, so the retention time will be longer.
Q. How does solubility in the liquid stationary phase affect the retention time of a compound in gas-liquid chromatography?
More soluble compounds will have longer retention times as they spend less time in the carrier gas mobile phase.
Q. How does temperature affect the retention time of a compound in gas-liquid chromatography?
The higher the temperature, the shorter the retention times of all compounds, as the molecules have more kinetic energy.
Q. How can a gas-liquid chromatogram be used to work out the percentage composition of a mixture?
The area under each peak is the relative amount of each substance. Area is calculated using 1/2 x base x height.
Q. Why can carbon-13 nuclei be used in NMR spectroscopy?
It can align with or against a magnetic field. It is less stable to oppose the magnetic field, meaning this has higher energy. Energy can be supplied to the nucleus in the form of radio waves and cause it to flip from the more stable alignment to the less stable alignment. This energy change depends on the carbon environment.
Q. What is a carbon environment?
The atoms/group of atoms that a carbon atom is bonded to.
Q. What can be deduced from a carbon-13 NMR spectrum? How?
Number of different carbon environments = number of peaks on the spectrum.
Types of carbon environments - compare the chemical shift values of the peaks to the data book to identify the carbon environments.
Q. Why can protons be used in NMR spectroscopy?
Hydrogen nuclei can align with or against a magnetic field. The direction of this alignment can be flipped using a specific frequency of radio waves (known as the resonance condition). This is because there is a difference in the energy of the two alignments. The frequency required for this change depends on the proton environment.
Q. What are equivalent protons?
Protons in the same environment. These may be on the same or different carbons.
Q. When doesn't n+1 rule not work?
Alcohols - the chemical shift for the hydrogen atom in -OH is variable, and this peak is always a singlet. -OH doesn't cause splitting in adjacent hydrogens.
Equivalent hydrogens - protons bonded to the same carbon (protons in the same environment) have no effect on each other in terms of splitting.
Benzene - the splitting pattern is very complicated (called a multiplet). The peaks for a benzene ring will be found in the range 6 to 9 ppm.
Q. Why is TMS (Tetramethylsilane) used as the standard for chemical shift measurements in NMR spectroscopy?
- It has 12 hydrogens in the same environment, meaning a single strong peak will be produced in proton NMR.
- It contains carbon and hydrogen atoms meaning it can be used in both carbon and proton NMR.
- It contains 1 carbon environmen,t meaning it produced a single peak in carbon-13 NMR.
- It's non-toxic
- It's inert so won't react with the compounds under analysis.
Q. What is a deuterated solvent? Why are they used in NMR spectroscopy?
A deuterated solvent contains deuterium (an isotope of hydrogen) instead of hydrogen-1. They are used so that they don't produce a peak on the proton NMR spectrum. This means the solvent will not interfere with the analysis.
Q. Why does proton exchange occur between an alcohol and D2O? Why does this cause the O-H peak to disappear from the NMR spectrum?
Alcohols are slightly acidic, meaning the hydrogen in the OH group transfers to one of the lone pairs on oxygen in D2O. The negative ion formed is likely to collide with D2O, which will reform the alcohol with an OD group instead of an OH group. Deuterium doesn't produce a peak onthe NMR spectrum, meaning the peak caused by the O-H proton disappears.
No comments:
Post a Comment