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1st attachment- Extra information on how to do mass spectra if needed
2nd attachment- Actual worksheet that needs to be completed
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SOME IDEAS TO CONSIDER WHEN INTERPRETING MASS SPECTRA
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.
AROMATIC HYDROCARBONS? often have
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-2016Price : $19