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The Molecular Modeling Workbook for Organic Chemistry - Hehre J.W.

Hehre J.W., Shusterman J.A. The Molecular Modeling Workbook for Organic Chemistry - Wavefunction, 1998. - 307 p.
Download (direct link): molecularmodelingworkbook1998.djvu
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2-methyl-2-butanol lacks a parent peak and contains strong peaks at M-15
(loss of CH3*) and M-l 8 (loss of H20).
First, consider loss of ethyl radical. Examine the geometry and
electrostatic potential map of 2-methyl-2-butanol radical cation. Are any
of the carbon-carbon bonds significantly longer than normal? (Compare the
geometry to that of 2-methyl-2-butanol.) Which portion of the molecule is
most positively charged? Which portion carries the least positive charge?
Assuming the elongated bond breaks, which fragment is likely to carry a
positive charge? Examine the spin density surface of 2-methyl-2-butanol
radical cation. Which atom(s) carries the unpaired electron? Assuming the
elongated bond breaks, which fragment is likely to carry this electron?
Write the chemical reaction that describes loss of ethyl radical and draw
complete Lewis structures for all of the reactants and products.
Next, consider loss of water. This involves loss of OH and a CH hydrogen,
and must yield a new radical cation (this is required because water is a
neutral, even-electron molecule). Examine the geometries, electrostatic
potential maps and spin density surfaces of the three radical cations
that might result {radical cation A, A, N). Draw a single Lewis structure
(or pair of equivalent Lewis structures) for each radical cation which
best describes both the location of the unpaired electron and the
positive charge. Which CH hydrogen must be lost in order to generate each
radical cation? Which radical cation is most stable? Why? Is the most
stable radical cation that in which the unpaired electron and positive
charge is most delocalized?
268 Chapter 20 Mass Spectrometry
Mass Spectra of Alkenes and Arenes. Resonance Stabilized Cations
In general, fragmentation in a mass spectrometer gives rise to the most
stable ions. For unsaturated compounds resonance stablized ions may be
possible.
The mass spectrum of trans-2-hexene shows a very strong peak at M-29.
Compare the geometry of trans-2-hexene radical cation to that of trans-2-
hexene. Is there any indication for pending loss of a neutral fragment of
mass 29? (Pay particular attention to elongated bonds.) Explain. If there
is, identify the ionic and neutral fragments. Examine both the spin
density surface and electrostatic potential map for trans-2-hexene
radical cation. Does this show evidence for any particular fragmentation?
Explain.
Repeat your analysis for y-propylbenzene. (Compare geometries of 1-
propylbenzene radical cation and
1-propylbenzene.) Where would you expect a strong peak in the mass
spectrum? Identify the ion responsible.

Spin density surface for 1-propylbenzene radical cation shows location of
unpaired electron.

Electrostatic potential map for Aaae.o-2-hexene radical cation shows most
positively-charged regions (in blue) and less positively-charged regions
(in red).
Chapter 20 Mass Spectrometry 269
i
3
Energy minima have all real frequencies, while molecules with one or more
imaginary frequencies are not minima.
ib
Spin density surface for butanal radical cation shows location of
unpaired electron.
Electrostatic potential map for butanal radical cation shows most
positively-charged regions (in blue) and less positively-charged regions
(in red).
McLafferty Rearrangement
Radical cations generated in a mass spectrometer from aldehydes and
ketones with o hydrogens undergo a rearrangement in which a o hydrogen is
first transferred and a carbon-carbon bond is then cleaved, e.g.
a
The McLafferty rearrangement, as it is known, is useful for structure
identification. For example, the mass spectrum of 2-methylbutanal shows a
peak at m/e 58, while that of
3-methylbutanal shows a peak at m/e 44.
m/e 86 m/e 44
Examine energies for butanal radical cation, the transition state for
hydrogen migration and the rearranged enol radical cation. Is the first
step, hydrogen transfer endothermic or exothermic? Is the activation
barrier low enough that the process will be fast in a mass spectrometer?
Is the rearranged cation an energy minimum?
Use geometries, electrostatic potential maps and spin densities to help
you draw Lewis structures for butanal radical cation, the transition
state and product. Where is the positive charge and the unpaired electron
in each? Is the positive charge (the unpaired electron) more or less
delocalized in the transition state than in the reactant? In the product?
270 Chapter 20 Mass Spectrometry
Pericyclic Reactions
1 Electrocyclic Reactions 272
2 The Diels-Alder Reaction. A Symmetry Allowed Process 273
3 Electron-Flow in Diels-Alder Reactions 274
4 Catalysis of Diels-Alder Reactions 275
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