topic 19 Flashcards
what can mass spectrometry be used for
mass spectrometry can be used to identify compounds
what is the molecular ion and how does it form
- in mass spectrometer → when molecule loses e- a molecular ion is formed
- the molecular ion produces a molecular ion peak on the mass spectrum
- the mass/charge value of the molecular ion peak will be the same as the molecular mass of the compound (if the ion has a +1 charge)
what is a high resolution mass spectrometer
mass spectrometers that can measure atomic and molecular masses extremely accurately to several decimal places are known as high resolution mass spectrometers
what can high resolution mass spec be used for
can be useful for identifying compounds that appear to have the same Mr when theyre rounded to the nearest whole number
how do you calculate precis molecular mass from high resolution mass spec
to calculate precise molecular mass use the full decimal number of atomic mass → aka molecular ion peak
what is NMR
is an analytical technique that you can use to work out the structure of an organic molecule
how does an NMR work
- a sample of a compound in placed in a strong magnetic field+ exposed to a range of different frequencies of radio waves
- the nuclei of certain atoms within a molecule absorbs energy from the radio waves
- the amount of energy that a nucleus absorbs depends on the environment its in
what can the pattern of the absorptions in NMR tell you
the pattern of these absorptions gives you info about the positions of certain atoms within the molecule and how many atoms of that type the molecule contains
- can use this and other data to work out the structure of a molecule
what are the 2 types of NMR spectroscopy
carbon-13 NMR
high resolution proton NMR
what is carbon-13 NMR
- gives info about the number of carbon atoms that are in a molecule
- also gives info on the environments that theyre in
what is proton NMR
gives info about the number of hydrogen atoms that are in the molecule and the environments theyre in
what affects the amount of energy that is absorbed by nuclei
- any other atoms and groups of atoms that are around a nucleus will also affect its amount of electron shielding
- e.g if a carbon atom bonds to a more electronegative atom → the amount of shielding around the carbon atoms nucleus will decrease
what is the effect of the surrounding e- on the nucleus on how much it will feel the magnetic field
nucleus is partly shielded from the effect of external magnetic fields by its surrounding electrons
this means that the nuclei in a molecule feel different magnetic fields depending on their environments and therefore absorb different amounts of energy at different frequencies
what do you look for in NMR
looking for these differences in absorption of energy between environments in NMR
what does an atoms environment depend on
an atoms environment depends on all the groups its connected to going along the molecule
what is the condition for two atoms to have the same environment
for two atoms to be in the same environment they has to be joined to the exact same groups
if an electronegative atom is bonded in the molecule -> it means that because the groups will be different distances away from the electronegative atom theyll have different environments
what does each peak on a carbon 13 NMR mean
- peaks show the frequencies at which energy was absorbed by the carbon nuclei
- each peak represents one carbon environment
what is used as a standard in NMR spectrums
the differences in absorption are measured relative to tetramethylsilane (TMS) → used as a standard substance
a small amount of TMS added to samples to give a reference peak on spectrum
why is TMS used as a standard in NMR spectrums
- its chosen as a standard because its absorption peak is at a lower frequency than almost everything else
- this peak is given a value of 0
all of the peaks in other substances are measured as chemical shifts relative to this
- this peak is given a value of 0
why does TMS only produce one absorption peak
TMS produced a single absorption peak in both types of NMR as all of its carbon and hydrogen nuclei are in the same environment
what is chemical shift
the difference in the radio frequency absorbed by the nuclei (hydrogen or carbon) in the molecule being analysed and that absorbed by the same nuclei in TMS
what is the symbol for chemical shift and how is it measured
chemical shift symbol → δ and measured in ppm
how do you interpret a carbon-13 NMR
1)count no. of peaks in spectrum
- this gives the number of carbon environments in the molecule
- if theres a peak at δ=0 → dont count it → TMS reference peak
2) look up chemical shifts in shift diagram
- will be given a diagram on a data sheet which shows the chemical shifts experienced by carbon nuclei/groups
- match up the peaks in the spectrum with the chemical shifts in the diagram given in the data sheet to see which carbon environment the peaks represent
3) draw compounds to see whether they match up with the molecular formula and the peaks
how can you predict what a carbon-13 NMR may look like
- identify number of different carbon environments
- use the chemical shift diagram in data booklet to work out where peaks of each carbon environment would appear
what does proton NMR spectra tell you
proton (or 1H) NMR spectra can tell you about hydrogen environments
what does each peak on a proton NMR represent
each peak represents one hydrogen environment
how do peaks in proton NMR s split
- peaks in proton NMR split according to how the hydrogen environments are arranged
- only the peaks of hydrogens bonded to carbon atoms split
only hydrogen nuclei on adjacent carbon atoms affect each other
what is splitting of peaks in proton NMR caused by
- splitting is caused by the influence of hydrogen atoms that are bonded to the neighbouring carbons
- this effect is called spin-spin coupling
what are split peaks called
split peaks are called multiples
what is the n+1 rule
peaks split into one more than the number of hydrogens on the neighbouring carbon atom
n+1
(number of hydrogens on adjacent hydrogen + 1)
how do you determine how many peaks a hydrogen that is attached to a carbon in the middle of a carbon chain will have
- it may have 2 neighbouring carbon atoms
- all of them contribute to the splitting
- so the number of split peaks will be the sum of all of the hydrogens on the adjacent carbon atoms + 1
what are integration traces
- these tell you the ratio of protons in each environment
- the relative area under each peak tells you the number of H atoms in each environment
areas can be shown using numbers above the peaks or with an integration trace
the height increases are proportional to the area under the peak
e.g if one environment has 3 hydrogens, then the height of that peak must equal to x3 of the peaks for the environment that has 1 hydrogen (3:1 ratio)
how can structures be worked out using proton NMR
- work backwards to predict proton NMR spectrum
- look at molecule and figure out the different hydrogen environments
- then identify their chemical shifts and then the split peaks if there are any
- draw the height of the peaks relative to how many hydrogens there are
what is chromatography used for
used to separate a mixture
what is a mobile phase
where the molecules can move (always liquid or gas)
the mobile phase moves through/over the stationary phase
what is a stationary phase
cant move (a solid or a liquid on a solid support
what does the distance that each substance moves over the stationary phase dependent on
the distance each substance moves up the plate depends on its solubility in the mobile phase or its retention/ adsorption to the stationary phase
how are different substances seperated out using chromatography
- differences in solubility and retention by the stationary phase that separate out different substances
what do more soluble components do in chromatography
travel further up the plate/ faster through the column
what are the different components in paper chromatography
- solvent such as ethanol → mobile phase
- piece of paper → stationary phase
- solvent moves over the paper
how do you calculate Rf
Rf= distance travelled by spot/ distance travelled by solvent
draw a labelled diagram of a paper chromatography set up
how can you work out what was in the mixture separated by paper chromatogaphy
can calculate the Rf value for each spot on the paper and looking them up in a table of known values to work out what was in the mixture
what may cause Rf value to change
- Rf values are always the same no matter how big the paper is/ how far the solvent travels
- however if the composition of the paper, the solvent or temp change → Rf value will change
what is HPLC
HPLC- high performance liquid chromatography
done under high pressure
what is the stationary and mobile phase of HPLC
- stationary phase → small particles of a solid packed into a column/tube
- often silica bonded to various hydrocarbons
- liquid mobile → polar mixture of ethanol and water
- is forced through the column under high pressure
how is HPLC carried out
the mixture that youre trying to separate is injected into the stream of solvent and is carried through the column as a solution
- as the liquid leaves the column UV is passed through it
- the UV is absorbed by parts of the mixture as they come through
- UV detector measured the UV light absorbed by the mixture
- chromatogram produced shows the retention times of the mixture
how is the mixture separated in HPLC
- the mixture is separated because the different parts are attracted by different amounts to the solid
- so they take different amounts of time to travel through the column
what is a retention time in HPLC
this is the time taken for a substance to pass through the column and reach the detector
what can retention times be used for
can compare experimental retention times with those from data booklets to identify substances in the mixture
when can HPLC be used
HPLC can be used where gas chromatography cant → e.g if the sample is heat sensitive/ has a high bpt
how does gas chromatography work
mobile phase-> gas
- sample is injected into a stream of gas which carries the sample through a coiled column coated with a viscous liquid e.g oil or a solid
- separates a mixture into its individual components
- the components of the mixture constantly dissolve in the oil or absorb onto the solid and evaporate back into the as and then redissolve as they travel through the column
- the different components can be identified by their time taken to travel through the column aka retention time
what is gas chromatography-mass spectrometry (GC-MS)
- combines benefit of GC and mass spec
- sample is separated using GC
- then the separated components are fed into a mass spectrometer
- the mass spectrometer produced a mass spectrum for each component which can be used to identify each one
- shows what the original sample was made up of
what can HPLC, GC and mass spec be used for
- HPLC, GC and mass spec combined are often used in forensics
- can be used to trace illegal substances in e.g blood
- drug testing
why do different amino acids have different Rf values
amino acids have different solubilities in the mobile and stationary phase
Draw the displayed formula of trimethylsaline
what is low resolution NMR
number of peaks -> number of hydrogen environment
no split peaks
what is integration
area under the peak
tells you the number of hydrogens
