29- chromatography and spectroscopy

Question 1

What is thin layer chromatography?

Answer 1

Quick and inexpensive technique that indicates how many components are in a mixture. The techniques use a TLC plate which are usually a plastic sheet or glass, coated in a thin layer of a solid adsorbent substance- usually silica.

Question 2

How does thin layer chromatography work?

Answer 2

the different components in the mixture have different affinities for the adsorbent and binds with differing strengths. Separation is achieved by the relative adsorptions of substances with the stationary phase.

Question 3

How do you carry out TLC?

Answer 3

1. Draw a pencil line across the plate about 1cm from the bottom.
2. Use a capillary tube to spot a small amount of solution of the sample onto the base line on the plate
3. Prepare a chromatography tank for the TLC plate. This can be made from a small beaker with a watch glass over the top. Put some solvent into the beaker with a depth of about 0.5cm
4. Place TLC plate in the beaker, making sure the solvent doesn’t cover the spot. Cover with watch glass and leave it undisturbed.
5. Allow solvent to rise up plate until it is about 1cm below the top of the plate, remove, and mark solvent front with a pencil.
6. Circle visible spots with pencil or hold UV lamp over the plate and circle the spots you can see.

Question 4

How does gas chromatography work?

Answer 4

Stationary phase is a high bp liquid adsorbed onto an inert solid support. The mobile carrier gas carries components of the volatile mixture through the capillary column which contains the stationary phase.
The components slow down as they interact with the liquid stationary phase inside the column. The more soluble the component is in the liquid stationary phase, the slower it moves through the capillary column.
Components are separated depending on their solubility in the liquid stationary phase. The compounds in the mixture reach the detector at different times depending on their interactions with the stationary phase. The compound retained in the column for the shortest time has the lowest retention time and it is detected first. The retention time is the time taken for each component to travel through the column.

Question 5

What information can be gained from a gas chromatogram?

Answer 5

Retention times can be used to identify the components present in the sample by comparing these retention times for known components
Peak integrations can be used to determine the concentrations of components in the sample.

Question 6

How do you determine the concentrations of a component within a sample from gas chromatography?

Answer 6

Comparing it’s peak integration with values obtained from standard solutions of the component
1. prepare standard solutions of known concs of the compound being investigated
2. obtain the gas chromatogram of each standard solution
3. plot a calibration curve of peak area against conc - external calibration. It offers a method for converting a peak area into a conc.
4. obtain a gas chromatogram of the compound being investigated under the same conditions
5. use the calibration curve to measure the concs of the compound.

Question 7

What is the test for alkenes and what observation is made?

Answer 7

Add bromine water drop wise. Bromine water decolourises from orange to colourless

Question 8

What is the test for haloalkanes and what observations are made?

Answer 8

Add silver nitrate and ethanol and warm to 50 degrees in a water bath
Chloroalkane - white precipitate
bromoalkane- cream precipitate
idoalkane - yellow precipitate

Question 9

How do you test for a carbonyl functional group? What observations would be made?

Answer 9

add 2,4-dinitrophenylhydrazine. Forms an orange precipitate

Question 10

what is the test for the aldehyde functional group and what observations would be made?

Answer 10

Add Tollen’s reagent and warm. A silver mirror will be formed

Question 11

What is the test for a primary and secondary alcohol and an aldehyde and what observations would be made?

Answer 11

Add acidified potassium dichromate and warm in a water bath. Colour change from orange to green.

Question 12

What is the test for a carboxylic acid and what observations would be made?

Answer 12

add aqueous sodium carbonate. Effervescence

Question 13

What is a deuterated solvent?

Answer 13

Usually used in which the H1 atoms have been replaced by H2 atoms- deuterium. It produces no NMR signal in the frequency ranges used in H1 and C13 spectroscopy.

Question 14

What are the 2 main things that determine the peaks made in C13 NMR spectroscopy?

Answer 14

• Carbon atoms that are bonded to different atoms or groups of atoms, have different environments and will absorb at different chemical shifts.
• If two carbon atoms are positioned symmetrically within a molecule, then they are equivalent and have the same chemical environment. They will then absorb radiation at the same chemical shift and contribute to the same peak.

Question 15

What things can be interpreted from a proton NMR spectrum?

Answer 15

• The number of different proton environments from the number of peaks
• The types of proton environments from the chemical shift
• The relative numbers of each type of proton- from integration traces or ratio numbers of the relative peak areas.
• The number of non-equivalent protons adjacent to a given proton from the spin-spin splitting pattern.

Question 16

What is the n+1 rule?

Answer 16

the splitting of a main peak into sub peaks in the spin-spin coupling and the number of sub peaks is one greater than the number of adjacent protons on the next carbon.

Question 17

What is proton exchange?

Answer 17

It is used to identify -OH and -NH protons;
1. A proton NMR spectrum is run as normal.
2. A small volume of deuterium oxide is added, the mixture is shaken and a second spectrum is run.
Deuterium exchanges and replaces the OH and NH protons in the sample with deuterium atoms.
The peak will then disappear on the spectrum.

Question 18

What is the order of analysis you use for combined techniques to identify a compound?

Answer 18

-Use percentage composition by mass to determine the empirical formula of a compound.
- Use molecular ion peak from mass spectra
- infrared spectra- bonds present and functional groups
- NMR spectra - to determine the number and types of carbon and hydrogen atoms