1334
Chemistry Letters Vol.35, No.12 (2006)
Intermolecular Proton Transfer from Formaldehyde Intermediate to Anisole
in Noncatalytic Pyrolysis: Phenol Produced without Hydrolysis
Yasuo Tsujino, Chihiro Wakai, Nobuyuki Matubayasi, and Masaru Nakaharaꢀ
Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011
(Received August 29, 2006; CL-060984; E-mail: nakahara@scl.kyoto-u.ac.jp)
New reaction mechanism has been found for the pyrolysis
(mol dmꢁ3) in the homogeneous gas phase (corresponding to
0.25 MPa at 400 ꢂC, according to the ideal-gas law). Dilute ace-
tone solution of anisole was used to sample a small amount of
anisole in a volumetric manner; solvent acetone was evaporated
prior to the reaction. Most of the experimental procedures taken
here are the same as before.2–6 Without opening the sample tube,
the quenched gas phase was subjected to 1H and 13C NMR meas-
urements at room temperature using ECA400 (JEOL). After the
NMR measurements on the gas phase, the reactor tube was
opened and acetone-d6 and TMS (tetramethylsilane; the internal
reference) were added to detect the products and residual reac-
tant. The integrated spectral intensities provide the yields, which
are defined as the concentration of each compound divided by
the initial concentration of anisole. To quantify all species and
check mass balance, 13C NMR without proton irradiation was
of anisole according to the total product analysis based on the
liquid- and gas-phase 1H and 13C NMR spectroscopy. Formalde-
hyde and benzene are produced in the first rate-determining step
by the intramolecular proton transfer, and phenol, methane, and
carbon monoxide in equal amounts successively through the fast
intermolecular proton and hydride transfers from hot formalde-
hyde to anisole.
For the past decades, not heterolytic but homolytic mecha-
nisms have been considered for the pyrolysis of organic com-
pounds in the gas phase because of the lower bond dissociation
energy.1 Homolytic bond cleavage is commonly considered in
the oxygen-used combustion and high-energy photochemical
reactions. Recently, however, we have revealed heterolytic py-
rolysis for the high-temperature neat and hydrothermal reactions
of ethers and aldehydes.2–6 These reactions are green and attrac-
tive as an alternative to the conventional because of the absence
of hazardous catalysts and organic solvents. Heterolytic reac-
tions are dominant and homolytic ones are negligible in the
high-temperature reactions of oxygen-containing ethers2,3 and
aldehydes.4–6 For symmetric aliphatic ethers previously studied,
it has been found that heterolytic or ionic reactions are induced
by intramolecular proton transfer in the neat gas conditions at
high temperatures. Here, we show that the intermolecular proton
transfer occurs as well as the intramolecular one in the high-
temperature neat gas reaction of methyl phenyl ether (methoxy-
benzene, anisole) in the absence of catalysts or solvent.
1
performed in addition to H NMR.
To establish the reaction pathways of anisole at 400–430 ꢂC,
we have analyzed all of the products. To disclose a new pathway,
it is necessary to identify and quantify all the species with and
without hydrogens and to check the mass balance. Thus, 1H
and 13C NMR spectra are measured in both the gas and liquid
phases as a function of reaction time (5–20 h). The NMR
elemental and structural analysis has led us to find that anisole
undergoes the high-temperature intermolecular proton transfer
to generate phenol without water.
Products of the 20-h anisole reaction at 400 ꢂC have been
identified for the initial concentration of 1.0 M according to
the 1H spectrum shown in Figure 1. Carbon monoxide was
detected at 185 ppm by 13C NMR. The main products (yields
parenthesized) are:
For a better understanding of ether decomposition kinetics
and pathway control, anisole is superior to the symmetric ethers
because it is an asymmetric chain ether and the intramolecular
proton transfer occurs only in one way. In consequence, reactive
formaldehyde (HCHOꢀ) and inert benzene are formed as the
oxidized and reduced fragments, respectively. This is a simplifi-
cation of the complex reaction kinetics and mechanisms of neat
ether reactions at high temperatures.2,3 Formaldehyde (HCHOꢀ)
then acts as a reactive intermediate for the subsequent reaction
step. In previous papers2–7 it has been shown that formaldehyde
can induce new noncatalytic hydrothermal disproportionations
with themselves and other molecules. These high-temperature
reactions, though not induced without water, suggest a high
reactivity of the hydrogens in monomeric formaldehyde or its
hydrate (methanediol) that exists only as an oligomer (formalin)
or a ring (1,3,5-trioxane) in ambient conditions. Here, we
demonstrate the proton-transfer mechanism of anisole by
applying NMR spectroscopy to the quenched neat gas reaction
mixture of anisole.
C6H6 (0.24) ꢃ C6H5OH (0.27) ꢃ CH4 (0.25)
ð1Þ
ꢃ CO (0.28)
These products are yielded in almost equal amounts. The yield
sum is close to unity; an excess of 4% is due to the experimental
uncertainty. Furthermore, there are detected formaldehyde,
toluene, and methanol in very small amounts. Formaldehyde is
found to play a key role as a reactive intermediate, and the others
are considered to be by-products. It is to be noted that phenol is
generated without hydrolysis. These products require new
reaction pathways for the high-temperature neat gas reaction.
All of the main products shown above can be understood by
considering the following reaction pathways:
C6H5OCH3 ! C6H6 þ HCHOꢀ
C6H5OCH3 þ HCHOꢀ ! C6H5OH þ CH4 þ CO
ð2Þ
ð3Þ
Here, HCHOꢀ denotes a hot formaldehyde molecule that has
an excess energy due to the bond-fission activation. As shown
below, the process given by eq 2 is a slow process and eq 3
corresponds to the fast. In eq 2, anisole is first fragmented into
Anisole (Nacalai; purity >99%) was used without further
purification. It was loaded into a volume-known quartz tube of
1.5-mm i.d. and 3.0-mm o.d. to a desired concentration 1.0 M
Copyright Ó 2006 The Chemical Society of Japan
Tsujino, Yasuo
