"Ipso" aromatic nitration in the gas phase

Article abstract of DOI:10.1021/jp951563d

Nitration of selected dialkylbenzenes, including o- and p-cymene, p-rerf-butyltoluene, p-diisopropylbenzene, and p-di-rert-butylbenzene, by protonated methyl nitrate, (CH₃OH-NO₂)⁺, has been investigated in the gas phase in the pressure range from 10~8 to 720 Torr by Fourier transform ion cyclotron resonance (FT-ICR) and collisional activated dissociation (CAD) mass spectrometry and by the radiolytic technique. The results provide the first demonstration of gas-phase electrophilic aromatic nitrodealkylation and a quantitative evaluation of its efficiency relative to the competing nitration process. Dealkylation occurs exclusively following attack of the nitrating cation at the "ipso" position, which promotes intramolecular migration of the alkyl substituent to the nitro group, yielding O-alkylated or O-protonated ions. The positional selectivity of the nitration, and the relative rate of the "ipso" attack by (CH₃OH-NO₂)⁺ are discussed, especially as regards the role of the intrinsic steric requirements of the alkyl group, which in the gas phase is unaffected by the many complicating factors, e.g., solvation, ion pairing, etc., operative in solution. The results are compared with those concerning condensed-phase nitrodealkylation and the related nitrodesilylation reaction.

Full text of DOI:10.1021/jp951563d

4424
J. Phys. Chem. 1996, 100, 4424-4429
“Ipso” Aromatic Nitration in the Gas Phase
M. Attina`, F. Cacace,* and A. Ricci
UniVersita` di Camerino, V.S. Agostino 1, 62032 Camerino, Macerata, Italy,
UniVersita` di Roma “La Sapienza”, P. le Aldo Moro 5, 00185 Rome, Italy
ReceiVed: June 6, 1995; In Final Form: December 20, 1995X
Nitration of selected dialkylbenzenes, including o- and p-cymene, p-tert-butyltoluene, p-diisopropylbenzene,
and p-di-tert-butylbenzene, by protonated methyl nitrate, (CH3OH-NO2)+, has been investigated in the gas
phase in the pressure range from 10-8 to 720 Torr by Fourier transform ion cyclotron resonance (FT-ICR)
and collisional activated dissociation (CAD) mass spectrometry and by the radiolytic technique. The results
provide the first demonstration of gas-phase electrophilic aromatic nitrodealkylation and a quantitative evaluation
of its efficiency relative to the competing nitration process. Dealkylation occurs exclusively following attack
of the nitrating cation at the “ipso” position, which promotes intramolecular migration of the alkyl substituent
to the nitro group, yielding O-alkylated or O-protonated ions. The positional selectivity of the nitration, and
the relative rate of the “ipso” attack by (CH3OH-NO2)+ are discussed, especially as regards the role of the
intrinsic steric requirements of the alkyl group, which in the gas phase is unaffected by the many complicating
factors, e.g., solvation, ion pairing, etc., operative in solution. The results are compared with those concerning
condensed-phase nitrodealkylation and the related nitrodesilylation reaction.
Introduction
Aromatic nitration at a ring position carrying a substituent
Results
Radiolytic Experiments. The nitrating cation has been
other than H has long been deduced from the formation of
products characterized by the loss of the substituent attached
to the nitrated site,1-3 but its nature was fully recognized only
in 1971 by Perrin and Skinner,4 who coined the prefix “ipso”
currently used to denote the reaction. Ipso nitration has since
been the subject of extensive studies, whose results opened up
new perspectives in the theory of aromatic substitution.5-7 The
mechanistic picture outlined by the above studies, all performed
in solution, is complicated by the influence of the medium
especially as concerns the fate of the primary ipso nitrated
Wheland intermediate, generally denoted as the Wi adduct.
Depending on the nucleophilicity of the medium, the presence
of specific anions and other nucleophiles, etc., the Wi adduct
can react in different ways, e.g., by migration of the ipso group,
by capture by the nucleophile, by substituent modification, or
by return to reactants.
obtained according to the well-established reaction sequence
+
CH4 Df CnH5
(n ) 1, 2)
(1)
CnH5+ + CH3ONO2 f CnH4 + (CH3OH-NO2)+ (2)
promoted by the radiation-induced ionization of methane
containing a small mole fraction of methyl nitrate.11 Following
thermalization by multiple collisions with CH4 molecules, the
cation reacts with the aromatic substrate S, present in trace
amounts in the system, yielding arenium ions, e.g.
+
(CH3OH-NO2)+ + C6H5X f CH3OH + C6H5XNO2 (3)
In the case of alkylbenzenes a fraction of the arenium ions
can undergo dealkylation. Eventually, the gaseous cations are
quenched by a gaseous base, B. The nature, yields, and isomeric
composition of the neutral end products are established by GC
or GC/MS. The absolute yields of the products reported in
Table 1, expressed by their G(+M) values, i.e., the number of
molecules formed per 100 eV, are affected by a relatively large
uncertainty, estimated to be as high as (20%, owing to the
many sources of errors, especially as regards the radiation dose
actually absorbed by the gas, the presence of adventitious or
radiolytically formed impurities, etc. Nevertheless, consider-
ation of the absolute yields is of interest, showing that nitration
and/or nitrodealkylation are indeed major reaction channels in
the gas phase, the combined G(+M) values of their products
ranging from 0.7 to 1.5 in systems containing no deliberately
added bases. On the basis of the G value for the formation of
the CnH5+ primary ions from the irradiation of CH4,16 the above
G(+M) values correspond to yields of 25-50%. The ionic origin
of the nitrated and nitrodealkylated products is suggested by
their very nature, by the presence of radical and thermal electron
scavengers, and especially by the depression of their yields
A considerable simplification can be expected by the exten-
sion of the study to the gas phase, an environment largely
free of the complicating effects typical of condensed media
and one which allows, in addition, more direct correlation
of the experimental results with those from theoretical ap-
proaches.
We report here on the gas-phase nitrodealkylation of selected
dialkylbenzenes promoted by protonated methyl nitrate, (CH3-
OH-NO2)+, whose most stable isomer, protonated on the
methoxy oxygen, is in essence a nitronium ion solvated by a
methanol molecule, as shown by mutually supporting theoretical
and experimental results.8-10 The reactivity of (CH3OH-NO2)+
toward aromatics has been previously investigated by a com-
bination of mass spectrometric and radiolytic techniques,11-13
complemented by a recent theoretical study.14 We have
followed the same experimental approach15 to investigate the
gas-phase nitration of substrates selected for their recognized
ability to undergo nitrodealkylation in condensed media.
X Abstract published in AdVance ACS Abstracts, February 15, 1996.
0022-3654/96/20100-4424$12.00/0 © 1996 American Chemical Society

“Ipso” Aromatic Nitration in the Gas Phase
J. Phys. Chem., Vol. 100, No. 11, 1996 4425
TABLE 1: Aromatic Nitrodealkylation by Radiolytically Fomed (CH3OH-NO2)+ Ions in Methane at 720 Torr
a All systems contained CH3ONO2 (10 Torr), a thermal electron scavenger (SF6, 5 Torr) and a radical scavenger (O2, 10 Torr). b Standard deviation
of the results, 10%. c Apparent substrate/mesitylene reactivity ratio. d Evaluated indirectly from the k(p-tert-butyltoluene/k(p-di-tert-butyltoluene) ratio.
caused by addition of a base, e.g., (C2H5)3N, that competes with
the substrate for the (CH3OH-NO2)+ cation, besides intercept-
+
ing a small fraction of the primary CnH5 ions. The experi-
mental dependence of the nitration yields on the base concen-
tration quantitatively fits the trend predicted by a simple model
which takes into account only the competition between the base
and the substrate,17 as illustrated in Figure 1, that refers to
nitration of p-diisopropylbenzene. Remarkably, even in the
presence of a 3-fold excess (0.70 Torr vs 0.24 Torr) of (C2H5)3N
over the substrate, which reduces the lifetime of the nitrated
arenium ions down to ca. 10-8 s, the relative rates of nitration
and nitrodealkylation are unaffected.
The isomeric composition of the nitrated products shows a
significant bias for substitution ortho to a methyl, rather than
to an isopropyl group, e.g., o-cymene yields the following
isomers, where the position of the NO2 group is italicized: 3
) 6%, 4 ) 42%, 5 ) 21%, 6 ) 31%, close to that obtained
Figure 1. Dependence of the yields on the concentration of added
using NO2+BF4- in sulfolane, i.e., 3 ) 1%, 4 ) 42%, 5 ) 19%,
(C2H5)3N in the nitration of p-diisopropylbenzene in CH4 at 720 Torr,
6 ) 37%, whereas a somewhat different composition is obtained
from mixed-acids nitration.19 The same trend characterizes the
gas-phase nitration of p-cymene and p-tert-butyltoluenes, as
apparent from Table 1.
315 K. G° is the yield measured in the absence of added bases.
Salient features of the isomeric composition of the nitrode-
alkylated products is that the nitro group is invariably found at

4426 J. Phys. Chem., Vol. 100, No. 11, 1996
Attina´ et al.
RC6H4NO2 + i-C3H7OH2+ f
a ring position formerly occupied by one of the alkyl substituents
and that the relative position of the latter ones is unchanged.
In addition to nitrated products, alkylated products were
detected, e.g., nitration of p-diisopropylbenzene gave triisopro-
H2O + RC6H4N(O)+O-i-C3H7 (4)
+
which is known to occur selectively at the nitro group, yielding
an O-alkylated ion.21 In a typical case, the ions (R ) CH3)
were prepared in the external source according to reaction 4
from the CH4/i-C3H7OH CI of o-nitrotoluene, introduced into
the resonance cell, isolated, and allowed to react with selected
bases/nucleophiles. Although kinetic measurements were pre-
vented by the low intensity of the O-alkylated ions, the nature
of the charged products, and their dependence on the strength
of the base proved informative. CH3C6H4NO2H+ ions, m/z )
138, were formed in the presence of bases of relatively low
pylbenzenes, whose yields accounted for ca. 50% of the i-C3H7
lost in the nitrodeisopropylation, suggestive of intermolecular
R+ transfer from the ipso-nitrated arenium ion. This inference
is supported by the effect of a gaseous base, (C2H5)3N, that
almost entirely suppresses the alkylated products. To verify
the mechanistically informative evidence for the occurrence of
transalkylation, experiments were performed where the gaseous
system contained mesitylene, a highly activated arene that was
expected to efficiently trap any R+ ions from the ipso-nitrated
arenium ions. In a typical experiment, nitration of p-diisopro-
pylbenzene (0.32 Torr) in the presence of mesitylene (0.41 Torr)
gave equal amounts of p-nitrocumene (G(+M) ) 0.18) and of
PA, e.g., C6H6, PA ) 181.3 kcal mol-1 22
Stronger bases, such
.
as mesitylene, PA ) 200.7 kcal mol-1, were protonated but
not alkylated, whereas an exceptionally strong base/nucleophile,
(CH3O)3PO, PA ) 212.0 kcal mol-1, was observed to undergo
both protonation and alkylation by CH3C6H4N(O)+O-i-C3H7
ions. All the above observations point to the propensity of the
ions from (4) to undergo dealkylation, either unimolecularly
by loss of propene or upon reaction with gaseous bases/
nucleophiles, under conditions typical of FT-ICR experiments.
The structure of the nitrated adducts from reaction 3 was also
+
isopropylmesitylene, showing that the i-C3H7 ions from the
nitrodeisopropylation were quantitatively trapped by mesitylene.
The temperature has a measurable effect on the nitrodealky-
lation rate, that is enhanced at higher temperatures with respect
to that of nitration. A detailed study has been performed only
in the case of p-diisopropylbenzene, where the ratio of the rate
constants for nitration (kN) and for nitrodealkylation (kND) in
CH4 at 720 Torr has been measured at 315, 323, 333, 343, and
353 K. The Arrhenius plot obtained fits the linear equation
ln(kN/kND) ) -4.09 + 1331/T, whose correlation coefficient R
) 0.989 is consistent with the analytical errors.
+
probed by FT-ICR techniques, e.g., the (CH3C6H4-i-C3H7)NO2
ions obtained from the CH4/CH3ONO2 CI of o-cymene in the
external ion source were isolated and allowed to react with
gaseous bases, e.g., (CH3O)3PO. Efficient proton transfer was
observed, which, according to the criterion customarily used to
discriminate between ion-neutral complexes and covalently
bound intermediates,23 provides strong evidence that the species
being probed contain a covalently bound nitro group and hence
have the arenium-ion and/or the O-alkylated ion structure.
Collisionally Activated Dissociation (CAD) Spectrometry.
Additional structural insight was sought by recording the CAD
spectra of the nitrated adducts obtained from selected dialkyl-
benzenes according to reaction 3. In general, the CH4/CH3-
ONO2 CI spectra of typical dialkylbenzenes such as the
cymenes, tert-butyltoluenes, diisopropylbenzenes, di-tert-bu-
tylbenzenes, etc., recorded at 100-150 °C at total pressures of
0.1-0.4 Torr display significant (M + NO2)+ peaks.
Attention was focused on p-cymene, recording the CAD
spectra of populations of C10H14NO2+ ions obtained from three
different reactions. Population A was prepared from the CH4/
CH3ONO2 CI of p-cymene according to reaction 3, as previously
established by the FT-ICR results outlined in the previous
section. Population B consisted of C10H14NO2+ ions from the
CH4/i-C3H7OH CI of p-nitrotoluene, according to the O-
alkylation process (eq 4). Population C was obtained by the
CH4 CI of o-nitro-p-cymene according to the unselective proton-
transfer process
Fourier Transform Ion Cyclotron Resonance (FT-ICR)
Mass Spectrometry. In the low-pressure range typical of FT-
ICR experiments, the study of the nitration and nitrodealkylation
reactions promoted by (CH3OH-NO2)+ is complicated by the
incursion of predominant, often overwhelming, charge-transfer
and proton-transfer processes, favored by the low ionization
potential and the high proton affinity (PA) of the dialkylben-
zenes, and whose rate critically depends on the excess energy
of the nitrating cation. For instance, (CH3OH-NO2)+ ions
obtained according to reactions 1 and 2 from the CI of CH4/
CH3ONO2 mixtures in the external source of a FT-ICR
spectrometer, driven into the resonance cell and allowed to react
with p-diisopropylbenzene undergo almost exclusively charge
exchange, yielding but traces of the expected nitrated and
nitrodealkylated adducts. A definite improvement was achieved
by application of the collisional thermalization technique
developed by Tho¨lmann and Gru¨tzmacher.20 Accordingly, the
(CH3OH-NO2)+ ions formed in the external CI source were
driven into the resonance cell, containing p-diisopropylbenzene
at pressures ranging from 10-8 to 10-7 Torr. Following a 2-s
delay, the (CH3OH-NO2)+ ions were isolated by ejecting all
other ions by low-power radiofrequency “shots” and CH4 was
introduced via a pulsed valve, reaching a peak pressure up to
10-4 Torr for a short time (1-5 ms). Under such conditions,
appreciable intensities of ions whose m/z ratios correspond to
those of the C12H18NO2+ nitrated adduct(s) (m/z ) 208.133 75)
+
and of the C9H12NO2 nitrodealkylated adduct(s) (m/z )
166.086 80) ions were detected and identified by exact mass
measurement.
Remarkably, the relative abundance of the nitrodealkylated
adduct(s), ca. 3% after the reaction time of 3 s in p-
diisopropylbenzene at 10-7 Torr, exceeds by an order of
magnitude that of the nitrated adduct, pointing to the facile
dealkylation of the latter under the low-pressure conditions
which is expected to yield O-protonated ions, as well as arenium
ions, some of which are protonated at the ring position carrying
the i-C3H7 group.
The spectra, compared in Table 2, are qualitatively similar,
the only conspicuous difference concerning the abundance of
the charged fragment at m/z ) 91, much higher from population
B. This is hardly surprising, in that the fragment is likely arising
+
prevailing in the FT-ICR experiments. No C3H7 or (CH3-
OC3H7)H+ ions were detected, suggesting that dealkylation
involves loss of a neutral species, most likely C3H6. To verify
such a hypothesis and to ascertain the nature of the species
undergoing dealkylation, nitrated adducts were prepared by a
different route according to the reaction
+
from the O-isopropylated ion, the most abundant C10H14NO2
isomer in population B, according to the simple bond-fission
process

“Ipso” Aromatic Nitration in the Gas Phase
J. Phys. Chem., Vol. 100, No. 11, 1996 4427
+
TABLE 2: CAD Spectra of C10H14NO2 Ionic Populations
Mechanism of the Alkyl Group Removal. The scheme
outlined in Chart 1 provides a satisfactory rationalization of the
results. Formation of the Wi intermediate from (CH3OH-
NO2)+ is necessarily accompanied by formation of CH3OH,
whose local concentration is very high, the two species being
initially contained in the same ion-neutral complex. Methanol,
from Different Reactions
rel intensities (%)a
fragment m/z
Ab
Bc
Cd
39
41
3.1
2.6
2.9
2.5
8.0
1.8
2.5
PA ) 181.9 kcal mol-1 22
, is undoubtedly capable of promoting
43
3.8
2.0
51
3.1
1.8
exothermic dealkylation of Wi according to reactions 10a and
10b. No evidence for these processes has been provided by
the mass spectrometric and radiolytic experiments, whose results
suggest instead that dealkylation requires bases/nucleophiles
stronger than CH3OH, e.g., mesitylene in the case of p-
diisopropylbenzene. A reasonable explanation, independently
supported by the evidence from CAD spectrometry, can be based
on the fast R+ migration (eq 13a) to the NO2 group, a process
which is strongly exothermic, owing to the much higher basicity/
nucleophilicity of the nitro group than of the ipso position of
the ring. The exothermicity of migration (eq 13a) can promote
fragmentation (eq 13b) of the O-alkylated ion, the only process
observable under the low-pressure conditions typical of FT-
ICR experiments.
63
3.6
3.8
8.2
2.3
30.6
1.9
65
4.0
2.7
77
5.6
4.9
91
12.7
4.8
11.9
4.4
103
105
107
115
117
121
148
162
10.8
3.7
10.4
13.7
M
4.9
6.2
3.7
4.5
11.3
2.7
10.7
15.8
M
3.7
8.3
31.1
a The intensities are normalized with respect to the sum of the
intensities of a spectrum that represents the average of 50 scans. The
intensities of fragments containing metastable contributions (M) have
not been taken into account. b From reaction 3. c From reaction 4.
d From reaction 5.
+
There is no evidence for intraannular NO2 migration (eq
11) nor for R+ migration (eq 12), which is ruled out by the
failure to detect products where the relative position of the R
and R′ substituents has changed. In summary, the radiolytic
and mass spectrometric results suggest intramolecular migration
of the alkyl group bound to the ipso-nitrated carbon to the nitro
group, the same process undergone by the H atom of the nitrated
ring position in gas-phase nitration.13 Both processes reflect
the strong tendency, typical of isolated ions in the gas phase,
to achieve the most efficient intramolecular solvation of the
charge, which in the case of interest favors binding of the R+
cation to more nucleophilic nitro group rather than to the
strongly deactivated aromatic ring.
Remarkably, however, the fragment of m/z ) 91 is present
as well, if in lower abundances, in the CAD spectra of
population A and C, obtained from reactions 3 and 5 which are
not expected to yield, at least as primary ionic intermediates,
O-isopropylated ions. A plausible explanation is that a fraction
of the arenium ions from nitration (eq 3) and protonation (eq
5) isomerizes into the O-isopropylated ion structure in the time
interval, ca. 10 µs, between formation and structural assay:
Positional Selectivity of the Nitration. It is apparent from
the foregoing that the extent of nitrodealkylation measured in
the radiolytic experiments depends exclusively on the rate of
ipso attack, being unaffected by the subsequent reactions
undergone by the Wi intermediate, as confirmed by the
insensitivity of the nitration/nitrodealkylation ratio to the addition
of bases/nucleophiles. Since the gaseous Wi intermediate is
bound to evolve into the nitrodealkylated product, the ratio of
ipso substitution corresponds to that of ipso attack and hence,
at variance with solution-chemistry studies, there is no need to
worry about the effects of possible subsequent reactions that
affect the composition of the products in solution, e.g., nucleo-
philic substitution or nucleophilic capture yielding cyclohexa-
dienyl derivatives.
Discussion
Previous mass spectrometric and radiolytic studies have
shown that the efficient gas-phase nitration reaction promoted
by protonated methyl nitrate involves formation of arenium ions
In principle, accurate evaluation of the intrinsic positional
selectivity of nitration is possible in the gas phase, an expectation
confirmed by the limited set of data from competition experi-
ments where mesitylene was employed as a convenient second-
ary standard.24 Although the fi values obtained in this way
should be regarded as grossly approximate, their comparison
with those of unsubstituted ring positions activated to a similar
extent is of interest, in that it allows evaluation of intrinsic steric
effects in unsolvated species.
which subsequently undergo intramolecular proton transfer to
the nitro group, considerably faster than the competing intraan-
nular migration, e.g., k0/kr ) 22.5 in the nitration of toluene in
CH4 at 720 Torr and 315 K:13
Consider, for instance, the following pairs of substrates:
Passing to nitrodealkylation, two mechanistic problems deserve
consideration, namely, the detailed course of the alkyl group
removal from the ipso-nitrated ring position and the evaluation
of the efficiency of the process relative to nitration, particularly
as regards the effect of steric factors.
It is apparent that the reactivity of the comparably activated

4428 J. Phys. Chem., Vol. 100, No. 11, 1996
Attina´ et al.
CHART 1
para positions is reduced by a factor of 3.3 upon replacement
of H by i-C3H7, whereas replacement by t-C4H9 causes a much
larger, almost 200-fold, decrease of reactivity. Such effects
point to the paramount role of steric hindrance of the substituent
to the ipso attack by (CH3OH-NO2)+, since the observed trend
can hardly be traced to factors other than the intrinsic steric
bulk of the alkyl group. By contrast, the steric acceleration
noted in solution, where o-cymene is nitrodealkylated from 3
to 5 times faster than p-cymene,18,19 is much less effective, if it
all, in the gas phase, where the two isomers undergo nitration
and nitrodealkylation at approximately the same relative rate
(Table 1).
Finally, the temperature dependence of the ratio of the rate
constants for nitration and nitrodealkylation of p-diisopropyl-
benzene, measured in the range from 315 to 355 K, is of interest
since, based on the preceding considerations, it corresponds to
the difference of the activation barriers for ipso attack and for
attack to the four equivalent unsubstituted positions of the
substrate. From the slope of the Arrhenius plot the Ea barrier
for ipso attack is higher by 2.6 ( 0.4 kcal mol-1 than that of
attack to each of the positions ortho to the isopropyl groups.
Nitrodealkylation vs Nitrodesilylation. Comparison of the
present results with those concerning the gas-phase nitration of
phenylsilanes by (CH3OH-NO2)+ 25 reveals significant differ-
ences. Owing to its enhanced basicity, the ring carbon bearing
the SiR3 group becomes an efficient proton sink, and hence
desilylation of positions other than the one substituted by the
nitro group can occur, e.g.
takes place (Table 1), whereas ipso nitrodesilylation accounts
for over 30% of the products from the p-tolylsilane.25 This trend
is supported by a comparison of the partial rate factors for de-
tert-butylation of p-di-tert-butyltoluene, fi ) 0.17, and for
desilylation of p-(trimethylsilyl)toluene, fi ) 18.3,25 pointing
to the much higher tendency of phenylsilanes to undergo ipso
attack and subsequent desilylation.
Experimental Section
Materials. The gases and most chemicals used in the mass
spectrometric and the radiolytic experiments, or as reference
standards in GC and GC/MS were research-grade products
obtained from commercial sources (Matheson Gas Products, Inc.
and Aldrich-Chemie GmbH). Other products e.g., isopropyl-
and diisopropylnitrobenzenes were prepared according to stan-
dard procedures and assayed by the same techniques used for
the analysis of the radiolytic products.
Mass Spectrometric Experiments. The FT-ICR study was
performed using a Bruker-Spectrospin Apex TM 47e spectrom-
eter equipped with an external CI ion source, a pulsed valve,
two separate inlets, and a XMASS TM data system. The
external CI ions source was operated at 150 °C, under a total
pressure of 10-5 Torr. The stationary pressure of the neutral
reagents in the resonance cell was in the range from 10-8
-
10-7 Torr. CAD spectra were recorded using a ZAB-2F
instrument (VG Micromass Ltd.) operated in the CI mode.
Typical experimental conditions were source temperature 180
°C, emission current 0.5 mA, repeller voltage 0 V, total pressure
in the ion source 0.1-0.4 Torr. MIKE spectra were recorded
at an energy resolution of ca. 5 × 103 and represent the average
of at least 40 scans. CAD spectra were taken at a lower energy
resolution by admitting He into the collision cell at such a
pressure to reduce the main beam intensity by ca. 30% with
respect to its initial value.
As to the ipso attack proper, the enhanced basicity/nucleo-
philicity of the position carrying the SiR3 substituent, combined
with the smaller steric hindrance of the Si(CH3)3 than of the
C(CH3)3 group, make nitrodesilylation much more efficient than
nitrode-tert-butylation, as apparent from a comparison of the
behavior of p-tert-butyltoluene and of p-(trimethylsilyl)toluene.
No appreciable nitrode-tert-butylation of the former substrate
Radiolytic Experiments. The gaseous samples were pre-
pared according to standard procedures using a greaseless
vacuum line, in 280-mL Pyrex vessels and irradiated to a total
dose of 5 × 103 Gy at a dose rate of 2 × 104 Gy h-1 in a
thermostatically controlled 220 Gammacell from Nuclear Canada
Ltd. The radiolytic products were extracted with methanol,

“Ipso” Aromatic Nitration in the Gas Phase
J. Phys. Chem., Vol. 100, No. 11, 1996 4429
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from Hewlett-Packard and a TRIO 1 quadrupole GC/MS
instrument from VG Micromass Inc., on the following col-
umns: (i) a 50-m × 0.20 mm × 0.5 µm capillary column, coated
with a cross-linked methylsilicone phase (PONA column from
HP), operated from 30 to 250 °C, and (ii) a 60-m × 0.25 mm
× 0.25 µm capillary column, coated with poly(20% diphenyl-
80% dimethylsiloxane) (SPB-20 from Supelco Co.), operated
from 60 to 230 °C. The identity of the products was established
by comparison of their retention time with those of authentic
standards and/or by GC/MS, whereas their yields were calcu-
lated from the area of the corresponding elution peaks, relative
to that of the reference standard, using individual calibration
factors to correct for the response of the different compounds
analyzed.
1994, 116, 9535.
(14) Szabo`, K. J.; Ho¨rnfeldt, A.-B.; Gronowitz, S. J. Am. Chem. Soc.
1992, 114, 6827.
(15) Cacace, F. Acc. Chem. Res. 1988, 21, 215.
(16) Ausloos, P.; Lias, S. G.; Gorden, R., Jr. J. Chem. Phys. 1963, 39,
3341.
(17) Given the large excess of CH3ONO2 over (C2H5)3N, the fraction
+
of CnH5 intercepted by the base can be ignored without introducing a
significant error.
(18) Olah, G. A.; Kuhn, S. J. J. Am. Chem. Soc. 1964, 86, 1067.
(19) Manglik, A. K.; Moodie, R. B.; Schofield, K; Tobin, G. D.;
Coombes, R. G.; Hadjigeorgiou, P. J. Chem. Soc., Perkin Trans 2 1980,
1606.
(20) Tho¨lmann, D.; Gru¨tzmacher, H.-Fr. ICR Ion Trap Newslett. 1992,
25, 17.
Acknowledgment. The authors are grateful to G. de Petris
and F. Pepi for useful discussions and the CAD measurements.
The financial support of the Italian Ministero per l’Universita`
e la Ricerca Scientifica e Tecnologica (MURST) and of the
Consiglio Nazionale delle Ricerche (CNR) is gratefully ac-
knowledged.
(21) Kruger, T. L.; Flammang, R.; Litton, J. F.; Cooks, R. G. Tetrahedron
Lett. 1976, 4555.
(22) Nitrobenzene is an oxygen base, having a PA of 193.4 kcal mol-1
,
according to the compilation of: Lias, S. G.; Bartmess, J.-E.; Liebman, J.
F.; Holmes, J. L.; Levin, R. D.; Mallard, W. G. J. Phys. Chem. Ref. Data
1988, 17, Suppl. No. 1. The basicity of the nitrobenzene ring is much lower,
e.g., that of the ipso carbon has been computed to amount to 157.4 kcal
References and Notes
(1) Wheeler, A. S.; Smithy, I. W. J. Am. Chem. Soc. 1921, 43, 2611.
(2) Wheeler, A. S.; Harris, C. R. J. Am. Chem. Soc. 1927, 49, 494.
(3) Nightingale, D. V. Chem. ReV. 1947, 40, 117.
mol-1 11
.
(23) Cacace, F.; Attina`, M.; Fornarini, S. Angew. Chem., Int. Ed. Engl.
1985, 34, 654 and references therein.
(24) From ref 11: kmesitylene/kbenzene) 8.1 ( 0.1.
(25) Attina`, M.; Cacace, F.; Ricci, A. Gazz. Chim. Ital. 1989, 119, 217.
(4) Perrin, C. L.; Skinner, G. A. J. Am. Chem. Soc. 1971, 93, 3389.
(5) Schofield, K. Aromatic Nitration; Cambridge University Press:
Cambridge UK, 1980, and references therein.
(6) Olah, G. A.; Malhotra, R.; Narang, S. C. Nitration; VCH Publish-
ers: New York, 1989.
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