Fragmentation pattern of Aromatic hydrocarbon in mass spectra

·        Molecular ion peak is fairly noticeable because an aromatic ring better stabilizes the molecular ion.

·        If the aromatic ring is substituted by an alkyl group, a prominent peak is formed at m/z 91. This highly stabilized fragment is result from rearrangements.

·        Next frequently observed peak at m/z 65 results from elimination of neutral acetylene molecule from tropylium ion.

·        When alkyl group is longer than two carbon peak at m/z 92 is observed, accounts for hydrogen migration with elimination of neutral alkene molecule.

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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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Mass Spectra of Alkene

Molecular ion peak is intense in unsaturated compounds. Molecular ion represents the most stable cation formed by the removal of an electron from the parent molecule. In conjugated alkene, molecular ion is formed by the removal of one of the π electrons. The molecular ion, thus formed, is stabilized by the resonance. It means the burden of positive charge is shared between all atoms. Here we will study normal alkene. Let's try to solve the spectra of 3-heptne.

The other intense peak corresponds to the ion formed by McLafferty rearrangement. Let’s see what McLafferty rearrangement is. Molecules those have gama γ Hydrogen can undergo this rearrangement.

What does it mean by γ Hydrogen? Carbon atoms are labeled on the basis of their position in molecule with respect to the functional group. Carbon atom which bears functional group is called α carbon and Carbon atom placed next to the α carbon is called β carbon and the second next carbon is called the γ Carbon. The Hydrogen atoms attached to these carbon atoms are respectively called as α, β and γ Hydrogen. In McLafferty rearrangement, molecular ion splits from β Carbon and functional group gains the γ Hydrogen. Thus formed cation is stabilized by resonance and is represented by an intense peak in mass spectra.

The next stable cation is allylic carbocation. Allyl group is three carbon molecule with one double bond. It is formed when alkene undergoes allylic cleavage that means a cleavage which results in formation of allyl carbocation. This allyl cation is stabilized by resonance and appears as next highest peak to the molecular ion peak.

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Fragmentation and Mass Spectra of Alkane

• Molecular ion peak is always present although height of peak decreases as the molecular mass increases in the homologous series.
• C-C bond is weaker than C-H hence cleavage is favored at C-C bond.
• Fragmentation pattern is characterized by clusters of peak, and the corresponding peaks of each cluster are 14 mass units (CH₂) apart.
• Largest peak in each cluster represents a C_nH_2n+1 fragment and occurs at m/z= 14n+1
• Most abundant fragments are C₃ and C₄
• Fragment abundance decrease for [M- C₂H₅]⁺ and [M- CH₃]⁺is very weak or missing.
• Compounds containing more than 8 carbons show similar spectra; identify them by M⁺peak.

Branched Alkane:

• Molecular ion peak is mostly not observed.
• Bond cleavage takes place preferably at the site of branching.
• The way a molecular ion fragments depends on the strength of its bonds and the stability of the fragments.
• Generally, largest substituent is eliminated readily as a radical. The radical achieves stability by the delocalization of lone electron. 

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