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MS-IR-NMR Worksheet

1st attachment- Extra information on how to do mass spectra if needed

2nd attachment- Actual worksheet that needs to be completed

3rd attachment- Used as choices for the worksheet that needs to be completed (2nd attachment)











First look at the peaks on the right of the spectrum, the ones with the highest m/z values. In


most cases, the highest large peak is the molecular ion, M+. Its mass is the same as that of


the parent molecule, so finding the molecular ion tells you the molecular weight of the


compound. Some molecules fragment very easily and may have a very small molecular ion or


no molecular ion at all.


For most organic compounds, the molecular ion appears at an even m/z number. The most


important exception occurs in compounds containing nitrogen. The nitrogen rule states that


a molecule containing an odd number of nitrogen atoms will have a molecular ion with an


odd m/z value.


Next, look for M+1 and M+2 peaks to the right of the molecular ion. These peaks are caused


by isotopes heavier than the predominant isotope. In particular, carbon in nature consists of


about 99% 12C and 1% 13C. This means that for a compound containing ten carbon atoms,


about 10% of the molecules will contain one 13C and the M+1 peak will be about 10% the size


of the M+ molecular ion. M+1 peaks are found in almost all mass spectra. M+2 peaks are


more unusual. M+2 peaks are found only in compounds which contain elements that have


have high percentages of an isotope with mass 2 higher than the predominant isotope. Two


such elements which are found in some organic compounds are chlorine and bromine.


Chlorine consists of 76% 35Cl and 24% 37Cl, and bromine consists of almost exactly equal


amounts of 79Br and 81Br. In a mass spectrum, a molecule containing one chlorine atom will


have M and M+2 peaks in a 3:1 ratio; a molecule containing one bromine atom will have M


and M+2 peaks in a 1:1 ratio.


Peaks in a mass spectrum with m/z values less than the molecular ion result from


fragmentation of the molecule. The exact fragmentation pattern of a molecule is difficult to


predict and to some extent depends on the experimental conditions. In general, the peaks


that are observed are due to the most stable carbocations that can be formed from the


molecule. The following generalizations can be made regarding fragmentation of various


types of molecules:



ALKANES- Simple alkanes tend to undergo fragmentation by the initial loss of a methyl group to


form a (m-15) species. This carbocation can then undergo stepwise cleavage down the alkyl


chain, expelling neutral two-carbon units (ethene). Branched hydrocarbons form more stable


secondary and tertiary carbocations, and these peaks will tend to dominate the mass spectrum.


ALKENES and ALKYNES? These compounds often have a strong molecular ion peak, formed by


removal of one ? electron. Prominent peaks include resonance-stabilized allylic (-CH=CH-CH2+)


and propargylic (-C?C-CH2+) cations.




a strong molecular ion peak resulting from


removal of one ? electron from the


aromatic ring. If the molecule contains a


benzyl (C6H5CH2-) unit, the major cleavage


will be to generate the benzyl carbocation,


which rearranges to form the tropylium


ion, C7H7+ .



ALCOHOLS ? Alcohols readily lose a molecule of water to form a peak with mass (M ? 18).


Therefore, they often have a very weak molecular ion or no detectable molecular ion. Alcohols


also cleave by loss of an alkyl radical from the carbon bearing the ?OH group to form an oxonium


ion with mass (M ? R). Ethers undergo a similar fragmentation, with loss of an alkyl radical.






















































CARBONYL COMPOUNDS, such as aldehydes, ketones, carboxylic acids, esters and amides,


fragment at the bonds of the carbonyl carbon to produce an acylium ion (alpha cleavage).




of an ester



If there is a sufficiently long carbon chain, McLafferty rearrangement can be very important. The


base peak can be the McLafferty rearrangement product.






R = alkyl group (ketones), H- (aldehydes), -OH (carboxylic acids), -OR (esters)



AMINES ? The most characteristic fragmentation of amines is ?-cleavage. (Remember that


compounds containing an odd number of nitrogen atoms have an odd-numbered molecular ion.)




of an amine
























ALKYL HALIDES - Organic halides fragment with simple expulsion of the halogen.



















The molecular ions of chlorine and bromine-containing compounds show multiple peaks because


these elements consist of two different isotopes in relatively high abundance. For chlorine, the




Cl/37Cl ratio is 3.08:1; for bromine, the 79Br/81Br ratio is 1.02:1. The molecular ion of a chlorinecontaining compound consists of two peaks, separated by two mass units, in the ratio ~3:1, and a


bromine-containing compound has two peaks, again separated by two mass units, having


approximately equal intensities.




Paper#9209906 | Written in 27-Jul-2016

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