V.M. Zelikman et al. / Journal of Molecular Catalysis A: Chemical 407 (2015) 60–66
65
а
60
40
20
0
b
0
2
4
6
8
10
Mole ratio n-PrOH : CBr4
tion of CBr4 with decane. (ɑ) – In the presence of the catalyst (I), 10 wt.%), (b) –
without a catalyst. The molar ratio CBr4:C10H22 = 1:10, reaction temperature 135 ◦C.
Fig.
9. Kinetics
of
bromination
in
C10H22-CBr4–chlorine-containing
catalys–n-propanol system. Initial conditions: reaction volume of 0.225 ml,
[CBr4]0 = 0.44 mol/L, [EtOH]0 = 1.6 mol/L, [n-PrOH]0 = 1.3 mol/L, catalyst weighed
2.6 × 10−2 g: 1 – catalyst (III) + n-PrOH, 2 – catalyst (III) + EtOH, 3 – catalyst (IV) +
EtOH, 4 – catalyst (V) + EtOH. Reaction temperature 135 ◦C.
reaction with temperature. Indeed, the reaction mixture in the
experiment at 135 ◦C (Fig. 8, curve 2) still contained a consider-
of CBr4. Another piece of evidence is that the alcohol added in a
moderate amount quantity in the absence of the catalyst did not
change significantly the product buildup pattern (curve 3 in Fig. 8).
As the temperature was elevated above the indicated values
(descending portion of curve 2 in Fig. 8), the contribution of
thermally induced dissociation of CBr4 increased. Under these con-
ditions, the “shielding” role of alcohols decreased and gradually lost
its value. Moreover, some associates can have even a detrimental
effect, transferring the radical chain via the hot radicals formed in
reaction (10) to the ligands of the active complex. The accelerated
(catalytic or thermally initiated) consumption of the donor additive
and the progressively increasing rate of catalyst deactivation (prob-
ably involving the alcohol in the process) dramatically decreased
3.5. Effect of alcohols on the catalytic bromination of decane with
tetrabromomethane
In our previous studies of the reaction of decane with tetra-
chloromethane in the presence of Cu(II) chloride complexes with
the chlorination products [22,23]. Ethanol and n-propanol, which
had exhibited the highest efficiency, considerably increased the
degree of bromination in the catalytic reaction with CBr4 as well. A
comparison of the data of Table 3 with those in Figs. 8 and 9 shows
that the yield of bromination products almost virtually doubled in
the C10H22-CBr4-catalyst-donor additive system. However, this
effect had some limitations, in particular, as regards the reaction
temperature and the amount of added alcohol.
It is noteworthy that as in case of the reactions in the absence of
alcohol (see above), there was no significant difference in activity
between the bromine- and chlorine-containing catalysts (Fig. 9).
The temperature dependence of the yield of monobromode-
canes in the presence of catalyst and n-PrOH (Fig. 8, curve 2) has a
distinct extremum. Such a shape of the curve can be explained as
ligand complexes with copper halide derivatives or/and formation
of the solvation shell around the polar moiety of the catalyst, but
also can directly participate in chain initiation (10) and propagation
(11) [22,23]:
An increase in the amount of the alcohol in the system resulted
in higher yields of bromodecanes but only to a certain limit. This
effect was observed for both catalyzed and non-catalyzed reactions.
Curve b in Fig. 10, which corresponds to the latter case, shows the
dependence of the yield of bromodecanes on the amount of n-
propanol added at 135◦C. In this case, the products were formed
under the conditions when the thermally initiated dissociation
of tetrabromomethane did not result in a considerable change
in the composition of the initial reaction system (see Table 2).
The occurrence of the reaction was due to the fact that a certain
amount of •Br and •CBr3 radicals was formed to have an energy
[CuBr4]2− + CH3CH2CH2OH → [CuBr3]2− + CH3CH2•CHOH + HBr(9)
sufficient to abstract the ␣-hydrogen atoms from alcohols (C
H
bond dissociation energy is 385 kJ/mol for ethanol [38]). The radi-
cal products thus formed are capable of initiating a new chain that
can involve alkane molecules as well (the C H bond energy for
secondary bonds of linear alkanes is 399–405 kJ/mol [39]). This
reaction pathway was confirmed by the observed formation of the
noticeable amount of the acetal CH3CH2CH(OCH2CH2CH3)2, which
resulted from the reaction of propionaldehyde with n-propanol.
The aldehyde is probably the product of the enol rearrangement of
nation of 1-bromopropan-1-ol, a quite likely process under the
given conditions. Hydrogen bromide appeared in this reaction can
serve as a catalyst for the acetal formation.
CH3CH2•CHOH + CBr4 → CH3CH2CH2(Br)OH + •CBr3
(10)
The alcohol bromination products (mono- and dibromo-
propanols) were detected by the GC/MS technique. Undergoing
catalytic or/and thermal bromination, an alcohol simultaneously
“shields” the organic ligands, thereby preventing the catalytic com-
plex from degradation.
As the temperature increased, such associates with an alcohol
(in particular, n-propanol) were destroyed, thus making the QABs
fragments containing C H bonds more accessible for an attack by
free radicals. However, in the temperature range up to 130–140◦C,
this factor is apparently counterbalanced by an increase in the
formation rate of bromodecanes (see below). Although the alco-
hol itself undergoes bromination, it insignificantly competes with
decane at a low or medium concentration. This conclusion is sup-
ported by an increase in the chain length of the major radical
The initial segment of curve b in Fig. 10 shows an increase in
the yield of bromodecanes with an increase in the alcohol concen-
tration (positive effect), which is due to increase in the number of
propagation steps upon the thermal dissociation of CBr4, result-