chromatography + qualitative analysis + NMR

Question 1

what is the purpose of chromatography

Answer 1

used to separate individual components from a mixture of substances

Question 2

2 components of chromatography

Answer 2

stationary phase and mobile phase

Question 3

what are the stationary and mobile phases in TLC

Answer 3

stationary - silica gel (SiO2) / TLC plate
mobile phase - solvent

Question 4

what are some uses of chromatography

Answer 4

analysis of drugs, flavourings, plastics, air samples, forensic science

Question 5

what does TLC stand for

Answer 5

thin layer chromatography

Question 6

in TLC how does separation occur

Answer 6

by relative adsorption - different components in the mixture adsorb differently to the surface of the stationary phase

Question 7

adsorption definition

Answer 7

how much interaction components have with the stationary phase - higher adsorption = lower Rf

Question 8

Rf equation

Answer 8

distance travelled by component / distance travelled by solvent -

Question 9

what are 4 limitations of TLC

Answer 9

• difficult to measure the exact middle of the component spot
• similar compounds have similar Rf values, can be difficult to tell them apart or spots may be overlapping
• difficult to find a solvent that dissolves all components in a sample
• there may not be a reference chromatogram that exists in the database e.g. for an unknown compound

Question 10

what is the purpose of gas chromatography

Answer 10

used to separate volatile compounds in a mixture - it is also useful for analysing organic compounds with low boiling points

Question 11

volatility definition

Answer 11

the tendency of a compound to vaporise / turn into a gas

Question 12

what are the stationary and mobile phases in gas chromatography

Answer 12

stationary - high boiling point liquid adsorbed onto inert solid support
mobile - inert carrier gas e.g. a noble gas

Question 13

how are substances separated in gas chromatography

Answer 13

by relative solubility

Question 14

retention time definition

Answer 14

time taken for a compound takes to travel trhough the column

Question 15

how are components represented on gas chromatograms

Answer 15

by peaks

Question 16

how are gas chromatograms interpreted

Answer 16

retention times can be used to identify compounds by comparing them to retention times for known components

peak integration can be used to determine the concentration of components - area of the peaks is proportional to the amount of the component in the sample

Question 17

formula for % of a component in the mixture

Answer 17

( area of peak / total area of all peaks ) x 100

Question 18

what are 3 limitations of gas chromatography

Answer 18

• compounds may have the same retention times
• small amount of one component may hide behind a component with a higher concentration
• unknown components will not have any retention time data to compare

Question 19

outline a chemical test to identify an alkene

Answer 19

react with bromine water
POSITIVE = decolourises

Question 20

outline a chemical test to identify a haloalkane

Answer 20

react with AgNO3(aq)
POSITIVE = a coloured precipitate forms
Cl - white precipitate
Br - cream precipitate
I - yellow precipitate

then dissolve in NH3
Cl - dissolves in dilute NH3
Br - dissolves in conc NH3
I - insoluble in NH3

Question 21

outline a chemical test to identify a carbonyl group

Answer 21

react with 2,4-DNP
POSITIVE = orange-yellow solution becomes an orange precipitate

Question 22

outline a chemical test to identify an aldehyde

Answer 22

react with tollens reagent
POSITIVE = silver mirror forms

Question 23

outline 2 chemical tests to identify a phenol

Answer 23

react with bromine water
POSITIVE = decolourises + white precipitate 2,4,6-tribromophenol forms

react with Na
POSITIVE = fizzing + effervescence will occur

Question 24

outline a chemical test for a carboxylic acid

Answer 24

react with NaCO3 then bubble this gas through limewater
POSITIVE = effervescence + gas turns limewater cloudy

Question 25

outline a chemical test for an acyl chloride

Answer 25

react with water POSITIVE = white fumes of HCl given off

Question 26

what does NMR stand for

Answer 26

nuclear magnetic resonance

Question 27

how does NMR work

Answer 27

- it uses a combination of very strong magnetic fielf strength and radio frequency radiation - with the right combination of the two, the nuclei of some atoms can absorb this radiation, and the energy for absorption can be measured and recorded as an NMR spectrum

Question 28

what are the 2 types of NMR

Answer 28

carbon 13 NMR and proton NMR

Question 29

nuclear spin definition

Answer 29

the nucleus has 2 different spin states with different energies this is significant for particles with an odd number of particles in the nucleus - this is why 13C isotope is used

Question 30

resonance definition

Answer 30

with the right combination of magnetic field and radio frequency radiation the nucleus can absorb energy and rapidly flip between the 2 states - this is resonance

Question 31

what reference chemical is used in NMR + why

Answer 31

TMS - it is given a shift value of 0ppm, so all shift values are relative to TMS - it is chemically unreactive - it has 12 equivalent protons and 1 carbon environment, giving a sharp easily measured peak in 13C and H NMR - nuclei are heavily shielded so rarely peaks below it - volatile so easily removed from the sample after

Question 32

what is the structure of TMS

Answer 32

an Si atom bonded to 4 methyl CH3 groups in a tetrahedral shape

Question 33

deuterated solvents definition + an example

Answer 33

these are solvents where 1H atoms are replaced with 2H atoms - 2H is known as deuterium e.g. CDCl3

Question 34

why are deuterated solvents used

Answer 34

solvents containing C or H atoms cannot be used in NMR as they would produce signals themselves deuterated solvents produce no NMR signal in frequency ranges in H NMR and could appear at specific points in 13C NMR, so it can be ignored

Question 35

what information can be obtained from a 13C NMR spectrum

Answer 35

number of carbon environments - number of peaks types of carbon environments - chemical shift

Question 36

what information can be obtained from a H NMR spectrum

Answer 36

number of proton environments - number of peaks types of proton environments - chemical shift number of neighbouring protons - splitting

Question 37

what peaks are hardest to detect on H NMR spectra + what is often added to make it easier

Answer 37

O-H or N-H peaks this is because they can appear over a wide range of shift values D2O is a deuterated solvent which will cause any peaks caused y O-H or N-H to be removed from the spectra, which can then be used to form a second spectra to compare to the original and determine if any OH or NH groups are present