Mechanism of reaction of n-butane with but-2-enes in the presence of LaCaX faujasites

Article abstract of DOI:10.1023/A:1016020430731

The reactions of n-butane and an n-butane (80 mol. percent)-isobutane (20 mol. percent) mixture with but-2-enes in the presence of polycationic PdLaCaX faujasites were studied. Quantum-chemical calculation of the enthalpies of formation of alkanes C4-C8 a

Full text of DOI:10.1023/A:1016020430731

Russian Chemical Bulletin, International Edition, Vol. 51, No. 5, pp. 783—788, May, 2002
783
Mechanism of reaction of nꢀbutane with butꢀ2ꢀenes
in the presence of LaCaX faujasites
A. L. Bachurikhin,aꢀ E. S. Mortikov, V. Yu. Gribanov, and I. A. Abronin
a
b
b
aN. D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences,
4
7 Leninsky prosp., 119991 Moscow, Russian Federation.
Fax: +7 (095) 135 5328. Eꢀmail: secretary@ioc.ac.ru
b
A. N. Kosygin Moscow State Textile Academy,
1
ul. Malaya Kaluzhskaya, 119991 Moscow, Russian Federation.
Fax: +7 (095) 952 1440. Eꢀmail: office@msta.ac.ru
The reactions of nꢀbutane and an nꢀbutane (80 mol.%)—isobutane (20 mol.%) mixture
with butꢀ2ꢀenes in the presence of polycationic PdLaCaX faujasites were studied. Quantumꢀ
chemical calculation of the enthalpies of formation of alkanes C —C and their cations
4
8
showed that the reaction [Bnn]+
[Bu ] is of crucial importance for the isomeric
i +
composition of the products of alkane alkylation. The general scheme of transformations of
the hydrocarbons in the alkylation of nꢀ and isoꢀparaffins was proposed based on the experiꢀ
mental data on the distribution of the C isomers in the catalyzate at different temperatures
8
and duration of the reaction.
Key words: nꢀbutane, isobutane, butꢀ2ꢀenes, isomerization, trimethylpentanes, dimethylꢀ
hexanes, Xꢀfaujasite, alkylation gasoline, quantumꢀchemical semiempirical calculations, alkyꢀ
lation.
The growth of the world automobile fleet and related
Experimental
ecological problems require the exclusion of toxic leadꢀ
based antiknock additives from gasoline and lowering
the content of aromatic hydrocarbons. Therefore, an inꢀ
crease in the production of alkylation gasoline, which
presently takes approximately one seventh of the total
gasoline produced in the world, is urgent. The most part
of alkylation gasoline is manufactured1 by the reaction
of isobutane with olefins С —С in the presence of H SO
4
The catalysts were based on zeolites X (faujasites), which
are well known in petrochemistry, much more sensitive to
elevated temperatures than highꢀsilica Yꢀfaujasites, and
7
characterized by the maximum activity in alkylation. The
La0.18—0.22Са0.21—0.15Na0.04[(Al Si1.40)O4.80]•nH O catalysts
1
2
—5
were prepared by ion exchange of the starting NaX form with
solutions of calcium and lanthanum nitrates.
2
5
2
Palladium (0.3—0.5 wt.%) was introduced into these samples
by ion exchange. A mixture of 70 wt.% zeolite and 30 wt.%
pseudoboehmite was extruded and thermally treated at 450 °С.
The resulting catalyst was tested in a riser reactor with the
ascendant stock flow.
The following starting products were used: (1) a mixture of
nꢀbutane and isobutane obtained by nꢀbutane isomerization on
zeolites with similar composition at 300 °С (content of isoꢀ
butane was 20—25 mol.%)8 with an additive of butꢀ2ꢀenes
(2 wt.%) with respect to the total weight of hydrocarbons and
or HF. These methods have a serious disadvantage: forꢀ
mation of poorly utilizable acidic tars and the use of only
isoꢀparaffins as raw materials. In this connection, the
development of new processes using heterogeneous cataꢀ
lysts becomes more important. Such processes are more
environmentally safe, and they can involve unbranched
paraffins, in particular, nꢀbutane, in alkylation reactions
especially because the supply of the latter substantially
6
exceed resources of deficient isobutane. A study of the
(
2) a mixture of nꢀbutane and butꢀ2ꢀenes with the 50 : 1
weight ratio.
Experiments were carried out in the 50—150 °С temperaꢀ
ture range. The pressure (Р) was maintained at a level of
.0—5.0 MPa to keep the raw materials in the liquidꢀphase
mechanism of transformations of nꢀparaffins in the presꢀ
ence of olefins is of special interest. A comparative analysis
of the behavior of nꢀ and isoꢀparaffins in alkylation makes
it possible to evaluate the present concepts on the speꢀ
cific features of particular stages in traditional mechaꢀ
nisms of these reactions and check the adequacy of the
description of the real routes of nꢀalkane transformaꢀ
tions by these concepts.
4
state in the reaction zone. The space feed rate for the alkylaꢀ
tion of a mixture of isobutane and nꢀbutane (hereinafter named
–
1
the isomerizate) was 4 h , and that for nꢀbutane alkylation
was 2 h–
1
.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 721—726, May, 2002.
066ꢀ5285/02/5105ꢀ783 $27.00 © 2002 Plenum Publishing Corporation
1

784
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
Bachurikhin et al.
Reaction products were analyzed by GLC using an
data allow one to compare the ease of hydride ion elimiꢀ
nation from the tertiary and secondary C atoms of the
nꢀbutane molecule.
Ekokhromꢀ93 programꢀapparatus complex consisting of a
Hewlett Packard 5709 chromatograph with a flameꢀionization
detector and an analogꢀtoꢀdigital converter. The CHROM 1.0
for Windows program was applied in calculations.
The features of particular stages of paraffin alkylation
by olefins have been studied in detail in recent deꢀ
The enthalpies of formation (∆H ) for isomeric paraffins С4
f
1—5,8—17
cades.
Many researchers agree that the mechaꢀ
and С8 and their cations were calculated by the quantumꢀ
chemical РМ3 method using the HyperChem PC program.
nism proposed by Schmerling9
—11
correctly reflects, as a
whole, the essence of processes during paraffin alkylaꢀ
tion by olefins despite some incompleteness and inaccuꢀ
racy. According to this mechanism, the alkylation of
isobutane with nꢀbutenes affords a mixture of different triꢀ
methylpentanes (TMP) (mainly 2,2,4ꢀTMP, 2,2,3ꢀTMP,
Results and Discussion
The temperature plots of the conversion, the relative
yield of the liquid product, and the content of hydrocarꢀ
2
,3,4ꢀTMP, and 2,3,3ꢀTMP). This conclusion was conꢀ
bons С in the reaction products are extremal, and the
firmed in many works on isobutane alkylation with
8
position of the extreme in the temperature scale are apꢀ
proximately the same in all cases (Fig. 1). A substantial
difference in the reaction temperatures at which the maxiꢀ
mum catalyst activity is achieved (125 °С for nꢀbutane
and 75 °С for the isomerizate) indicates that nꢀbutane is
much less reactive than its mixture with isobutane. These
Y (wt.%)
40
a
2
3
0
Parameter (%)
1
a
20
2
5
8
6
0
0
1
0
6
3
3
4
7
8
1
1
2
3
4
5 t/h
Y (wt.%)
b
4
0
2
5
0
75
100
125 T/°C
Parameter (%)
00
b
1
1
2
3
30
8
6
4
2
0
0
0
0
2
0
3
1
5
10
6
8
7
4
1
2
3
4
5 t/h
5
0
75
100
125
T/°C
Fig. 2. Content (Y) of 2,2,4ꢀ (1), 2,3,3ꢀ (2), 2,3,4ꢀ (3),
2,2,3ꢀTMP (4), 3,4ꢀ (5), 2,4ꢀ (6), 2,5ꢀ (7), and 2,3ꢀDMH (8)
Fig. 1. Conversion (1), yield of the liquid products (2), and
content of hydrocarbons С in the reaction products (3) as a
in the fraction С as a function of time of the catalyst working
8
8
function of the temperature of alkylation of the isomerizate (a)
and nꢀbutane (b) with butꢀ2ꢀenes on PdLaCaXꢀfaujasites
(t) for the alkylation of nꢀbutane (a) and isomerizate (b) with
butꢀ2ꢀenes on PdLaCaXꢀfaujasites at 125 (a) and 75 °С (b)
(Р = 4.0 MPa).
(
Р = 4.0 MPa, the first h of the catalysts working).

Mechanism of alkylation of nꢀparaffins
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
785
Table 1. Content of trimethylpentanes (TMP) and dimethylhexanes (DMH) in products С formed by the alkylation of nꢀbutane
8
and isomerization (figures in parentheses) with butꢀ2ꢀenes on PdLaCaX faujasites
Т/°C
Content of isomer (wt.%)
2,3,4ꢀTMP 2,2,3ꢀTMP 3,4ꢀDMH
2
,2,4ꢀTMP
2,3,3ꢀTMP
2,4ꢀDMH
2,5ꢀDMH
2,3ꢀDMH
5
7
1
1
1
0
5
00
25
50
3.2 (11.4)
3.3 (15.8)
3.6 (16.3)
5.5 (17.5)
6.2
38.0 (42.4)
37.9 (36.1)
39.1 (34.7)
39.2 (32.1)
38.3
12.7 (23.6)
12.4 (18.5)
15.5 (18.1)
15.5 (17.1)
15.1
3.3 (4.7)
3.8 (5.4)
5.5 (5.0)
5.9 (4.1)
6.3
14.4 (4.3)
14.8 (4.9)
14.9 (5.3)
15.3 (6.0)
16.2
11.9 (4.3)
12.2 (8.1)
13.8 (10.0)
14.3 (11.9)
15.2
6.6 (2.2)
6.2 (4.8)
5.5 (5.1)
2.1 (5.7)
1.9
3.9 (6.1)
3.5 (5.5)
2.1 (5.3)
1.4 (5.0)
0.6
nꢀbutenes in the presence of liquid and heterogeneous
catalysts.1
Accepting such a scheme for nꢀbutane, we see that
the main reaction products should be dimethylhexane
In order to explain this unusual, at first glance, reꢀ
sult, we advanced several assumptions concerning parꢀ
ticular pathways of nꢀbutane transformations, which could
give the experimentally observed isomeric composition
of the reaction products.
—5,8—17
(
DMH) isomers (mainly 3,4ꢀ, 2,4ꢀ, 2,2ꢀ, and 2,5ꢀDMH).
However, according to the data in Table 1 and Fig. 2, a,
the total amount of TMP in nꢀbutane alkylation with
butꢀ2ꢀenes on PdLaCaXꢀfaujasites is 1.2—2.0 times
greater than the amount of DMH isomers. Thus,
nꢀbutane alkylation affords the products, whose isomeric
composition does not correspond to the Schmerling
mechanism.
It can be assumed that nꢀbutane is partially isomerꢀ
ized to isobutane, which is much more reactive than
nꢀbutane, and this is precisely isobutane which really
reacts. However, this assertion meets the objection that
the TMP/DMH ratio increases with the temperature inꢀ
crease to ∼125 °С (Fig. 3, curve 1) (along with this, the
yield of the liquid products also considerably increases,
see Fig. 1, b), although the temperature increase should
be accompanied by the enhancement of TMP isomerꢀ
ization to more thermodynamically stable DMH. Thereꢀ
fore, the TMP/DMH ratio should more likely decrease
with temperature, as it takes place in the alkylation of
the isomerizate (see Fig. 3, curve 2).
1. nꢀButane is isomerized to isobutane before its transꢀ
formation into the nꢀbutyl cation. Thus, the isobutyl
cation formed from isobutane reacts instead of the nꢀbutyl
cation (e.g., according to the Schmerling mechanism).
The DMH isomers can form by both the direct alkylaꢀ
tion of nꢀbutane with butꢀ2ꢀenes and isomerization of
all possible TMP and TMP (TMP+ are isomerized to
DMH , and the latter are transformed into DMH), as
well as by the alkylation of isobutane with nꢀbutꢀ1ꢀene
formed from butꢀ2ꢀenes.
+
+
2. The TMP isomers are formed due to the isomerꢀ
+
+
ization of all possible DMH (DMH are isomerized to
+
TMP , and those are transformed into TMP), which, in
turn, are the products of addition of the nꢀbutyl cation to
the butꢀ2ꢀene molecules. The formation of TMP from
DMH is improbable because the most of DMH isomers
are thermodynamically more stable than TMP under the
experimental conditions.
3. The nꢀbutyl cation is isomerized at the moment of
formation (when a hydride ion is eliminated from the
nꢀbutane molecule) to the isobutyl cation. The further
transformations of the isobutyl cations occur in approxiꢀ
mately the same manner as it is described in point 1 (like
the formation of the DMH isomers).
TMP/DMH
5
The first assumption is rejected by the results of studyꢀ
ing8 nꢀbutane isomerization, according to which it
does not virtually occur on the catalysts at temperaꢀ
tures <200 °С.
In order to verify two other assumptions, we comꢀ
pared the experimental data (see Figs. 1—3, Table 1)
with the results of quantumꢀchemical calculation of the
4
2
3
2
enthalpy of formation of the isomeric paraffins С , DMH,
TMP, and their cations (Table 2).
4
1
As known, the rearrangements of the carbocations
under discussion are characterized by rather low activaꢀ
tion energies (∼10 kcal mol ). Therefore, based on the
enthalpy of rearrangements of different carbocations, one
can predict (with an accuracy sufficient for qualitative
5
0
75
100
125
T/°C
–
1
Fig. 3. TMP/DMH ratio as a function of the temperature of
alkylation of nꢀbutane (1) and isomerizate (2) with butꢀ2ꢀenes
on PdLaCaX faujasites (P = 4.0 MPa).

786
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
Bachurikhin et al.
n +
i +
Table 2. Enthalpies of formation (∆H ) of isomeric
of the enthalpy change (|∆Н |) in the [Bu ] /[Bu ] catꢀ
f
paraffins С , DMH, TMP, and their cations calcuꢀ
4
ion rearrangement (with the charge on the C(2) atom) is
the highest among the corresponding values for all the
most possible rearrangements and equals 12.02 kcal mol–
Thus, the butyl cations formed by hydride ion elimiꢀ
nation from the nꢀbutane molecule can isomerize to
isobutyl cations yet before addition to the butꢀ2ꢀene molꢀ
ecules. The rate of approaching the reversible reaction
lated by the PM3 method
1
.
Paraffin
∆H /kcal mol–1
f
Molecule
Cation*
nꢀButane
Isobutane
–29.76
–31.41
–51.65
178.59 (2)
190.61 (2)
155.59 (2),
n +
i +
[
Bu ]
[Bu ] (with the charge on the C(2) atom)
2
,3ꢀDMH
to the thermodynamic equilibrium increases with temꢀ
perature. This results in an increase in the conversion of
the butyl cation to the thermodynamically more stable
isobutyl cation. However, since the addition of the isobuꢀ
tyl cation to butꢀ2ꢀenes and subsequent transformations
afford trimethylpentanes (unlike these, nꢀbutyl affords
dimethylhexanes), their content in the alkylate increases
with temperature. The TMP/DMH ratio also increases
1
1
1
55.88 (3),
69.43 (4),
65.98 (5)
2
,4ꢀDMH
–52.40
154.19 (2),
1
1
69.58 (3),
55.17 (4),
1
66.22 (5)
153.05 (2),
67.11 (3)
168.14 (2),
57.50 (3)
156.44 (3),
69.47 (4)
171.63 (3),
53.70 (4)
156.54 (2),
58.03 (3)
157.64 (2),
71.26 (4)
2
3
2
2
2
2
,5ꢀDMH
,4ꢀDMH
,2,3ꢀTMP
,2,4ꢀTMP
,3,4ꢀTMP
,3,3ꢀTMP
–53.02
–51.07
–52.41
–53.62
–51.92
–51.68
1
(
see Fig. 3).
The further temperature increase is accompanied by
1
n +
i +
the maximum approach of the [Bu ]
[Bu ] transꢀ
1
formation to the thermodynamic equilibrium, due to
i +
n +
which the [Bu ] /[Bu ] ratio stops increasing. Howꢀ
1
ever, the degree of isomerization of all possible TMP+
and TMP isomers to thermodynamically more stable
DMH+ and DMH increases. This results in a decrease
the TMP/DMH ratio and the appearance of an insignifiꢀ
cant maximum in the plot of this quantity vs. temperaꢀ
ture of nꢀbutane alkylation by butꢀ2ꢀenes (see Fig. 3).
However, numerous studies did not reveal the speꢀ
cific routes of the mutual transformations of the cationic
TMP and DMH+ intermediates, which finally defines
the isomeric composition of the reaction products. Acꢀ
cording to the data in Table 3, the enthalpies of formaꢀ
1
1
*
The position of the C atom on which the charge is
localized in indicated in parentheses.
+
evaluation) the main reaction routes and prevailing isoꢀ
mers in the reaction products.
Several important conclusions can be drawn from the
data presented in Table 2.
First, carbocations, whose charge is localized on the
tertiary C atom, are, as a rule, more stable than the
corresponding cations with the charge on the secondary
C atoms (by at least 10 kcal mol ). Therefore, these
cations should isomerize, immediately on their formaꢀ
tion, to cations with the charged tertiary C atoms. Reꢀ
+
+
tion of all the most stable TMP and DMH cations are
very close, which does not allow a certain conclusion
about the predomination of particular reaction pathways.
At the same time, it is seen in Table 1 and Fig. 2 that
when either nꢀbutane or the isomerizate are used as the
raw materials, a correlation between the concentrations
of 2,3,3ꢀTMP and 2,3,4ꢀTMP in the reaction products
is observed. Meanwhile, no similar correlation between
the contents of these isomers and 2,2,4ꢀTMP exists. This
fact allows us to conclude that these hydrocarbons are
formed via different reaction routes (Schemes 1 and 2,
–
1
9
—11
mind that according to the Schmerling mechanism,
+
the 2,2,3ꢀTMP cation with the charge on the C(4) rapꢀ
idly isomerizes at the moment of formation to the more
stable tertiary TMP cations.
Second, our results reject the assumption that the
TMP isomers are formed from the cationic DMH. Alꢀ
most all DMH ions with the charged tertiary C atom
are more stable than similar TMP ions (except for the
structures in which the methyl groups in DMH are next
to each other), although the difference is usually only
–
1
∆H/kcal mol for cation rearrangements are indicated
by figures in Scheme 1).
+
The addition of the isobutyl cation with the charge
on the C(2) atom to the butꢀ2ꢀene molecule affords the
2,2,3ꢀTMP cation with the charge on the C(4) atom,
which can further transform via three main routes (see
Scheme 1). The most probable are pathways (2) and (3),
which provide the maximum energy gain at the very
first stages (∆Н for them are –13.03 and –11.44 to
+
+
–
1
1
—2 kcal mol . Therefore, it is assumed that the
TMP to DMH+ rearrangements do not either preꢀ
+
dominate.
–
1
The third and most important conclusion the comꢀ
pletely confirms our third assumption. The absolute value
–12.93 kcal mol , respectively). This conclusion is conꢀ
firmed by our experimental data, according to which the

Mechanism of alkylation of nꢀparaffins
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
787
Scheme 1
alkylation of nꢀbutane and the isomerizate with butꢀ2ꢀ
enes affords most 2,3,3ꢀTMP of the TMP isomers
isobutane) and 2,2,4ꢀTMP in the reaction products. Howꢀ
ever, a certain contribution of reaction pathway (1) in
Scheme 1 cannot either be excluded.
(
2
route (2)) and somewhat less (by 1.5—3.0 times)
,3,4ꢀTMP (route (3)). The latter can be explained by
The DMH isomers are also formed via different routes,
depending on a paraffin reacting with olefins at the first
stage of the reaction. Scheme 3 mainly corresponds to
the formation of the DMH isomers when nꢀbutane is
used as a paraffin. This is indicated by the high content
of 3,4ꢀDMH in the reaction products (see Table 1,
Fig. 2, a), which increases with temperature due to the
intensification of alkylation (see Table 1).
the fact that for similar rearrangements the rate of hydride
transfer is much higher than that of methyl transfer (this
is precisely the difference between routes (2) and (3)).
The existence of a single intermediate through which
2
,3,3ꢀ and 2,3,4ꢀTMP are formed gives an insight into
the same ratio between these isomers during the whole
testing period of catalysts (see Fig. 2).
Unlike 2,3,3ꢀ and 2,3,4ꢀTMP, the most amount of
2
,2,4ꢀTMP is formed, most likely, according to Scheme 2
Scheme 3
involving isobutylene as an olefin. This is indicated by
the absence of a correlation between the amounts of
2
,3,3ꢀ and 2,3,4ꢀTMP that formed, on the one hand,
and 2,2,4ꢀTMP, on the other hand, and also by the
parallel change in the content of isobutylene (as well as
3
,4ꢀDMH+
+
DMH and DMH isomers
Scheme 2
When the isomerizate containing a sufficient amount
of isobutane is alkylated, the DMH isomers are likely
formed predominantly according to Scheme 4. This is
indicated by the high content of 2,3ꢀDMH in the alkylaꢀ
tion products (see Table 1, Fig. 2, b) because 2,3ꢀDMH
is formed immediately from 2,2ꢀDMH with the charge
on the C(3) atom.
Thus, the study of the alkylation of nꢀbutane and
nꢀbutane—isobutane mixture obtained by nꢀbutane
2
,2,4ꢀTMP

788
Russ.Chem.Bull., Int.Ed., Vol. 51, No. 5, May, 2002
Bachurikhin et al.
Scheme 4
formation (main intermediate is the 2,2ꢀDMH cation
with the charge on the C(3) atom).
This work was financially supported by the Russian
Foundation for Basic Research (Project No. 99ꢀ03ꢀ
2
,2ꢀDMH+
3
2145).
+
DMH and DMH isomers
References
isomerization (isomerizate) in the presence of the LaCaX
faujasite catalyst suggests that the traditional models of
alkylation mechanisms cannot adequately describe the
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1
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Petroleum Hydrocarbons], Gostoptekhizdat, Moscow, 1959,
, 305 (Russ. Transl.).
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3
2
of formation (∆H ) of several isooctanes and their catꢀ
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f
ions, which play an important role in the stages of transꢀ
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1
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4
5
6
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3
3 (Russ. Transl.).
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,
7
8
. W. E. Garwood and P. B. Venuto, J. Catal., 1968, 11, 175.
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Transl.)].
was concluded. This transformation affects both the isoꢀ
meric composition of the reaction products and the qualiꢀ
tative parameters of the process (conversion of olefins,
the yield of the liquid product, TMP/DMH ratio, and,
as a consequence, the octane number of the alkylate
obtained). The increase in the TMP/DMH ratio with
temperature of the reaction on PdLaCaXꢀfaujasites is
caused by the shift of the indicated equilibrium toward
the thermodynamically more stable isobutyl cation. The
9
. L. Schmerling, J. Am. Chem. Soc., 1945, 61, 1152.
1
1
0. L. Schmerling, Ind. Eng. Chem., 1948, 40, 2072.
1. K. I. Patrilyak, Yu. N. Sidorenko, and V. A. Bortyshevskii,
Alkilirovanie na tseolitakh [Alkylation on Zeolites], Naukova
Dumka, Kiev, 1991, 176 pp. (in Russian).
2. J. E. Hofmann and A. Schriesheim, J. Am. Chem. Soc.,
1962, 84, 953.
13. J. E. Hofmann, J. Org. Chim., 1964, 29, 1497.
4. L. F. Olbrait, B. M. Dosin, and M. A. Ferman, in
Alkilirovanie. Issledovanie promyshlennoe oformlenie
protsessa [Alkylation. Study and Industrial Design of the Proꢀ
cess], Khimiya, Moscow, 1982, 87 (in Russian).
5. A. Martines, A. Martines, and C. Martines, Catal. Rev.,
Sci. Eng., 1993, 35, 483.
6. J. Weitkamp and S. Maixner, Chemie, 1983, 36, 11.
7. J. Weitkamp, Proc. Intern. Symp. on Zeolite Catalysis,
Zeocat´85, Siofok (Hungary), 1985, 690.
1
1
2
2
,3,3ꢀ and 2,3,4ꢀTMP isomers, on the one hand, and
,2,4ꢀTMP, on the other hand, are formed via different
i
reaction routes but, in all cases, the single initial interꢀ
mediate exists, viz., 2,2,3ꢀTMP cation with the charge
on the C(4) atom, which is the product of the direct
addition of the isobutyl cation to the butꢀ2ꢀene molꢀ
ecule. The DMH isomers are also formed through difꢀ
ferent reaction pathways. When nꢀbutane is the starting
material, the reaction involves 3,4ꢀDMH and its catꢀ
ions. When isobutane in the composition of the isoꢀ
merizate is used, the products are formed via 2,2ꢀDMH
1
1
1
Received July 25, 2001;
in revised form January 13, 2002