Chemistry Reference
In-Depth Information
release of ring strain when the double bond is attacked. The higher reactivity
of the norbornene double bond compared to 1-octene was also reported by
other authors.
31
The rate-limiting step is the hydrogen transfer (chain trans-
fer),
25,30
indicating there is no apparent dependence on the ene concen-
tration, but a dependence on the thiol concentration. cis-2-Hexene is much
more reactive than trans-2-hexene. The higher reactivity of a primary alkene
compared to internal double bonds led to the use of undecylenic acid and its
esters for the bio-based thiol-ene reaction. Undecylenic acid is a product of the
pyrolysis of ricinoleic acid, the main FA in castor oil.
Ferreri et al.
17
showed that the isomerization of the double bonds in
vesicles depends on the proximity of the double bonds to the vesicle-solvent
interface.
17
The hydrophilic HOCH
2
CH
2
S
radical, used in their study,
isomerized all the double bonds (oleic, linoleic, or arachidonoic) at the same
rate, independent of their position, when the FAs were freely dissolved.
When the lipid chains were within large unilamellar vesicles, the double
bonds, closer to the vesicle-water interface (closer to the head -COO
group)
were isomerized much faster than those further from it.
Bantchev et al.
32
reacted butanethiol with corn and canola oil under UV
light. One of the observations was that the reaction is faster at lower tem-
peratures - an observation that is in agreement with the negative activation
energy for the thiol-ene reaction reported by Sivertz et al.
5
Again, the authors
explained the observation with the reversibility of the first step of the re-
action and with shifting the equilibrium of the first step towards the
unreacted materials with the increase in temperature.
4.3.2.2 The Effect of bis-Allylic Hydrogens
The reaction of thiyl radicals with compounds having two double bounds
(like linoleic or linolenic acid) can be influenced by the side-reaction of a
thiyl radical abstracting a bis-allylic radical. The kinetics of this reaction
were first studied by Schoneich et al.
26
They reported the kinetic rates to be
on the order of (0.3-3.1)
10
7
M
1
s
1
. The more lipophilic thiols showed a
higher rate and the rate constants were higher in polar solvents.
The abstraction of bis-allylic hydrogens by thiyl radicals can change the
thiyl radical concentration and, correspondingly, the reaction rate.
Samuelsson et al.
33
and Desroches et al.
34
observed that linoleate reacts
much more slowly than oleate when they react separately with thiols.
Bantchev et al.
32
observed that the oleate double bond and the linoleate
double bond in corn oil react at the same rate. These two observations can be
reconciled using the results for phospholipids produced by Ferreri et al.:
14
the oleic double bonds reacted with the thiyl radical more quickly than the
linoleic double bonds did, when there were no linoleic groups present.
When both oleic and linoleic moieties were present simultaneously in the
system, the rate of reaction of the linoleic moiety did not change, but the rate
of reaction of the oleic moiety decreased significantly, to a rate corres-
ponding to the same rate of reaction per double bond as the linoleic moiety.
