Chemistry Reference
In-Depth Information
Table 4.1 Bond dissociation energies (BDEs). BDEs in the current work
were calculated using structures that had been geometry
optimized with the Parameterized Model number 3 (PM3)
and Unrestricted Hartree-Fock/PM3 methods. The BDEs are
calculated in heat of formation energies.
D/kJ mol
1
Bond
Reference
Me(Et)(H)C-H
405
1
Allylic C-H
344
2
363
1
Bis-allylic C-H
305
3
Bis-allylic C-H
313
2
MeS-H
360
1
BuS-H
356
Current work
H
2
P-H
345
1
(MeO)
2
(O)P-H
366
Current work
C
ΒΌ
C-C: :C
614
4
C-C
347
4
BuS-CH
3
272
Current work
(MeO)
2
(O)P-CH
3
266
Current work
the overall decomposition process leads to isomerization of the double
bonds to their thermodynamic equilibrium cis-trans mixture of
30/70%.
The importance of thiol molecules in biological systems (the most note-
worthy being cysteine) has generated significant academic interest in the
impact of this reaction, especially since it can lead to in vivo formation of
trans-FAs from cis-FAs. Sivertz et al.
5
seem to be the first to report that the
apparent activation energy of the reaction of a thiol with an alkene is
negative (i.e. the rate is faster at lower temperatures) for some solvents. From
this observation, they deduced that Step 1 of the cycle (the addition of the
thiyl radical to the double bond) is reversible. Later, Walling and Helmreich
6
and Sivertz
7
reported experimental data for the cis-2-butene isomerization to
trans-2-butene in the presence of thiyl radicals. Neureiter and Bordwell
8
published a series of experiments with the addition of thiol-acetic acid to
2-chloro-2-butene. From the ratio of the erythro- to threo-addition products,
it was concluded the thiol radical adds to the double bond without forming a
bridged radical intermediate.
Kircher
9
was the first to investigate the isomerization of lipids in the
presence of thiols. The reaction was faster under direct sunlight, and influ-
enced by the presence of oxygen, which strongly indicated a radical mech-
anism. The isomerization rate was independent of the oleate concentration
in most of the measurements. Isomerization of double bonds was faster for
methyl oleate and olive oil than for methyl linoleate and methyl linolenate.
Schwab et al.
10-13
were the first to use the thiol-ene reaction to obtain
bio-based products. They showed that under UV light irradiation, H
2
S
adds more slowly to linseed oil than to methyl oleate.
10
Later,
11
they showed
the thiol-ene reaction to be zero-order with respect to the double bond
concentration and the reaction rate was oleic4
B
B
2
linoleic4
B
3
linolenic.
