CH908, Problem set 3
1. Interpret the following spectra, draw reasonable structures and
assign as many peaks as possible:
a.
b.
Answer: 3-pentanone
c. note: this is not the same as b.
Answer: 2,3-butanedione.
Answer: 3-methyl, 2-butanone
d. Elemental composition is given as C8H18O
Notes: alkane/alkene series. 31, 45 indicate an oxygen, probably
not an alcohol, from the intensity. Something special about 57,
indicating either a very stable neutral radical loss, or a very stable
even electron product ion, or very high symmetry – so that this
species can be formed several ways.
Partial answer: C4H9-O-C4H9 several isomers are possible, but
intense 41 suggests branching. 87 peak suggest the branch (on
one side at least) is not at the first carbon. To determine exactly
which isomer, it’d be necessary to have the whole series of isomer
spectra.
Answer: diisobutyl ether
e.
Notes: no nitrogens. methyls and ethyls and propyls are present.
Even electron 58 ion is strange. 31, 45 indicates an oxygen, but
intensity says it’s not an alcohol, probably not even an ether –
probably carbonyl.
Answer: 4-methyl, 2 pentanone. 58 peak comes from the
McLafferty rearrangement.
f.
Same as above, but the McLafferty rearrangement even electron
ion peak is heavily suppressed, and shifted by 14 Da to 72 Da.
Also, 57 peak is much higher, probably C4H9+.
Answer: 3-methyl, 2 pentanone.
2. A product ion mass or neutral loss of 91 Da indicates what
moiety? Why? Draw the structures.
C7H7•, Toluyl or tropylium (7 membered aromatic ring).
3. In EI ions, where are the charge site and the radical site located
initially? Why? Do they stay together during fragmentation?
First non-bonding electrons from heteroatoms, then pi bonding
electrons, then sigma bonding electrons. This simply follows the
energetics of electron binding. The charge site and radical site
almost always move apart in the fragmentation because the
electron-pushing rules are different for charges and radicals
(hemolytic vs. heterolytic bond cleavage)
4. Define the terms 'distonic ion' and 'charge remote fragmentation'.
Distonic ion: an radical ion where the charge and radical are at
different positions.
Charge remote fragmentation: fragmentation that is in a distant part
of the molecule from the charge, usually caused by long-range
radical rearrangements (hydrogen abstraction)