How to Interpret Mass Spectra?

Here we learn how to interpret mass spectra?
As you know mass spectrum is a presentation of the masses of the positively charged fragments versus their relative concentrations. The most intense peak in the spectrum, called the ‘Base peak’ is assigned value of 100%, and the intensity of other peaks, are reported as percentage of the base peak.
The commonly used technique in mass spectrometry is ‘Electron Impact’. On electron impact, the molecules are energized sufficiently to eject an electron, thus produces molecular ion "M+"; which further fragments to more daughter ions.
Fragmentation fallow general principles of stability of organic molecule.
Interpretation of mass spectrum is just like to solve puzzle. You have to think all possible ways in which molecular ion can fragment, and form more stable daughter cation. More stable daughter cation gives more abundant peak.
Lets work out this spectrum of alkane.

1. Identify molecular ion peak. Here it is 72. 
2. find out carbon skeleton corresponds to molecular ion. Here it may be C5H(12-1)ion
3. Find out all possible carbon frames. Here we find 3 possibilities..
4. Identify base peak in given spectra. Here it is 43 and corresponds to the major fragment ion. 
5. Now its puzzle time:) you have to work with every possible skeleton. 

1. n-pentane -The C2-C3 bond is more likely to break than the C1-C2 bond because C2-C3 fragmentation leads to a primary carbocation and a primary radical which is more stable than primary carbocation and methyl radical. Means major fragment will be (m/z=43) and other fragments may be (m/z=29, 15, 57). 
2. 2-methylbutane- The C2-C3 bond is more likely to break and forms secondary carbocation (m/z =43). And next probable fragment will form due to loss of methyl radical which leads to formation of secondary carbocation (m/z= 57). 
3. 2,2-dimethylpropane- major fragment will form by loss of methyl radical which leads to tertiary carbocation(m/z =57). 
6. Assess all possibilities.
1. n-pentane- base peak will be 43, and other major peaks will be 57, 29 and 15.
2. 2-methylbutane- base peak will be 43 and peak 57 should also have high abundance.
3. 2,2-dimethylpropane-base peak will be 57.
7. Correct answer will be n-pentane.

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