Organic Synthesis and Analysis

Question 1

What is the mobile phase?

Answer 1

What carries the soluble components of the mixture with it.

Question 2

What is the stationary phase?

Answer 2

What holds back the components in the mixture that are attracted to it.

Question 3

Explain how chromatography works?

Answer 3

Every component of a mixture has its unique balance between its affinity for the stationary phase (retention) and the mobile phase (solibility) due to their different structures and polarities. The more affinity the component has for the stationary phase, the slower it moves. The more affinity it has for the mobile phase, the faster it moves.

Question 4

Difference between TLC and paper chromatography

Answer 4

Filter paper replaced by glass, metal or plastic sheet coated in silica gel or Al2O3.

Question 5

Advantages of TLC over papier chromatography

Answer 5

Runs faster. Smaller amounts of mixtures can be separated. Spots usually spread out less. Plates are more robust than paper.

Question 6

How can colourless spots be located in TLC?

Answer 6

Shining UV light on the plate. Or spray the plate with a locating agent (like ninhydrin) which reacts with the components of the mixture forming coloured compounds.

Question 7

How to calculate Rf

Answer 7

Distance moved by spot over distance moved by solvent.

Question 8

What is the eluent?

Answer 8

The solvent in the mobile phase.

Question 9

Describe column chromatography and how it works?

Answer 9

Uses powder such as silica, aluminium oxide, or resin as stationary phase. This is packed in narrow tube (column) and the eluent is added at the top. As eluent runs down column, the components of the mixture move at different rates and can be collected separately in flasks underneath column. Has mineral wool plug at bottom of column.

Question 10

How can better separation be achieved in column chromatography?

Answer 10

Can use more than one eluent.

Question 11

Describe Gas-liquid chromatography

Answer 11

Stationary phase is a powder coated with oil. This is either packed into or coated onto the inside of a capillary tube (very long and thin and coiled up). Capillary tube is in oven whose temperature can be varied. Mobile phase is usually an unreactive gas (eg He or N2).

Question 12

How does Gas-liquid chromatography work?

Answer 12

After injection, the sample is carried along by the gas and the mixture separates as some components move with the gas and some are retained by the oil, each to a different degree. Components therefore leave column at different times after injection (have different retention times). Results may be presented on a graph where the area under each peak is proportional to the amount of that component.

Question 13

Advantage of Gas-liquid chromatography

Answer 13

Is extremely sensitive so can separate minute traces of substances.

Question 14

How does GCMS work?

Answer 14

Stands for gas chromatography-mass spectrometry. Mass spectrometer used as detector for gas chromatography system. As each component comes out of the GC column, it’s retention time is noted. Each component is fed into a mass spectrometer so the compound can be identified by either its fragmentation pattern or by measuring its accurate mass.

Question 15

How does general NMR work?

Answer 15

A magnetic field is applied to the sample which is surrounded by a source of radio waves and a radio receiver. This generates an energy change in the nuclei of atoms in the sample that can be detected. Electromagnetic energy is emitted which can be interpreted by a computer.

Question 16

Describe tetramethylsilane (TMS) and how it is used.

Answer 16

It is a silicon atom bonded to 4 methyl groups. The chemical shift of each of the hydrogens is 0 by definition and that are used as the standard that all other chemical shifts are measured by reference to. It is inert, non-toxic and easy to remove from a sample. A little TMS can be added to the sample to calibrate the spectrum.

Question 17

What is chemical shift?

Answer 17

It is related to the difference in frequency between the resonating nucleus and that of TMS. Measured in parts per million from a defined 0 related to TMS.

Question 18

Why does chemical shift of different atoms of the same element occur?

Answer 18

All nuclei are shielded from the external magnetic field by the electrons around them. Nuclei with more electrons around them are better shielded so feel a smaller magnetic field and have a lower resonant frequency. Lower resonant frequency means less chemical shift. The amount of shielding depends on the environment the atom is in.

Question 19

Explain how an atom being bonded to a more electronegative atom can affect its chemical shift

Answer 19

A more electronegative atom draws electrons away from the atoms it is bonded to. The atom (C or H) is deshielded and feels a greater magnetic field so resonates at a higher frequency so has a greater chemical shift.

Question 20

Which way does chemical shift increase on an NMR spectrum?

Answer 20

Right to left.

Question 21

What is the area under the peaks for proton NMR proportional to?

Answer 21

The number of hydrogens in the environment that corresponds to that peak.

Question 22

How does the integration trace on a proton NMR spectrum work?

Answer 22

The relative heights of the steps of this trace give the relative number of each type of hydrogen. Appears as a line.

Question 23

What is spin-spin coupling (spin-spin splitting) and how does it work?

Answer 23

The peaks can be split into particular patterns. The applied magnetic field felt by any hydrogen atom is affected by the magnetic field of the hydrogen atoms on the neighbouring carbon atoms.

Question 24

What is the n+1 rule?

Answer 24

n hydrogens on an adjacent carbon atom will split a peak into n+1 smaller peaks. Singlet for n=0. Duplet for n=1 (height 1:1). Triplet for n=2 (1:2:1). Quartet for n=3 (1:3:3:1). Multiplet for n>3.

Question 25

Criteria for solvent for proton NMR

Answer 25

Must not contain any hydrogen atoms otherwise the signal from the hydrogen atoms in the solution would swamp the signals from the hydrogen atoms in the sample because there are vastly more of them.

Question 26

How much carbon is C-13?

Answer 26

About 1%

Question 27

Why doesn’t NMR work with C-12?

Answer 27

It has no nuclear spin

Question 28

What does the height and area under peaks in carbon-13 NMR tell you?

Answer 28

Absolutely nothing.

Question 29

Does splitting occur in carbon NMR?

Answer 29

No

Question 30

Solvents for NMR

Answer 30

Deuterated solvents (using deuterium) or CCl4

Question 31

Essential things when doing TLC

Answer 31

Wear plastic gloves tonprevent contamination from hands to plate. Developing solvent is added to a depth below the pencil line otherwise it would dissolve the mixture from the plate. If the solvent is toxic, the chromatography is carried out in a chamber with a lid and the plate is dried in a fume cupboard.