Further Synthesis and Analysis

Question 1

What do chemists do to make synthesis routes safe and efficient?

Answer 1

• They use non-hazardous starting materials to limit the potential for accidents and environmental damage
• They try to make the process less wasteful by doing reactions with high atom economies and percentage yields.
• They also try to reduce waste by designing synthesis routes that have the fewest steps as possible

Question 1

What is the importance of synthesis routes?

Answer 1

It allows chemists to make desired compounds from another. It is useful for things like designing medicines and making imitations of natural substances when the real thing is hard to extract.

Question 2

What are the dangers/wastes of solvents in synthesis routes?

Answer 2

Avoiding using solvents wherever possible is one way of reducing both the hazards associated with a process and the amount of waste created by a synthesis route. Solvents are often flammable and toxic so can pose safety risks. If the solvent has to be disposed of after the reaction is complete, that can also create a lot of waste.

Question 3

What are the two types of NMR?

Answer 3

Carbon 13 NMR gives you information about how the carbon atoms in a molecule are arranged, and proton NMR tells you how the hydrogen atoms in a molecule are arranged.

Question 4

What causes a nuclear spin and why is it important in NMR?

Answer 4

Any atomic nucleus with an off number of nucleons in its nucleus has a nuclear spin. This causes it to have a weak magnetic field- a bit like a bar magnet. NMR spectroscopy looks at how this tiny magnetic field reacts when you put in a much larger external magnetic field.

Question 5

What happens to nuclei when exposed to a strong external magnetic field?

Answer 5

They will all align either with the field or against it.
The nuclei aligned with the magnetic field are at a slightly lower energy level than the opposed nuclei

Question 6

What do radio waves do to nuclei?

Answer 6

Radio waves of the right frequency can give the nuclei that are aligned with the external magnetic field enough energy to flip up to a higher energy level. The nuclei opposed to the ecternal field can emit radio waves and flip down to the lower energy level.

Question 7

Why will nuclei in different environments absorb different amounts of energy at different frequencies?

Answer 7

Nuclei are partially shielded from the effects of external magnetic fields by their surrounding electrons and the groups of atoms around a nucleus will affect its electron shielding.

Question 8

What is the reference molecule in NMR?

Answer 8

Tetramethylsilane (TMS)
It has 12 hydrogen atoms in identical environments, so it produces a single absorption peak, well away from most other absorption peaks.

Question 9

What do the peaks in carbon NMR mean?

Answer 9

Each peak on the spectrum corresponds to a different carbon environment

Question 10

What do the peaks in proton NMR mean?

Answer 10

Each peak on the spectrum corresponds to a different hydrogen environment

Question 11

What do the numbers above the peaks in proton NMR mean?

Answer 11

They tell you the ratio of the areas under the peaks. This relative area under each peak also tells you the relative number of H atoms in each environment.

Question 12

How do you work out splitting patterns in proton NMR?

Answer 12

Count the number of H atoms on neighboring carbons + 1
singlet
doublet
triplet
quartet
quintet

Question 13

Why must samples be dissolved in hydrogen free solvents in NMR?

Answer 13

• the H atoms would show up on the spectrum as they have a spin state
• instead should use deuterated solvents
• CCl4 can also be used as a solvent

Question 14

What is chromatography?

Answer 14

It is a process used to separate stuff in a mixture and then identify it.

Question 15

What is the basic principle of chromatography?

Answer 15

• the mobile phase moves through or over the stationary phase
• the distance each substance moves up the plate depends on its solubility in the mobile phase and its retention by the stationary phase
• components that are more soluble in the mobile phase will travel further up the plate
• it’s these differences in solubiltiy and retention by the stationary phase that separate out the different substances

Question 16

What is the mobile phase and stationary phase in chromatography?

Answer 16

Mobile phase - Where the molecules can move, this is always a liquid or gas
Stationary phase - where the molecules cannot move, this must be a solid, or a liquid on a solid support

Question 17

What is the stationary phase in TLC?

Answer 17

The stationary phase is a thin layer of silica or alumina fixed to a glass or an aluminium metal plate

Question 18

How do you carry out TLC?

Answer 18

• Draw a line in pencil near the bottom of the TLC plate and put a very small drop of each mixture to be separated on the line
• allow the spots on the plate to dry
• place the plate in a beaker with a small volume of solvent. The solvent level must be belwo the baseline so it doesn’t dissolve your samples away
• the solvent will start to move up the plate. As it moves, the solvent carries the subtances in the mixture with it/
• leave the beaker until the solvent has moved almost to the top of the plate. Then remove the plate from the beaker. before it evaporates, use a pencil to mark how far the solvent has traveled up the plate
• place the plate in a fume cupboard to let it dry
• the result is a chromatogram

Question 19

How do you use iodine vapor to make TLC plates show colorless chemicals?

Answer 19

Expose, the chromatogram to iodine vapor. Iodine vapor is a locating agent- it sticks to the chemicals on the plate and they’ll show up as brown/purple spots.

Question 19

How do you use UV light to make TLC plates show colourless chemicals?

Answer 19

Many TLC plates have a special fluorescent dye added to the silica or alumina layer that glows when UV light shines on it. You can put the plate under a UV lamp and draw around the dark patches to show where the spots of chemical area.

Question 19

What are rf values?

Answer 19

They identify a solvent on a chromatogram based on its solubility in a solvent and can be looked up on a database of standard rf values.

Question 20

What factors affect rf values and how do you mitigate this in a chromatography experiment?

Answer 20

• the composition of the TLC plate
• the solvent
• the temperature
  Run the plate with spots of known substances that you suspect to be in the unknown so they are all ran in the same conditions at the same time.

Question 21

What is column chromatography used for?

Answer 21

It is mostly used for purifiying an organic product

Question 22

How is column chromatography used?

Answer 22

- It involves packing a glass column with a slurry of an absorbent material such as aluminum oxide, coated with water. This is the stationary phase. - The mixture to be separated is added to the top through the column and allowed to drain down into the slurry. A solvent is then run slowly and continually through the column. This solvent is the mobile phase. - As the mixture is washed through the column, its components separate out according to how soluble they are in the mobile phase and how stronly they are adsorbed onto the stationary phase - Each different component will spend some time adsorbed onto the stationary phase and some time dissolved in the mobile phase. The more soluble each compound is in the mobile phase, the quicker it will pass through the column.

Question 23

How is gas chromatography used?

Answer 23

- If you have a mixture of volatile liquids then gas chromatography is the best way to separate them out so that you can identify them - The stationary phase is a solid or a solid coated by a viscous liquid, such as oil packed into a long tube. - The tube is coiled to save space and built into an oven. The mobile phase in an unreactive carrier gas such as nitrogen - Each component takes a different amount of time from being injected into the tube to be recorded at the other end. - Each separate substance will have a unique retention time, so they can use the retention time to identify the components of the mixture. - The area under each peak tells you the relative amount of each component that is present in the mixture.

Question 24

What are some of the uses of gas chromatography?

Answer 24

GC can be used to find the level of alcohol in blood or urine, the results are accurate enough to be used as evidence in court. It is also used to find the proportions of various esters in oils used in paints this lets picture restorers know exactly what paint was originally used.

Question 25

What is Gas chromatography mass spectrometry?

Answer 25

Gas chromatography-mass spectrometry combines the benefits of gas chromatography and mass spectrometry to make a super analysis tool. The sample is separated using gas chromatography, but instead of going to a detector, the separated components are fed into a mass spectrometer.

Question 26

What are the advantages of GC/MC?

Answer 26

The advantage of this method over normal GC is that the components separated out by the chomatography can be positively identified, which can be impossible from a chromatogram alone. Computers can be used to match up the mass spectrum for each component of the mixture against a database, so the whole process can be automated.