Description of this paper

Loading

SOME IDEAS TO CONSIDER WHEN INTERPRETING MASS-(Answered)

Description

Instant Solution ? Click "Buy button" to Download the solution File


Question

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)


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.

 

R

 


 

.+

 

..

 


 

R??C?O?H

 


 

?????>

 


 

R?

 


 

+

 


 

..

 


 

+

 


 

R??C=O?H

 


 

R?

 


 

<??????>

 


 

..

 

..

 


 

+

 


 

R??C?O?H

 


 

R?

 


 

R?

 


 

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).

 

?-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.

 

McLafferty

 

rearrangement

 

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.)

 

beta-cleavage

 

of an amine

 


 

H

 


 

H

 


 

N

 


 

N

 


 

+

 


 

+

 


 

.

 


 

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

 


 

.

 

+

 


 

+

 

X

 


 

+

 


 

.X

 


 

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

 

35

 

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

Price : $19
SiteLock