4856 J. Phys. Chem. A, Vol. 107, No. 24, 2003
Lifshitz et al.
B. Ring Expansion. a. Introductory Remarks. As can be seen
vicinity of the radical site to the nitrogen atom in the ring. As
can be seen in Figure 3, the yield of 2-picoline is somewhat
higher than that of 5-picoline, indicating that the insertion of
the methylene radical into a C-C bond is somewhat easier that
the insertion into a C-N bond.
in the product distribution (Figure 3), the products of ring
expansion are among the products of the highest concentration.
It has also been shown in the study on the decomposition of
8
N-methylpyrrole that ring expansion toward the formation of
pyridine is one of the major processes in its thermal reactions.
It was assumed that ring expansion takes place from methylene
pyrrole radicals rather than from methyl pyrrole itself. In the
expansion process, the hydropyridyl radical is formed, and by
a fast ejection of a hydrogen atom (∆H ∼ 23 kcal/mol) that
follows, pyridine is obtained. This assumption was later verified
by detailed quantum chemical calculations for ring expansion
We have demonstrated in the past that the ring expansion of
N-methylene pyrrole was much faster than the process in the
- and the 3-methylene pyrrole isomers, having a barrier of only
2
∼
35 kcal/mol, compared to 61 and 74 kcal/mol for the other
7
two. We have also found (yet unpublished) that the isomer-
ization barrier for the isomerization from the 2 position to the
N position is equal to ∼50 kcal/mol. We have therefore
concluded that the ring expansion will always take place from
N-methylene pyrrole, regardless of what is the reacting isomer.
From the 2- and 3-methylene pyrrole, this will take place
following the isomerization process that is the rate determining
step. For this reason, we used an activation energy of 48 kcal/
mol for reactions 33 and 34 that can be considered to some
extent as global reactions.
7
from N-methylene and 2 and 3-methylene pyrrole. On the other
hand, the first transition state on the potential energy surface
of the ring expansion process, when N-methylpyrrole is the
starting molecule, lies about 140 kcal/mol above the energy level
7
of N-methylpyrrole. In the transition state, in addition to N-C
bond breaking, one of the hydrogen atoms originating from the
methyl group is already far removed from its original site on
the carbon atom to a distance of 2.35 Å. This means that the
energy level of the transition state is determined by both H-atom
C. Free Radical Reactions vs Unimolecular Decomposi-
tions. As can be seen in Table 1 and Figure 2, there is a
considerable yield of low molecular weight products, both with
and without nitrogen. However, a reaction scheme based on
free radical reactions alone, where the free radical concentrations
are determined by the initiation steps discussed previously, could
not account for the yields of almost all of the decomposition
products. We thus believe that unimolecular ring cleavage
processes that produce unstable intermediates play also an
important role in the decomposition mechanism. Unimolecular
decompositions can take place from both the reactant itself and
from unstable species that are formed after the reactant has lost
a methyl group or a hydrogen atom (reactions 1-4 and 5-8).
The various unimolecular ring cleavage processes starting from
the reactant require much higher activation energies even just
owing to thermochemical considerations. In fact, sensitivity
analyses show that the reactant dissociation is much less
important than its dissociation after losing a hydrogen atom or
a methyl group. Only the latter was thus considered in the final
kinetic scheme. There are many possible reaction channels for
unimolecular cleavage of five membered heterocyclics. We
introduced into the kinetic scheme steps that are known in
7
ejection and N-C bond cleavage. We have assumed that here
too, the ring expansion process takes place from methylene
methyl pyrrole rather than dimethylpyrrole itself.
There are three possible ring expansion processes in 2,4-
dimethylpyrrole. Two processes produce two isomers of picoline
(methyl pyridine), 2-picoline, and 5-picoline. One process yields
pyridine. All of the three reactions take place from radical
species. The two picoline isomers are formed from methylene
methylpyrrole. This is a species that is formed after removing
a hydrogen atom from a methyl group attached to the ring.
Pyridine is formed from a species that is formed after the
removal of a methyl group from the molecule.
b. Mechanism. The ring expansion mechanism is based on
insertion of a methylene group into a C-C or a C-N bond in
the pyrrole ring. It can be expressed schematically as
1
4-17
similar systems
and several assumed channels. A few of
these reaction steps are listed in Scheme 1.
It should be mentioned, however, that by no means these
reactions should be considered as one concerted step each. They
probably involve a series of consecutive reactions that can only
be established by quantum chemical calculations. For the
purpose of modeling, we have estimated an apparent rate
constant for the entire process which is expressed by a simple
unimolecular reaction.
in methylene methylpyrrole, and as
D. Computer Modeling. a. Reaction Scheme. To model the
observed product distribution, we have constructed the reaction
scheme that is shown in Table 3. The scheme contains 36 species
and 69 elementary reactions. The symbol (R) at the end of a
reaction in the scheme indicates that, after a reaction time of 2
ms (or earlier), the reaction proceeds in the reverse direction.
The rate constants listed in the table are given as k ) A exp-
in methyl pyrrolyl and in methylene pyrrole.
Each methylene group in methylene methylpyrrole can, in
principle, be inserted into two different locations in the ring,
but the final products of the ring expansion are the same.
However, owing to the presence of double bonds in the molecule
that are not in full resonance with the single bonds, the insertion
into C-C or C-N single bonds has a lower barrier than the
insertion into a CdC double bond.
n
(-E/RT) or k ) A′T exp(-E/RT) when the rate constant taken
from the database fits a wide temperature range. The units are
3
-1
-1
cm , s , kcal, and mol . The Arrhenius parameters for the
reactions in the scheme are either estimated or taken from
various literature sources. These sources are specific articles
relevant to the present system and databases, mainly the NIST-
The only difference between the two radicals that are obtained
by removing hydrogen atoms from the methyl groups is in the
18
Kinetic Standard Reference Data Base 17. The parameters for
Lifshitz, Assa
