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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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(CH3)2C-N==N-C(CH3)2 heat-
free radicals
AIBN N
111
N
Heating AIBN causes homolytic cleavage
of the weakest bond(s) in the molecule. Consider cleavage of the two
types of CC bonds, the CH bond, and the CN single bond. Write the
reactions that would result from homolytic cleavage of each of these
bonds, then use the energies of the various radicals and AIBN to
calculate reaction energies (the energies of H, CH3, and CN are shown on
the left). Which bond cleavage should occur most readily? Considering
only this bond cleavage, what resonance contributors must be drawn to
describe the two radicals adequately?
Homolytic cleavage of AIBN generates
two radicals, either of which might initiate a radical chain reaction. If
we assume that the next step is for one of these radicals to abstract a
hydrogen atom from another molecule, we would expect the "reactive"
radical to be the one that makes a stronger bond to hydrogen. Use the
energies of H (shown on left), the relevant radicals, and the radicals+H
products to calculate bond energies. Which radical is more likely to
abstract hydrogen and initiate a chain reaction?
240 Chapter 17 Free Radicals and Carbenes
Free Radicals Add to Double Bonds
Electrophilic addition of hydrogen bromide to alkenes follows
Markovnikov's rule, leading to the product with halogen on the more-
substituted position. However, trace amounts of hydroperoxides (among
other "impurities") may initiate a reaction that gives rise to the "anti-
Markovnikov" product, with bromine in the less-substituted position.

Br
HBr
CH3CH=CH2 HBr
Markovnikov
Br
(trace ROOH) CH3CH2CH2 anti-Markovnikov
One possible interpretation is a change to a free radical chain
mechanism. Bromine radical is first produced which then adds to the
alkene. The resulting free radical reacts with hydrogen bromide to yield
the final alkyl bromide and regenerate bromine radical.
ROOH + HBr -- Br'
Br' + CH3CH=CH2 --CH3CHCH2Br CH3CHCH2Br + HBr-"- CH3CH2CH2Br
+ Br'
Examine spin density surfaces for l-bromo-2-propyl radical and 2-bromo-l-
propyl radical (resulting from bromine atom addition to propene). For
which is the unpaired electron more delocalized? Compare energies for the
two radicals. Is the more delocalized radical also the lower-energy
radical? Could this result have been anticipated using resonance
arguments?
Does the lower energy radical intermediate lead to the lower energy
product? Compare energies for 1-propyl bromide and 2-propyl bromide to
tell.
fl

Spin density surface for l-bromo-2-propyl radical shows location of the
unpaired electron.
Chapter 17 Free Radicals and Carbenes 241
Spin Traps and Radical Scavengers
Spin density surface for phenoxy radical shows location of the unpaired
electron.
ilectrostatic potential map for vitamin E radical distinguishes ;harged
regions (in red and )lue) which are likely to nteract strongly with polar
;nvironments from uncharged egions (in green) which are mlikely to
interact.
The hydroxyl hydrogen in phenol is particularly susceptible to
abstraction by a free radical.
The process is exothermic, suggesting that the phenoxy radical is
particularly stable. Display the spin density surface for phenoxy
radical. Is the unpaired electron localized or delocalized over several
centers? Is the unpaired electron in the a or e system? Draw appropriate
Lewis structures that account for your data.
Phenol is a "radical scavenger". Other radical scavengers include 3,5-di-
fe?t-butyl-4-hydroxytoluene (butylated hydroxytoluene or BHT) and vitamin
E.
Examine the spin density surface for BHT radical. Is the unpaired
electron localized or delocalized? Examine BHT radical as a space-filling
model. What effect do the bulky tert-butyl groups have on the "chemistry"
of the species? (Hint: BHT radical does not readily add to alkenes or
abstract hydrogens from other molecules.)
Compare the spin density surface for vitamin E radical to those of
phenoxy and BHT radicals (see also Chapter 16, Problem 2). Are there
significant differences among the three? If so, elaborate. What is the
function of the long alkyl chain in vitamin E? Examine an electrostatic
potential map for vitamin E radical. Do you expect it to be soluble in
aqueous (polar) or non-aqueous (non-polar) environments, or both?
!42 Chapter 17 Free Radicals and Carbenes
Singlet and Triplet Methylene
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