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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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relationship between the CH bond lengths in 3-ethylpentane and the
stabilities of the radicals resulting from bond dissociation? Elaborate.
Calculate CH bond dissociation energies inpropene and in toluene, leading
to ally I and benzyl radicals, respectively. (The energy of hydrogen atom
is given at right.) Is bond dissociation easier or more difficult in
these systems relative to bond dissociation in 3-ethylpentane (methyl
NI)? Examine spin density surfaces for allyl and benzyl radicals. Draw
Lewis structures that account for the electron distribution in each
radical. Does spin delocalization appear to stabilize radicals in the
same way charge delocalization stabilizes ions?
R*+ FT

Spin density surface for benzyl radical shows location of the unpaired
electron.
E(H') = -0.4962 au
Chapter 17 Free Radicals and Carbenes 237
3
Free Radical Chlorination of Alkanes
A(NA) = -457.3711 au
iiHi
Spin density surface for chlorine atom+methane transition state shows
location of impaired electron.
Chlorine gas reacts directly and highly exothermically with alkanes,
giving rise to alkyl chlorides and hydrogen chloride, e.g., for addition
to methane.
ci2
CH4 ---> CH3CI + HCI
The process is believed to initiate with formation of chlorine atom
(either thermally or photochemically), which then abstracts a hydrogen
from methane.
Cl2 ---- Cl* + Cl*
Cl* + CH4 -- *CH3 + HCI
The resulting methyl radical abstracts a chlorine atom from Cl2, leading
to product and generation of another chlorine atom.
*CH3 + Cl2 -- CH3CI + Cl'
Construct a reaction energy diagram starting from chlorine atom and
methane and ending with methyl chloride and chlorine atom. (Models for
methane, methyl radical, hydrogen chloride, chlorine and methyl chloride
are available; the energy of chlorine atom is given at left.) Is each
step exothermic? Is there an overall thermodynamic driving force for the
reaction?
Examine the structures of the two transition states (chlorine
atom+methane and chlorine+methyl radical). For each, characterize the
transition state as "early" (close to the geometry of the reactants) or
as "late" (close to the geometry of the products)? In light of the
thermodynamics of the individual steps, are your results anticipated by
the Hammond Postulate? Explain.
Examine spin densities for the two transition states. Draw a "Lewis
structure"(or sequence of Lewis structures) for each which properly
conveys the location(s) of the unpaired electron.
238 Chapter 17 Free Radicals and Carbenes
I
Chlorination of Toluene
Free radical initiators play an important role in many
_
chemical reactions (see also Chapter 17, Problem 5).
ll
For example, combustion of gasoline is assisted by compounds such as
tetraethyl lead, heating of which results in bond breaking and generation
of ethyl radical.
(CH3CH2)3Pb-CH2CH3 (CH3CH2)3Pb' + CH3CH2
In the presence of chlorine gas and a free-radical initiator, three of
toluene's hydrogens are sequentially replaced by chlorine atoms.
Cl2 Cl2 Cl2
C7H8 . ... t " C7H7CI-----> C7H6CI2 ------- C7H5CI3
8 initiator 11 / u a
The first step in the overall process is believed to involve abstraction
of hydrogen by chlorine atom, followed by reaction of the ensuing radical
with Cl2.
C7Hg C7H7 + HCI C7H7CI + Cl
Draw resonance structures for the possible radicals resulting from
hydrogen atom abstraction from toluene.
Which would you anticipate to be the most stable? Why?
Compare energies for the different radicals {radical A, radical B, ...).
Is the lowest-energy radical that which you anticipated? Are any of the
alternatives significantly better than any of the others? Explain your
reasoning.
What are the three products resulting from free-radical chlorination of
toluene? Why are only three hydrogens replaced?
Chapter 17 Free Radicals and Carbenes 239
Radical Initiators
Free radical chain reactions depend on an
easily generated free radical to initiate the chain. One way to generate
this radical is to irradiate halogens, such as Cl2 and Br2.
111111...111 Another way is to add a small amount of
an "initiator"
molecule to the reaction mixture, such as AIBN. This molecule,
when heated, decomposes into free radicals that E(CH3') = -39.3426 au
react with other molecules to initiate a chain reaction.
E(H-) = -.4962 au
E(CN')= -91.6848 au N
111 N
E(N2) = -108.3010 au
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