Structure Determination notes

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Structure determination
Mass spectrometry is used to measure relative atomic masses of isotopes and relative
molecular masses
Vaporisation: sample turned into gas
Ionisation: gas particles bombarded with high-energy electrons- electrons knocked off ­
leaving positive ions
Acceleration: positive ion accelerated by an electric field
Deflection: ions deflected by an magnetic field- deflection depends on mass/charge ratio
Detection: ions reach detector and a mass spectrum is produced
particles lose an electron- free radical cation formed-[C2H5OH]+
Some ions fragment- bonds break/ some remain intact and give a peak corresponding to thr
relative molecular mass, Mr, of the compound
Ionised molecules- molecular ions- peak furthest to the right corresponds to the molecular
ion (highest mass)
Mass spectrometry is the most important technique of measuring Mr
Molecular ion breaks up ­ one fragment will carry a positive charge and the other an
unpaired electron- only charged fragment detected because uncharged fragment will not be
Other peaks come from fragments of the molecule- useful when two compounds have the
same relative molecular mass ­ but the spectrum will be different as the fragments of the
two molecules will be different
Butane, shows the following main peaks: 2
Mr = 58 molecular ion, CH3CH2CH2CH3+
Mr = 43 CH3CH2CH2+, formed when CH3 is lost
Mr = 29 CH3CH2+, formed when C2H5 is lost
Any unchanged species are removed by the vacuum pump
Methylpropanone, shows the following main peaks: 3
Mr = 58 molecular ion, CH3CH(CH3)CH3+
Mr = 43 CH3CHCH3+ formed when CH3 is lost
It is not possible to get a peak of Mr = 29 from methylpropanone by breaking just one bond.
Fragmentation is more likely to take place at weaker bonds
The more stable the fragment that forms, the greater its abundance in the spectrum
Stability of carbocations: tertiary > secondary >primary
Fragmentation of carbonyl compounds

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Typically fragment at the carbonyl group to give an alkyl radical and a stable RCO+ cation
(acylium ion)- stable because the RCO+ ion has a positive charge localised on the carbon
atom, the carbonyl group is polarised Cdelta+=Odelta- because oxygen is more electronegative
than carbon:
The abundant peak at m/z=43 suggests a CH3CO group in the parent molecule- can be seen
in a propanone spectrum
Isotope Peaks
Some molecules containing isotopes will produce more than one molecular ion
Example, butane, C4H10, will…read more

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