INSTRUMENTACION QUIMICA III ESPECTROMETRIA DE MASAS LABORATORIO 2, FEBRERO 23 DE 2010 1) Dissociation or fragmentation of ions yields a fragment ion and a neutral. Depending on whether thefragmentation ion is an even-electron or an odd-electron species certain fragmentation pathways may be allowed or forbidden. The “even electron rule” serves as a guideline for ion fragmentation.Explain the rule and give an example of each case. 2) Apply the nitrogen rule to the compounds below. Do not calculate molecular weights. Simply fill “even” or “odd” into the second column!Compound Methane Acetone Chloroform Ammonia Urea Pyridazine Triazole Monoisotopic mass is …
3) ?-cleavage is a very important fragmentation pathway for molecules with heteroatoms. Explainthe fragmentation pathway for the following molecules: Acetone, butanone, N-ethyl-N-methyl-propanamine (C6H15N), methyl propyl ether, diethyl ether, ethylisopropylthioether (extra credit). Usethe Chemistry Web Book to get the EI spectra (http://webbook.nist.gov/chemistry/). 4) Cleavage of the benzylic bond in phenylalkanes. Molecular ions containing aromatic rings are stable dueto the charge-stabilizing properties of aromatic rings. Explain the fragmentation pathway of the following molecules: n-propylbenzene and toluene. What is the origin of the signal at m/z 91, socommon in the EI spectra of phenylalkanes? Use the Chemistry Web Book to get the EI spectra (http://webbook.nist.gov/chemistry/). 5) In a molecular ion of straight chain such an n-alkanethere is not preferred position to localize the charge, and the primary step of dissociation is unspecific ?-bond cleavage. Compare the EI spectra of the isomers n-decane and 2,7-dimethyl octane.Use the Chemistry Web Book to get the EI spectra (http://webbook.nist.gov/chemistry/). 6. Explain with two examples the McLafferty rearrangement. Who is McLafferty? Is he still alive?
