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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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"nucleophilic addition", how would you expect CC12 to approach propene?
Were the reaction an "electrophilic addition", how would you expect CC12
to approach propene? Which interpretation is more consistent with the
geometry of the transition state?
A
HOMO and LUMO of dichlorocarbene
characterize the molecule's electrophilic and nucleophilic behavior,
respectively.
Chapter 17 Free Radicals and Carbenes 245
Polymers
1
Nylon....................................................................
248
2 Synthetic
Polymers.......................................................249
3
Rubber...................................................................
250
4 Alkene
Polymerization....................................................251
5 Stereoregularity of
Polypropylene........................................252
short strand of polystyrene (see
problem 2)
Nylon
One of the oldest and (still) commercially most important polymers is
nylon. Actually, nylon does not refer to a single polymer, but rather to
an entire class of amide polymers or "polyamides". Two of the most common
are Nylon 6 and Nylon 6,6.
Commercially, Nylon 6 is made from caprolactam.
i
caprolactam
Examine the structure of the short strand of Nylon 6 in which all amide
bonds are Z. What is the monomer unit? How many monomers are in the
strand? Note: Each end of the polymer strand has been "capped" by one or
more atoms. Do not count these "caps" as monomers. Compare the strand to
that of a simple polypeptide, for example, polyglycine (see also Chapter
16, Problem 9), and point out any obvious similarities. Pay particular
attention to hydrogen bonds.
Examine the structure of a strand of Nylon 6 in which all amide bonds are
E. Describe how this differs from the strand in which the amide linkages
are Z. In particular, are the same hydrogen-bond patterns found?
Examine the structure of Nylon 6,6 (amide bonds have been assumed to
adopt E geometries). What is the repeating unit? How many monomers are in
the strand? Nylon 6,6 is made by combining two different molecules, a
diacid and a diamine. Draw these molecules.
Synthetic Polymers
A wealth of important materials fall under the general category of
synthetic polymers. All share a common theme of being made up of
sequences of one or more monomer units.
One after the other, examine the structures of a number of common
monomers. What features, if any, do they have in common? What relevance
is this to the polymerization process?
One after the other, examine the structures of a number of common
polymers. For each, draw the repeating unit, and indicate the chain
length (number of repeating units in the strand). Note: Each end of a
polymer strand has been capped by adding extra atoms. Do not count these
atoms as repeating units. Also, use the smallest possible repeating unit.
AT'
I

It is unrealistic to expect that any single conformer of a polymer will
adequately represent the overall size and shape of the polymer. The low-
energy conformer for each polymer strand shown here is merely meant to
allow identification of the polymer in terms of its components.
Chapter 18 Polymers 249
Rubber
The name vulcanized (from Vulcan, the Greek god of fire) comes from the
manner in which the polymer was first prepared by heating rubber in the
presence of sulfur.
Condensation of isoprene (2-methyl-1,3-butadiene) either leads to a
polymer in which all double bonds are trans ("natural rubber") or in
which they are cis (gutta-percha).
/ \
natural rubber
Examine structures of the different forms of rubber provided. Which is
natural rubber and which is guttapercha? How many monomers are in each
strand?
Natural rubber is known to be more "elastic" (deformable) than gutta-
percha. Is there any obvious difference in the structures in the two
strands which might lead to a difference in the properties of the real
polymers?
Rubber used in practical applications is crosslinked through disulfide (-
S-S-) bonds, and is known as "vulcanized" rubber. Can you name another
important class of "polymers" which are crosslinked through disulfide
bonds? Examine vulcanized rubber. How many individual strands does it
comprise? Are these strands of natural rubber or of gutta-percha? What is
the percentage (by weight) of sulfur incorporated into the polymer? (The
molecular weight of the sample is 1701 amu.) Does this classify as a low-
sulfur polymer (<3%), a high-sulfur polymer (>10%) or in between?
Can you provide a qualitative explanation of why natural rubber becomes
more and more hard (less deformable) as the percentage of incorporated
sulfur is increased?
250 Chapter 18 Polymers
Alkene Polymerization
Stable transition-metal complexes may act as homogenous catalysts in
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