Fragmentation and mass spectra of Esters

• Molecular ion peak is weak but noticeable.
• The most characteristic peak results from McLafferty rearrangement. 
  
• Other important Peaks results from bond cleavage next to C=O.
  
• gives an easily recognizable peak for esters.
• Easters, in which the acid portion is the predominant portion of molecule, the fragmentation pattern is same as described for the fragmentation pattern of free acid.
  
• Easters, in which the alcohol portion is the predominant portion of molecule, eliminate a molecule of acid on fragmentation.
• lets check spectrum of methyl octanoate
  

Fragmentation and mass spectra of Aliphatic Acids

• Molecular ion peak is weak but noticeable.
• In short chain acids, cleavage of bonds next to C=O results prominent peaks at M-OH and M-CO₂H.
• In long chain acids, the spectrum consists of two series of peaks resulting from cleavage at each C-C bond with retention of charge either on the Oxygen containing fragment or the alkyl containing fragment.
• The most characteristic peak is m/z 60 resulting from the McLafferty rearrangement. Branching at α carbon enhance this cleavage.
  

lets examine spectrum of decanoic acid 

Fragmentation and mass spectra of Aldehydes

• Molecular ion peak is very weak.
• M-1 peak is good diagnostic peak, even for long chain Aldehyde.
• In lower aldehydes, α-cleavage is prominent with retention of charge on oxygen.
• In straight chain Aldehydes, the other diagnostic peaks are at 

1. M-18 because loss of water
2. M-28 because loss of ethylene
3. M-43 because loss of CH2=CH-O
4. M-44 because loss of CH2=CH-OH

Fragmentation and mass spectra of Ketone

• The mass spectrum of a ketone generally has an intense molecular ion peak.
• Ketones fragment homolytically at the C-C bond adjacent to the C=O bond, which results in the formation of a cation with a positive charge shared by two atoms. The alkyl group leading to the more stable radical is the one that is more easily cleaved.

• If one of the alkyl groups attached to the carbonyl carbon has γ hydrogen, a cleavage known as a McLafferty rearrangement may occur.
  

lets interpret this spectrum

Fragmentation and mass spectra of Ethers

• Molecular ion peak is small.
• The presence of oxygen atom can be deduced from strong peaks at m/z31,45,59,73 ect. These peaks represents the RO⁺ and ROCH₂⁺fragments.
• Fragmentation of the resulting molecular ion occurs in two principal ways:

1. A C-O bond is cleaved heterolytically, with the electrons going to the more electronegative oxygen atom. 
   
2. A C-C bond is cleaved homolytically at the position because it leads to a relatively stable cation in which the positive charge is shared by two atoms (carbon and oxygen).
   

Now try to this interpret spectrum

Fragmentation pattern and mass spectra of Alcohols

·        Molecular ion peak of primary and secondary alcohol is quite small and for tertiary alcohol is undetectable.

·        Molecular ion peak is formed by the removal of one electron from the lone pairs on the oxygen atom of primary and secondary alcohol.

·        Cleavage of C-C bond next to the oxygen (α cleavage) is of general occurrence.

o   Thus primary alcohol  show a prominent peak resulting from oxoniumion (m/z 31)

o   secondary alcohol show a prominent peak resulting from (m/z 45,59,73 etc)

o   tertiary alcohol show a prominent peak resulting from(m/z 59, 73, 87 etc)

·     A distinct and sometimes prominent peak can usually be found at M-18 from loss of H₂O.

·     In primary alcohol elimination of water, together with elimination of alkene, accounts for the presence of a peak at M-(alkene+ H₂O) ie (m/z 46, 74, 102 etc)

lets examine spectrum of 1-pentanol

examine spectrum of  secondary alc. 2-pentanol

now examine spectrum of  tertiary alc. 2-methyl-2-butanol

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