Chapter 29 Key Concepts

Question 1

Chromatography

Answer 1

Used to separate individual components from a mixture of substances. All forms have a stationary phase and a mobile phase

Question 2

What is chromatography used for?

Answer 2

Analysis of 
drugs
Plastics
Flavourings 
Air samples 
And has applications in forensic science

Question 3

Theory behind TLC?

Answer 3

Quick and inexpensive
Tells you how many components are in a mixture. For this method you use a TLC plate that is usually a plastic sheet or glass coaxed in a thin layer of a solid adsorbent substance
*Usually SILICA

Adsorbent is the STATIONARY PHASE
Different components have different affinities for the adsorbent and bind with different strengths to its surface.
Separation happens because of the relative adsorption

Often run the TLC of sample alongside pure samples of what might be present so it’s easier to identify the unknown sample visually without calculating any Rf values

Question 4

How to identify the component using the RF value

Answer 4

Rf=distance moved by the component/distance moved by solvent front

Compare the Rf value with known values recorded using the same solvent system and adsorbent

Question 5

What’s gas chromatography used for?

Answer 5

Useful for separating and identifying volatile organic compounds present in a mixture

Question 6

Describe gas chromatography

Stationary phase
Mobile phase

Answer 6

Stationary phase
-high boiling liquid absorbed onto an inert solid support

Mobile phase
-inert carrier gas like Helium or Neon

Question 7

Describe the stages of the gas chromatograph

Answer 7

1. small amount of volatile mixture is injected into the apparatus.
   * APPARATUS IS CALLED GAS CHROMATOGRAPH
2. Mobile carrier gas carries the components in the sample through a capillary column
   * COLUMN CONTAINS THE LIQUID STATIONARY PHASE ABSORBED TO THE SOLID SUPPORT
3. Components slow down as they interact with the liquid stationary phase
   * THE MORE SOLUBLE THE COMPONENT IS IN THE LIQUID STATIONARY PHASE THE SLOWER IT MOVED THROUGH THE CAPILLARY COLOMN
4. Compound reaches the detector at different times depending on interaction with the stationary phase

Question 8

How does separation occur in gas chromatography

Answer 8

Components of the mixture are separated depending on their solubility in the liquid stationary phase.

Question 9

Two things we can get from the gas chromatogram

Answer 9

each component is detected as a peak

• by comparing with known RF values, you can identify the components present
• peak integrations/peak area can be used to determine the concentrations if the components

Question 10

How do you work out the concentration of a component?

Answer 10

Basically Compare the peak area/peak integration with values obtained from the standard solutions of the component

1. prepare standard solutions of known concentrations of the compounds being investigated
2. get a gas chromatogram for each of the standard solutions
3. plot a calibration curve of peak area against concentration in EXTERNAL CALIBRATION
4. get a gas chrom of the compound being investigated under the same conditions
5. Use the calibration curve to measure the concentration of the compound.

Question 11

Identifying alkenes?

Answer 11

Add bromine water dropwise

Decolorises from orange to colourless

Question 12

Identifying haloalkanes

Answer 12

Add silver nitrate and ethanol and warm to to 5O degrees

Chloroalkane white ppt
Bromoalkane cream ppt
Iodoalkane yellow ppt

Question 13

Identifying carbonyl group

Answer 13

2,4-DNP

Orange ppt

Question 14

Identifying aldehyde

Answer 14

Add tollens reagent and warm

Silver mirror

Or add acidified potassium dichromate. Colour change orange to green

Question 15

Identifying primary or secondary alcohol

Answer 15

Add K2Cr2SO4

Colour change from orange to green

Question 16

Identifying Carboxylic Acids

Answer 16

Add aq sodium carbonate

You see effervescence

Question 17

How does NMR work

Answer 17

Uses a the right combo of very strong magnetic field and radio frequency radiation which some atoms then in turn absorb. The energy for the absorption can be measured and recorded as an NMR spectrum

Question 18

When is nuclear spin significant?

Answer 18

When there’s a an odd number of NUCLEONS

Question 19

How do the electrons surrounding the nuclei of atoms affect the NMR

Answer 19

Electrons surrounding the nucleus of an atom shift the energy and radio frequency needed for NMR to occur.

Frequency shift is measured on a scale called chemical shift.(delta) in ppm parts per million.

Amount of shift is determined by chemical environment.
*especially the presence of electronegative atoms

Question 20

What is TMS and what is it used for

Answer 20

TMS is tetramethylsilane (CH3)4Si is the standard reference chemical against which all chemical shifts are measured on a scale called chemical shift. It’s chemical shift value is 0ppm

Question 21

How does chemical environment affect absorption peaks

Answer 21

Depending on chemical environment, NMR requires a different energy and frequency making absorption peaks at chemical shifts.

Question 22

Pros of NMR

Answer 22

Allows CH arrangement in a molecule to be mapped out without having to carry out conventional chemical tests

Doesn’t destroy the organic compound that you’re testing

Question 23

Describe the steps of the NMR technique

Answer 23

1. Sample dissolved in solvent and placed in a narrow NMR sample tube with a small amount of TMS
2. place tube inside the NMR spec where it is spun to even out imperfections
3. Zero the spec against the TMS standard and give the sample a pulse of radiation containing the range of radio frequencies whilst maintaining a constant magnetic field
4. any absorptions from energy resulting from resonance are detected and displayed on a computer screen
   * can recover sample by evaporating the solvent

Question 24

Why is Deuterium used

Answer 24

Solvents containing deuterium isotopes eg CDCl3 are used as they don’t make signals in the frequency ranges used in H1 and C13 NMR cuz DEUTERIUM HAS AN EVEN NUMBER OF NUCLEONS

Question 25

What does C 13 NMR tell you about a molecule?

Answer 25

- the number of different carbon environments which is the number of peaks - the chemical shift tells you the type of carbon environment present

Question 26

Describe how the position of the atom within the molecule determines the chemical environment of a carbon atom

Answer 26

Carbon attached to different atoms or group of atoms have different environments and therefore WILL ABSORB AT DIFFERENT CHEMICAL SHIFTS two Carbons positioned symmetrically within a molecule are EQUIVALENT and will ABSORB AT THE SAME CHEMICAL SHIFT contributing to the same peak

Question 27

What does proton NMR tell us

Answer 27

-number of peaks tells us the number of different proton environments -chemical shift tells us the types of proton environments present just like CNMR BUT PNMR also tells us -relative numbers of each type of protons from integration traces or ratio numbers of the relative peak areas - spin spin splitting pattern gives us the number of non equivalent protons adjacent to a given proton * chemical shift range of 12ppm but for CNMR its 200ppm

Question 28

Factors that change the range of the chemical shift

Answer 28

Solvents Concentration Substituents

Question 29

What is proton exchange ? +think of spot the difference

Answer 29

Used to identify OH and NH -run the pNMR -add a small volume of deuterium oxide, shake and run a second spectrum . -The deuterium exchanges and replaces OH and NH protons in the sample with the deut and sets up an equilibrium eg with methanol CH3OH + D2O <==> CH3OD + HOD Deut nah absorb in this chemical shift range so OH peak disappears

Question 30

Structure determination for identification steps

Answer 30

Elemental analysis -Use percentage composition to work out the empirical formula Mass spectra -work out molecular formula Infrared spec -use absorption peaks to work out functional group NMR -work out the order of atoms

Question 31

A small amount of TMS

Answer 31

(tetramethylsilane) is added to the sample to calibrate the spectrum The same calibration compound is used for both H and C NMR TMS is used because: •its signal is away from all the others •it only gives one signal •it is non-toxic •it is inert •it has a low boiling point and so can be removed from sample easily