chromatography + qualitative analysis + NMR Flashcards
what is the purpose of chromatography
used to separate individual components from a mixture of substances
2 components of chromatography
stationary phase and mobile phase
what are the stationary and mobile phases in TLC
stationary - silica gel (SiO2) / TLC plate
mobile phase - solvent
what are some uses of chromatography
analysis of drugs, flavourings, plastics, air samples, forensic science
what does TLC stand for
thin layer chromatography
in TLC how does separation occur
by relative adsorption - different components in the mixture adsorb differently to the surface of the stationary phase
adsorption definition
how much interaction components have with the stationary phase - higher adsorption = lower Rf
Rf equation
distance travelled by component / distance travelled by solvent -
what are 4 limitations of TLC
- 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
what is the purpose of gas chromatography
used to separate volatile compounds in a mixture - it is also useful for analysing organic compounds with low boiling points
volatility definition
the tendency of a compound to vaporise / turn into a gas
what are the stationary and mobile phases in gas chromatography
stationary - high boiling point liquid adsorbed onto inert solid support
mobile - inert carrier gas e.g. a noble gas
how are substances separated in gas chromatography
by relative solubility
retention time definition
time taken for a compound takes to travel trhough the column
how are components represented on gas chromatograms
by peaks
how are gas chromatograms interpreted
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
formula for % of a component in the mixture
( area of peak / total area of all peaks ) x 100
what are 3 limitations of gas chromatography
- 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
outline a chemical test to identify an alkene
react with bromine water
POSITIVE = decolourises
outline a chemical test to identify a haloalkane
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
outline a chemical test to identify a carbonyl group
react with 2,4-DNP
POSITIVE = orange-yellow solution becomes an orange precipitate
outline a chemical test to identify an aldehyde
react with tollens reagent
POSITIVE = silver mirror forms
outline 2 chemical tests to identify a phenol
react with bromine water
POSITIVE = decolourises + white precipitate 2,4,6-tribromophenol forms
react with Na
POSITIVE = fizzing + effervescence will occur
outline a chemical test for a carboxylic acid
react with NaCO3 then bubble this gas through limewater
POSITIVE = effervescence + gas turns limewater cloudy
outline a chemical test for an acyl chloride
react with water
POSITIVE = white fumes of HCl given off
what does NMR stand for
nuclear magnetic resonance
how does NMR work
- 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
what are the 2 types of NMR
carbon 13 NMR and proton NMR
nuclear spin definition
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
resonance definition
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
what reference chemical is used in NMR + why
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
what is the structure of TMS
an Si atom bonded to 4 methyl CH3 groups in a tetrahedral shape
deuterated solvents definition + an example
these are solvents where 1H atoms are replaced with 2H atoms - 2H is known as deuterium
e.g. CDCl3
why are deuterated solvents used
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
what information can be obtained from a 13C NMR spectrum
number of carbon environments - number of peaks
types of carbon environments - chemical shift
what information can be obtained from a H NMR spectrum
number of proton environments - number of peaks
types of proton environments - chemical shift
number of neighbouring protons - splitting
what peaks are hardest to detect on H NMR spectra + what is often added to make it easier
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
