Br i p b si as

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Unformatted text preview: ' + $ ' + 1 = 5 unsaturations #2& #2& #2& •  Possible structures: O O NH2 H H2N CHM3122 Interpreta$on of mass spectra 9 Fragmenta$on •  fragmenta$on peaks can give us clues about the molecular structure CHM3122 Interpreta$on of mass spectra 10 Common fragmenta$ons Appendix B: Common ionic fragments CHM3122 Appendix C: Commonly lost fragments Interpreta$on of mass spectra Interpreta$on of fragmenta$on pagerns 11 Predic$ng major peaks Rules for predicEng major peaks in EI spectra: 1.  The rela$ve height of the molecular ion peak is greatest for a linear compund and decreases as the degree of branching increases. 2. The rela$ve height of the molecular ion peak genrally decreases as the molecular weight increases (in a homologous series). CHM3122 Interpreta$on of mass spectra 12 Predic$ng major peaks 3.  Homolysis is favoured for highly subs$tuted carbons in alkyl groups – the greater the subs$tu$on, the greater the likelyhood of cleavage. This is a consequence of the carboca$on stability. RC R C The order of stability of the carboca$ons is: CH3+ < RCH2+ < R2CH+ < R3C+ CHM3122 Interpreta$on of mass spectra 13 Predic$ng major peaks 4.  Double bonds, cyclic compounds, and especially aroma$c rings stabilize the molecular ion, and thus improve the probability of seeing its peak. 5.  Double bonds favour allylic cleavage to yield a resonance ­ stabilized carboca$on. CHM3122 Interpreta$on of mass spectra 14 Predic$ng major peaks 6.  Saturated rings tend to lose alkyl side chains via cleavage at the alpha posi$on (a special case of rule #3). Unsaturated rings may undergo a retro ­Diels ­Alder reac$on: CHM3122 Interpreta$on of mass spectra 15 Predic$ng major peaks 7.  In alkyl ­subs$tuted aroma$c compounds, cleavage at the beta posi$on is most likely, yielding a resonance ­stabilized benzyl ion , known as the tropyllium ion: CHM3122 Interpreta$on of mass spectra 16 Predic$ng major peaks 8.  C ­C bonds alpha to a heteroatom are frequently cleaved, leaving behind the charge on the fragment containing the heteroatom (which stabilizes the ca$on via resonance effects). CHM3122 Interpreta$on of mass spectra 17 Predic$ng major peaks 9.  Cleavage is oJen associated with the loss of small neutral stable molecules, such as CO, alkenes, water, ammonia, etc. O + CO NOTE: all of these rules apply to EI spectra! CHM3122 Interpreta$on of mass spectra 18 Rearrangements •  Peaks that cannot be explained via simple fragmenta$on can oJen be caused by molecular rearrangements Example : the McLaff...
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