Chemistry Reference
In-Depth Information
Oils with conjugated FAs showed lower reactivity than oils without conju-
gated FAs. The same group
13
demonstrated the reaction between H
2
S and
olefinic bonds in fatty compounds can be carried out not only under radical
initiation conditions, but also under nucleophilic catalytic conditions.
Nevertheless, the radical version of the thiol-ene reaction has been used
exclusively for bio-based material synthesis.
4.3.2 Kinetics of the Thiol-ene Reaction
4.3.2.1 Kinetics of the Thiol-ene Reaction (Single Ene)
An in-depth study of the mechanism and kinetics of the isomerization of
lipids in the presence of thiyl radicals was carried by Chatgilialoglu et al.
14-19
Since their focus was on understanding a live biological system, they carried
out their experiments with low thiol concentrations where the addition
reaction is not favored. A summary of their rate constants is shown in
Table 4.2. A schematic of the reactions, whose rate constants are included in
Table 4.2, is shown in Scheme 4.2. The table also includes related data from
other authors.
20-26
Biermann et al.
27
reported that the thiol and alkene can form an electron-
donor-electron-acceptor complex, which reacts with another thiol molecule
to generate radicals that initiate the thiol-ene chain.
Other studies investigated the kinetics of the reaction, without reporting
rate constants. Claudino et al.
28
investigated the reaction between a trifunc-
tional thiol (2-ethyl-(hydroxymethyl)-1,3-propanediol trimercapto propionate)
and methyl oleate and methyl elaidate. They monitored the reaction with
time-resolved FT-IR, Raman and NMR spectroscopy. They determined that the
reaction of the addition of the thiyl radical to the double bond is fast and
reversible, leading to cis-trans isomerization of the double bond, and that the
cis
-/
trans
- isomerization of the double bonds:
k
1
Z
k
H
R
2
H
H
1
E
H
H
R
1
H
R
1
R
2
R
1
R
2
k
k
-1
Z
-1
E
+
+
R
S
RS
RS
RSH
R
1
'
R
2
'
k
d
RS
R
k
H
H
S
R
1
'
R
2
'
H
H
k
k
d
R
1
H
H
R
1
R
1
R
2
R
2
n
R
S
n
-
2)
R
2
+(
R
S
R
1
R
2
Homopolymerization
(formation of telomers)
Chain transfer
Scheme 4.2 The propagation reactions of the thiol-ene reaction. R
1
and R
2
are the
remainder of the FA (most often oleic, R
1
¼
n-C
8
H
17
,R
2
¼
-CH
27
COOR
0
or vice versa).
