topic 19: spectroscopy and chromatography

Question 1

what are the different types radiation in a molecule

Answer 1

infrared in analysis
microwaves for heating
radiowaves in NMR
ultraviolet in initiation of reactions

Question 2

when does the NMR spectroscopy involve

Answer 2

involves interaction of materials with the low- energy radiowave region of the electromagnetic spectrum

Question 3

what so the radiowaves used in the proton NMR cause

Answer 3

causes the hydrogen nucleus to change its spin state

Question 4

NMR use the same technology as what

Answer 4

magnetic resonance imaging (MRI)

Question 5

in an hydrogen NMR spectrum what does one signal represent

Answer 5

each set of equivalent H atoms
eg. ethanol has 3
groups of different
hydrogen atoms

Question 6

in H NMR, what is the intensity (integration value) of each signal proportional to

Answer 6

the number of the equivalent H atoms it represents

Question 7

in H NMR, what are the solvents used

Answer 7

solvents without any 1H atoms
eg. CCl4, CDCl3

Question 8

the solvent used dont have 1H atoms what does this mean

Answer 8

this means that in the H NMR the solvent will not give any peaks

Question 9

state the use of CCl4 as a solvent

Answer 9

a non polar compound compound that is a good solvent for non-polar organic molecules

Question 10

what the use of CDCl3

Answer 10

a polar covalent molecule that is a good solvent for polar organic molecules

Question 11

a small amount of what solution is added to the sample to calibrate the spectrum

Answer 11

TMS (tetramethylsilane)

Question 12

why is tetramethylsilane (TMS) used

Answer 12

*its signal is away from all the others
*it only gives one signal *gives strong signal so only a small amount needed
*it is non-toxic
*it is inert
*it has a low boiling point and so can be removed
from sample easily

Question 13

the spectra is recorded on what scale and what does it do

Answer 13

the chemical
shift (δ), which is how much the field has shifted away from the
field for TMS

Question 14

what is δ a measure in

Answer 14

parts per million (ppm)

Question 15

what is the δ a scale of

Answer 15

a relative scale of how far the frequency of the proton signal has shifted away from that for TMS

Question 16

what does the δ depend on

Answer 16

what other atoms/groups are near the
H – more electronegative groups gives a greater shift

Question 17

splitting of peaks

Answer 17

= number of inequivalent H’s on neighbouring C atoms + 1

Question 18

describe singlet

Answer 18

• 1 splitting peak
• 0 neighbouring inequivalent H atoms

Question 19

describe doublet

Answer 19

• 2 splitting peaks
• 1 neighbouring inequivalent H atoms
• 1:1 —> relative size

Question 20

describe triplet

Answer 20

• 3 splitting peaks
• 2 neighbouring inequivalent H atoms
• 1:2:1 —> relative size

Question 21

describe quartet

Answer 21

• 4 splitting peaks
• 3 neighbouring inequivalent H atoms
• 1:3:3:1 —> relative size

Question 22

describe quintet

Answer 22

• 5 splitting peaks
• 4 neighbouring inequivalent H atoms
• 1:4:6:4:1 —> relative size

Question 23

what is it called when there is more than 5 splitting peaks

Answer 23

multiplet

Question 24

what is chromatography

Answer 24

an analytical technique that separates
components in a mixture between a mobile phase and a stationary phase

Question 25

what does separation by column chromatography depend on

Answer 25

the balance between solubility in the moving phase and retention in the stationary phase

Question 26

how does solid stationary phase separate by

Answer 26

adsorption

Question 27

how does liquid stationary phase separate by

Answer 27

relative solubility

Question 28

what is the state of the mobile phase

Answer 28

liquid or gas

Question 29

what is the state of the stationary phase

Answer 29

- may be a solid (as in thin layer chromatography, TLC) - a liquid or solid on a solid support (as in gas chromatography, GC)

Question 30

if the stationary phase was polar......

Answer 30

the moving phase was non- polar - e.g. hexane then non- polar compounds would pass through the column more quickly than polar compounds as they would have a greater solubility in the non-polar moving phase

Question 31

what does HPLC stand for

Answer 31

high performance liquid chromatography

Question 32

what is the stationary phase of HPLC

Answer 32

a solid silica

Question 33

what is the mobile phase of HPLC

Answer 33

a liquid

Question 34

what is the mobile phase in gas - liquid chromatography

Answer 34

a inert gas such as nitrogen, helium, argon

Question 35

what is the stationary phase in gas - liquid chromatography

Answer 35

a liquid on an inert solid

Question 36

what is gas - liquid chromatography be used for

Answer 36

be used to separate mixtures of volatile liquids

Question 37

what is retention time

Answer 37

The time taken for a particular compound to travel from the injection of the sample to where it leaves the column to the detector

Question 38

what is GC-MS used for

Answer 38

in analysis, in forensics, environmental analysis, airport security and space probes

Question 39

if gas liquid chromatography is added to IR or NMR machine what does it enable

Answer 39

enabling all the components in a mixture to be identified

Question 40

what is the method of TLC

Answer 40

a) wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line b) use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry c) add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth d) place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. replace the lid to get a tight seal e) when the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. allow the plate to dry in the fume cupboard f) place the plate under a UV lamp in order to see the spots. draw around them lightly in pencil g) calculate the Rf values of the observed spots

Question 41

what are the precautions of using TLC

Answer 41

- wear plastic gloves to prevent contamination from the hands to the plate - pencil line –will not dissolve in the solvent - tiny drop – too big a drop will cause different spots to merge - depth of solvent– if the solvent is too deep it will dissolve the sample spots from the plate - lid– to prevent evaporation of toxic solvent - will get more accurate results if the solvent is allowed to rise to near the top of the plate but the Rf value can be calculated if the solvent front does not reach the top of the plate - dry in a fume cupboard as the solvent is toxic - UV lamp used if the spots are colourless and not visible

Question 42

if using amino acids, what can be used instead of a UV lamp

Answer 42

ninhydrin spray to locate spots

Question 43

equation of Rf

Answer 43

= distance moved by amino acid ---------------------------------------------- distance moved by the solvent

Question 44

The dipeptide formed is hydrolysed under acidic conditions and the resulting mixture is analysed by column chromatography. the column uses a polar stationary phase. explain why lysine leaves the chromatography column after alanine

Answer 44

lysine ion has 2 positive charges whereas alanine has 1 so lysine has greater attraction for the stationary phase

Question 45

explain why different compounds will have different retention times in the same column, under the same conditions

Answer 45

- depends on the solubility of the component of the stationary phase. the greater the solubility of the component of the stationary phase, the greater the retention time - depends on the boiling temp of the compounds. high boiling temp compounds spend less time in the gas phase/ mobile phase so have longer retention time

Question 46

in the tripeptide, giving a reason for the lack of a third spot on the TLC

Answer 46

one amino acid is present twice OR another amino acid has the same Rf value as the other amino acids

Question 47

give two reasons why different amino acids have different Rf values

Answer 47

amino acids have different solubility to the stationary phase amino acids have different solubility in the mobile phase