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Gas phase reaction of nitrous acid and methyl nitrite with arenium ions. A new route to electrophilic aromatic nitrosation
~" 4 " . . .
26 April 1996
.~
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters253 (1996) 184-188
Gas phase reaction of nitrous acid and methyl nitrite with arenium ions. A new route to electrophilic aromatic nitrosation F. Cacace, A. Ricci Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive Universit~ "La Sapienza" di Roma, P.le A. Moro, 5, 00185 Rome, Italy
Received 17 November1995;in final form 22 January 1996
Abstract
The occurrence of an alternative route to aromatic nitrosation, based on the reaction of gaseous RC6H ~- arenium ions (R = H, CH 3) with XONO neutrals (X = H, CH3), has been demonstrated by FT-ICR mass spectrometry.
1. Introduction
Studies concerning the nitric oxide chemistry, and in particular the nitrosation processes, are currently an active area of research, owing to their intrinsic fundamental interest and to their impact on fields such as atmospheric chemistry, biochemistry, physiology, toxicology, etc. [1]. Aromatic nitrosation by a variety of reagents, including nitrous acid, the nitrous acidium ion H2NO~-, nitrosonium salts, alkyl nitrites, nitrosyl acetate etc., is a typical electrophilic reaction thoroughly investigated in solution [2-5]. In the gas phase aromatic nitrosation has been studied mainly by ICR mass spectrometry utilizing various charged reagents, according to the general equation XNO ÷ + C6H 6 ~ X + C6H6NO +,
Elsevier ScienceB.V. All rights reserved S0009-261 4(96)00240-0
RC6H ~ + XONO kI
" [RC6H~XONO ]
k_ l
I
(1)
where X = i-C3H7C1, C H 3 0 , CH3OH, and CH3ONO [6-9]. The structure of the C6H6NO + adduct has been probed by photodissociation experiments, whose results suggest that the product from
PII
reactions (1) is a ~r complex showing no tendency to isomerize into a c r complex, nor into O-protonated nitrosobenzene [9,10]. Irrespective of the reagent utilized, gas-phase nitrosation (1) is a conventional electrophilic reaction, whereby formation of the charged reagent and its interaction with the aromatic molecule are temporally and spatially distinct events. An alternative path is conceivable, whereby nitrosation takes place in the ion-neutral complex (INC) formed upon addition of a gaseous arenium ion to a neutral nitrosating species, e.g.
'2 , [RC6Hs(XONO)H+] k_ 2 k3
2 XOH + RC6HsNO +.
(2)
F. Cacace, A. Ricci / Chemical Physics Letters 253 (1996) 184-188
The distinctive feature of sequence (2) is that the nitrosating cation is formed, and the nitrosation occurs, within the INC without separation of the reagents. Here we report experiments aimed at demonstrating the operation of the alternative path (2) and to establish the structure of the nitrosated adduct formed. The experimental approach is based on FT-ICR mass spectrometry, a technique eminently suitable to achieve sharp discrimination between the alternative path (2) and the conventional route (1), the only nitrosation mechanism so far known both in solution and in the gas phase.
185
100 m
r3
80
E]
60 I% 4O
20
0
. . . .
0
4"
A
A
. . . .
I
0.5
/x . . . .
1
I
. . . .
1.5
I
2
. . . .
I
2.5
t(s)
Fig. 1. Time profile of the ionic intensities following introduction of H3 O+ ions into a H 2 0 / H N O 2 / N O x gaseous mixture at a total pressure of 5.0X10 - s Torr. ( m ) H3 O+, m / z = 19; ( * ) H ~ N O f , m / z = 48; (zx) NO + , m / z = 30.
2. Experimental Arenium ions were produced according to the reaction RC6H 5 + CnH ~- ~ CnH 4 + RC6H ~(R = H, CH3) (n = 1, 2),
(3)
by C H J C I in the external ion source of a model 47e APEX FT-ICR mass spectrometer from Bruker Spectrospin AG, fed with diluted R C 6 H s / C H 4 mixtures at a total pressure of = 10 -5 Torr. The arenium ions, excited by the considerable exothermicity of reaction (3), AH ° = - 49.8 kcal m o l - l for n = 1, R = H [11], were driven into the resonance cell and thermalized by collision with Ar, introduced via a pulsed valve and reaching a peak pressure up to - 10 -5 Torr [12]. After a pumping-down period of 3 s, the surviving RC6H ~" ions were isolated by ejecting all other ions by low-power radiofrequency 'shots', and allowed to react with the neutral XONO species, contained in the cell at pressures from 10 -8 tO 10 - 7 Torr. Whereas CH3ONO posed no particular problems, serious experimental difficulties were encountered in the case of HNO 2, which in the gas phase cannot be studied in the pure state, but only in the presence of its decomposition products, i.e. NOx oxides, water vapor, etc. [13,14]. The rapid, and pressure-dependent decomposition of nitrous acid makes it impossible to estimate with any degree of accuracy its actual concentration from total-pressure measurements in the resonance cell. Hence, the HNO 2 concentration, whose knowledge is required
to evaluate the rate constant of process (2), was estimated by an ICR 'titration' method, based on the proton transfer H 3 O + -I- HNO 2 --+ H 2 0
+ H2NO ~"
(4)
characterized by a significant exothermicity, 22.7 kcal mol -] , corresponding to the difference between the PA of water and of nitrous acid, 165.0 [11] and 187.7 kcal m o l - l [15,16], respectively. Proton-transfer reactions such as (4), highly exothermic and involving n-type, unhindered bases, are expected to occur at the rate of collision [17-19], which in turn can be estimated with an accuracy adequate to the purposes of this work using the ADO theory [19], or in the more recent theory by Su and Chesnavich [20]. Thus, measuring the rate of the [H30] + decrease in a gaseous H N O 2 / H 2 0 / N O x mixture affords, in principle, a way to estimate [HNO 2 ], provided of course that there are no 'sinks' other than nitrous acid for the H 3 0 + ions. This is the case, as indicated by the nature of the products formed, i.e. H2NO ~- accompanied by traces of NO + (Fig. 1), showing that indeed reaction (4) can be used to 'titrate' nitrous acid. Accordingly, H 3° ÷ ions produced, thermalized and isolated as described above, were allowed to react with the same gaseous mixtures employed for the study of reaction (2), and the rate of [H3 O+] decrease was used to evaluate [HNO 2 ].
186
F. Cacace, A. Ricci / Chemical Physics Letters 253 (1996) 1 8 4 - 1 8 8 100
3. Results and discussion
Two salient features of the experimental results concur in demonstrating operation of the alternative route (2) to aromatic nitrosation. First, RC6HsNO + nitrosated adducts are invariably formed from the reactions of RC6H ~" ions with XONO neutrals, as illustrated by the typical plots reported in Figs. 2 and 3. In all cases the nitrosated adduct is by far the major product, generally accompanied by minor amounts of (XONO)H + ions. Second, the experimental conditions typical of FT-ICR mass spectrometry deny the role of conventional nitrosation (1) by free (XONO)H + ions, namely RC6H ~ + XONO --} RC6H s + (XONO)H +,
(la)
80
6O I% 40
© o
o
o
o
20
o o 0
.
.
• .
.
.
.
.
.
.
i
•
•
,-L
.
.
5
.
.
•
.
i
.
.
10
.
.
.
.
.
.
.
• 7 15
t{s}
Fig. 2. Time profile of the ionic intensities following introduction of C6H ~- ions into a H 2 0 / H N O : / N O x mixture at a total pressure of 5.0X10 - s Tom ( O ) C6H~-, m / z = 7 9 ; (©) C6H6NO +, m / z = 108; ( A ) H2NO~, m / z = 48.
(XONO)H+ + RC6H 5 ~ XOH + RC6HsNO +. (lb) In fact, no arenes are deliberately introduced into the resonance cell, and although traces may leak from the external ion source, efficient differential pumping reduces their partial pressure well below l 0 - 9 T o r r , far too low to make (lb) a significant source of the RC6HsNO + adducts observed. Furthermore, as apparent from the plots of Figs. 2 and 3, the time profile of the (XONO)H + ions intensity excludes in all cases their role as the precursors of the RC6HsNO + adducts. For comparison purposes we have reexamined the direct nitrosation (1) of benzene by (CH3ONO)H +, previously reported by Reents and Freiser [9,10], utilizing ions obtained in the external source by CH4/CI of methyl nitrite. Table 1 summarizes the thermochemistry and the rate constants of the reactions studied. It is apparent that a particularly large
error bar is attached to the rate constant of the reaction of C6H ~- with HNO 2, which reflects a conservative estimate of the uncertainty affecting the measurements of [HNO 2 ] in the resonance cell, discussed in Section 2. From the results of Table 1 the efficiency of nitrosation sequence (2) ranges from 12 to 26% of the ADO collision rate, whereas direct nitrosation (1) of C 6 H 6 by (CH3ONO)H + occurs at collision rate. Such a trend can be rationalized according to the schematic energy diagram of Fig. 4, assuming that once formed complex 2 is bound to evolve into the RC6HsNO + product, without significant dissociation into (XONO)H ÷. This accounts for the high efficiency of path (1), whereby complex 2 is formed directly upon collision of the reagents. On the other hand, in the alternative path (2) complex 2 is not formed directly, arising instead from intracomplex proton transfer, a process that undergoes compe-
Table 1 Kinetics and thermochemislry of the reactions observed Process
10 ,o X k (cm 3 melee- l s - t)
Collision a efficiency (%)
AH° (kcal mol- ')
C6H ~- + HONO C6H ~" + CHaONO C7 H+ + CHaONO (CH3ONO)H + + C6H 6
1.7 4- 0.5 4.6 + 0.7 2.0 + 0.2 14.0 + 1.4
12 26 12 100
-31.1 b - 11.8 b -45.0 b - 10.1 c
a Based on the ADO collision rate, see text. b From the NO + BEs reported in Ref. [10], aad the: PA of benzene from Ref. [11]. c From the NO + BEs in Ref. [10].
F. Cacace, A. Ricci / Chemical Physics Letters 253 (1996) 184-188
187
100
E RCsHs+ + XONO
80
\
0
/
RCeHs + (XONO)H+
©
60
~o
I%
O
40
RCeH~NO+ + XOH)
0 0
20
0
Fig. 4. Schematic diagram of reaction sequences (1) and (2).
0 0
) . . . .
@ i
. . . .
~
05
. . . .
1
@
•
•
i
. . . .
1.s
l
) . . . .
i
2
@ . . . .
2.5
i
3
4) . . . .
i
3.5
t(s) Fig. 3. Reaction o f C6H ~- with CH3ONO at 4.6X 10 -8 Tort.
Time profile of the ionic intensities: ( O ) C6H ~ m / z = 79; ( © ) C6H6NO +, m / z = 108; (4k) (CH3ONO)H +, m / z = 62.
tition by back dissociation of complex 1. Thus, the overall efficiency of route (2) reflects primarily the k 2 / k _ ~ branching ratio, affected in turn by two distinct factors, namely the RC6H ~-- - - X O N O binding energy (BE) in complex 1, and the APA difference between XONO and RC6H 5. The overall inefficiency of path (2) can reflect a low BE, which enhances k_ 1 by favoring back dissociation of 1, a n d / o r a small APA, which can make intracomplex proton transfer a slow process [18], depressing k 2. Significantly, the alternative nitrosation (2) is most efficient in the C6H~-/CH3ONO system, characterized by the highest BE, and the largest APA among the pairs investigated. It should be mentioned that an alternative mechanism is conceivable, involving preliminary formation of a structured, proton-bound INC, capable of evolving directly into the product via a concerted process, e.g.
t C.~HI+ + XONO
)
H
~
•
"n
CsHGNO*+ XOH
(2b)
Although process (2b) cannot be ruled out, it appears less likely than sequence (2), owing to the unfavorable activation entropy associated with its cyclic transition state. Moreover, one should assume that the nitrosation promoted by the arenium ions and the direct nitrosation (1) occur via two different mechanisms, since the latter process is most likely is simpe
ligand exchange, without any preliminary protonation of the arene. Finally, there is a strong evidence that strictly related reactions of arenium ions with other proelectrophiles such as olefines and alcohols do not proceed via a concerted mechanism [24,25]. The structure of the R6HsNO ÷ adducts from sequence (2) has been probed by reaction with strong gaseous bases/nucleophiles e.g. pyridine. In all cases, one observes only NO + transfer, whereas no protonated base could be detected, ! RC6HsNO+ + CsHsN
!
) RC6H5 + CsHsN-NO+
(5)
~ RC6H4NO + C6HsNH+
(6)
The evidence from these experiments points to the ~-complex structure I of the RC6HsNO + adduct from the reaction sequence (2), since isomeric structures such as the or-complexes II and III, and O-protonated nitrosobenzene IV would be expected to efficiently protonate pyridine, a strong gaseous base. Such expectation is born out by the results of control experiments, showing that the (C6HsNO)H + ionic population obtained from the unselective proton transfer from CnH ~- to nitrosobenzene undergoes efficient proton transfer, but not NO + transfer, to gaseous bases such as pyridine. Thus, it appears that the product from the alternative sequence (2) has the same structure as the adduct from direct nitrosation (1), convincingly characterized as a "rr complex [9,10l. 0 N+
R - - ~ I
R
NO
~ ~
H lI
NO / ~ H
~_/X R In
k
RC6H4NOH + H Iv
188
F. Cacace, A. Ricci / Chemical Physics" Letters 253 (1996) 184-188
4. Conclusions The present results demonstrate the operation of the alternative nitrosation process (2) occurring in the INC formed upon addition of a gaseous arenium ion to nitrous acid, or its methyl ester, leading to formation of the same RC6HsNO + rr-complexes obtained from direct nitrosation. The results have a direct bearing on an area of active current interest concerning the kinetic and mechanistic role of INCs in gas-phase ion chemistry [21-23], and extend to another aromatic substitution the 'inverse' route to aromatic alkylation, the so-called 'Crafts-Friedel reaction', previously demonstrated in the gas phase [24,25].
Acknowledgement Support of the Ministero per l'Universith e la Ricerca Scientifica e Tecnologica (MURST) and of Consiglio Nazionale delle Ricerche (CNR) is gratefully acknowledged.
References [1] D.H.L. Williams, in: Organic reactivity. Physical and biological aspects, eds. B.T. Golding, R.J. Griffm and H. Maskill (Royal Society of Chemistry, London, 1995) p. 320. [2] J.H. Ridd, Quart. Rev. 15 (1961) 418. [3] D.H.L. Williams, Nitrosation (Cambridge Univ. Press, Cambridge, 1988).
[4] G.A. Olah, R. Malhotra and S.C. Narang, Nitration (VCH Publishers, New York, 1988) p. 75. [5] R. Taylor, Electrophilic aromatic substitution (Wiley, New York, 1990) p. 261, and references therein. [6] A.D. Williamson and J.L. Beauchamp, J. Am. Chem. Soc. 97 (1975) 5714. [7] T. McAllister and P. Pitman, Intern. J. Mass Spectrom. Ion Phys. 19 (1976) 241. [8] R. Farid and T.B. McMahon, Intern. J. Mass Spectrom. Ion Phys. 27 (1978) 163. [9] R.W. Reents Jr. and B.S. Freiser, J. Am. Chem. Soc. 102 (1980) 271. [10] R.W. Reents Jr. and B.S. Freiser, J. Am. Chem. Soc. 103 (1981) 2791. [11] J.E. Szulejko and T.B. McMahon, J. Am. Chem. Soc. 115 (1993) 7839. [12] D. ThiSlmann and H. Fr.-Griitzmacher, ICR Ion Trap Newsletter 25 (1992) 17. [13] R.T. Hall and G.C. Pimentel, J. Chem. Phys. 38 (1963) 1889. [14] D.M. Walford and A.L. Babb, J. Chem. Phys. 39 (1963) 432; 40 (1964) 1165 (E). [15] M.A. French, L.P. Hills and P. Kebarle, Can. J. Chem. 51 (1973) 456. [16] G. de Petris, A. Di Marzio and F. Grandinetti, J. Phys. Chem. 95 (1991) 9782. [17] S.G. Lias, J.E. Bartmess, J.F. Liebman, J.L. Holmes, R.D. Levin and W.J. Mallard J. Phys. Chem. Ref. Data 17 (1988) Suppl. 1. [18] H. Bficker and H. Fr.-Griitzmacher, Intern. J. Mass Spectrom. Ion Processes 109 (1991) 95. [19] M.T. Bowers and T. Su, in: Gas phase ion chemistry, Vol. l, ed. M.T. Bowers (Academic Press, New York, 1979) p. 96. [20] T. Su and W.J. Chesnavich J. Chem. Phys. 76 (1982) 5183. [21] R.D. Bowen, Accounts Chem. Res. 24 (1991) 364. [22] P. Longevialle, Mass Spectrom. Rev. 11 (1992) 157. [23] D.J. McAdoo and T.H. Morton, Accounts Chem. Res. 26 (1993) 295. [24] M. Aschi, M. Attin~ and F. Cacace, Angew. Chem. lntem. Ed. Engl. 34 (1995) 1589. [25] M. Aschi, M. Attinh and F. Cacace, J. Am. Chem. Soc., in press.
26 April 1996
.~
CHEMICAL PHYSICS LETTERS ELSEVIER
Chemical PhysicsLetters253 (1996) 184-188
Gas phase reaction of nitrous acid and methyl nitrite with arenium ions. A new route to electrophilic aromatic nitrosation F. Cacace, A. Ricci Dipartimento di Studi di Chimica e Tecnologia delle Sostanze Biologicamente Attive Universit~ "La Sapienza" di Roma, P.le A. Moro, 5, 00185 Rome, Italy
Received 17 November1995;in final form 22 January 1996
Abstract
The occurrence of an alternative route to aromatic nitrosation, based on the reaction of gaseous RC6H ~- arenium ions (R = H, CH 3) with XONO neutrals (X = H, CH3), has been demonstrated by FT-ICR mass spectrometry.
1. Introduction
Studies concerning the nitric oxide chemistry, and in particular the nitrosation processes, are currently an active area of research, owing to their intrinsic fundamental interest and to their impact on fields such as atmospheric chemistry, biochemistry, physiology, toxicology, etc. [1]. Aromatic nitrosation by a variety of reagents, including nitrous acid, the nitrous acidium ion H2NO~-, nitrosonium salts, alkyl nitrites, nitrosyl acetate etc., is a typical electrophilic reaction thoroughly investigated in solution [2-5]. In the gas phase aromatic nitrosation has been studied mainly by ICR mass spectrometry utilizing various charged reagents, according to the general equation XNO ÷ + C6H 6 ~ X + C6H6NO +,
Elsevier ScienceB.V. All rights reserved S0009-261 4(96)00240-0
RC6H ~ + XONO kI
" [RC6H~XONO ]
k_ l
I
(1)
where X = i-C3H7C1, C H 3 0 , CH3OH, and CH3ONO [6-9]. The structure of the C6H6NO + adduct has been probed by photodissociation experiments, whose results suggest that the product from
PII
reactions (1) is a ~r complex showing no tendency to isomerize into a c r complex, nor into O-protonated nitrosobenzene [9,10]. Irrespective of the reagent utilized, gas-phase nitrosation (1) is a conventional electrophilic reaction, whereby formation of the charged reagent and its interaction with the aromatic molecule are temporally and spatially distinct events. An alternative path is conceivable, whereby nitrosation takes place in the ion-neutral complex (INC) formed upon addition of a gaseous arenium ion to a neutral nitrosating species, e.g.
'2 , [RC6Hs(XONO)H+] k_ 2 k3
2 XOH + RC6HsNO +.
(2)
F. Cacace, A. Ricci / Chemical Physics Letters 253 (1996) 184-188
The distinctive feature of sequence (2) is that the nitrosating cation is formed, and the nitrosation occurs, within the INC without separation of the reagents. Here we report experiments aimed at demonstrating the operation of the alternative path (2) and to establish the structure of the nitrosated adduct formed. The experimental approach is based on FT-ICR mass spectrometry, a technique eminently suitable to achieve sharp discrimination between the alternative path (2) and the conventional route (1), the only nitrosation mechanism so far known both in solution and in the gas phase.
185
100 m
r3
80
E]
60 I% 4O
20
0
. . . .
0
4"
A
A
. . . .
I
0.5
/x . . . .
1
I
. . . .
1.5
I
2
. . . .
I
2.5
t(s)
Fig. 1. Time profile of the ionic intensities following introduction of H3 O+ ions into a H 2 0 / H N O 2 / N O x gaseous mixture at a total pressure of 5.0X10 - s Torr. ( m ) H3 O+, m / z = 19; ( * ) H ~ N O f , m / z = 48; (zx) NO + , m / z = 30.
2. Experimental Arenium ions were produced according to the reaction RC6H 5 + CnH ~- ~ CnH 4 + RC6H ~(R = H, CH3) (n = 1, 2),
(3)
by C H J C I in the external ion source of a model 47e APEX FT-ICR mass spectrometer from Bruker Spectrospin AG, fed with diluted R C 6 H s / C H 4 mixtures at a total pressure of = 10 -5 Torr. The arenium ions, excited by the considerable exothermicity of reaction (3), AH ° = - 49.8 kcal m o l - l for n = 1, R = H [11], were driven into the resonance cell and thermalized by collision with Ar, introduced via a pulsed valve and reaching a peak pressure up to - 10 -5 Torr [12]. After a pumping-down period of 3 s, the surviving RC6H ~" ions were isolated by ejecting all other ions by low-power radiofrequency 'shots', and allowed to react with the neutral XONO species, contained in the cell at pressures from 10 -8 tO 10 - 7 Torr. Whereas CH3ONO posed no particular problems, serious experimental difficulties were encountered in the case of HNO 2, which in the gas phase cannot be studied in the pure state, but only in the presence of its decomposition products, i.e. NOx oxides, water vapor, etc. [13,14]. The rapid, and pressure-dependent decomposition of nitrous acid makes it impossible to estimate with any degree of accuracy its actual concentration from total-pressure measurements in the resonance cell. Hence, the HNO 2 concentration, whose knowledge is required
to evaluate the rate constant of process (2), was estimated by an ICR 'titration' method, based on the proton transfer H 3 O + -I- HNO 2 --+ H 2 0
+ H2NO ~"
(4)
characterized by a significant exothermicity, 22.7 kcal mol -] , corresponding to the difference between the PA of water and of nitrous acid, 165.0 [11] and 187.7 kcal m o l - l [15,16], respectively. Proton-transfer reactions such as (4), highly exothermic and involving n-type, unhindered bases, are expected to occur at the rate of collision [17-19], which in turn can be estimated with an accuracy adequate to the purposes of this work using the ADO theory [19], or in the more recent theory by Su and Chesnavich [20]. Thus, measuring the rate of the [H30] + decrease in a gaseous H N O 2 / H 2 0 / N O x mixture affords, in principle, a way to estimate [HNO 2 ], provided of course that there are no 'sinks' other than nitrous acid for the H 3 0 + ions. This is the case, as indicated by the nature of the products formed, i.e. H2NO ~- accompanied by traces of NO + (Fig. 1), showing that indeed reaction (4) can be used to 'titrate' nitrous acid. Accordingly, H 3° ÷ ions produced, thermalized and isolated as described above, were allowed to react with the same gaseous mixtures employed for the study of reaction (2), and the rate of [H3 O+] decrease was used to evaluate [HNO 2 ].
186
F. Cacace, A. Ricci / Chemical Physics Letters 253 (1996) 1 8 4 - 1 8 8 100
3. Results and discussion
Two salient features of the experimental results concur in demonstrating operation of the alternative route (2) to aromatic nitrosation. First, RC6HsNO + nitrosated adducts are invariably formed from the reactions of RC6H ~" ions with XONO neutrals, as illustrated by the typical plots reported in Figs. 2 and 3. In all cases the nitrosated adduct is by far the major product, generally accompanied by minor amounts of (XONO)H + ions. Second, the experimental conditions typical of FT-ICR mass spectrometry deny the role of conventional nitrosation (1) by free (XONO)H + ions, namely RC6H ~ + XONO --} RC6H s + (XONO)H +,
(la)
80
6O I% 40
© o
o
o
o
20
o o 0
.
.
• .
.
.
.
.
.
.
i
•
•
,-L
.
.
5
.
.
•
.
i
.
.
10
.
.
.
.
.
.
.
• 7 15
t{s}
Fig. 2. Time profile of the ionic intensities following introduction of C6H ~- ions into a H 2 0 / H N O : / N O x mixture at a total pressure of 5.0X10 - s Tom ( O ) C6H~-, m / z = 7 9 ; (©) C6H6NO +, m / z = 108; ( A ) H2NO~, m / z = 48.
(XONO)H+ + RC6H 5 ~ XOH + RC6HsNO +. (lb) In fact, no arenes are deliberately introduced into the resonance cell, and although traces may leak from the external ion source, efficient differential pumping reduces their partial pressure well below l 0 - 9 T o r r , far too low to make (lb) a significant source of the RC6HsNO + adducts observed. Furthermore, as apparent from the plots of Figs. 2 and 3, the time profile of the (XONO)H + ions intensity excludes in all cases their role as the precursors of the RC6HsNO + adducts. For comparison purposes we have reexamined the direct nitrosation (1) of benzene by (CH3ONO)H +, previously reported by Reents and Freiser [9,10], utilizing ions obtained in the external source by CH4/CI of methyl nitrite. Table 1 summarizes the thermochemistry and the rate constants of the reactions studied. It is apparent that a particularly large
error bar is attached to the rate constant of the reaction of C6H ~- with HNO 2, which reflects a conservative estimate of the uncertainty affecting the measurements of [HNO 2 ] in the resonance cell, discussed in Section 2. From the results of Table 1 the efficiency of nitrosation sequence (2) ranges from 12 to 26% of the ADO collision rate, whereas direct nitrosation (1) of C 6 H 6 by (CH3ONO)H + occurs at collision rate. Such a trend can be rationalized according to the schematic energy diagram of Fig. 4, assuming that once formed complex 2 is bound to evolve into the RC6HsNO + product, without significant dissociation into (XONO)H ÷. This accounts for the high efficiency of path (1), whereby complex 2 is formed directly upon collision of the reagents. On the other hand, in the alternative path (2) complex 2 is not formed directly, arising instead from intracomplex proton transfer, a process that undergoes compe-
Table 1 Kinetics and thermochemislry of the reactions observed Process
10 ,o X k (cm 3 melee- l s - t)
Collision a efficiency (%)
AH° (kcal mol- ')
C6H ~- + HONO C6H ~" + CHaONO C7 H+ + CHaONO (CH3ONO)H + + C6H 6
1.7 4- 0.5 4.6 + 0.7 2.0 + 0.2 14.0 + 1.4
12 26 12 100
-31.1 b - 11.8 b -45.0 b - 10.1 c
a Based on the ADO collision rate, see text. b From the NO + BEs reported in Ref. [10], aad the: PA of benzene from Ref. [11]. c From the NO + BEs in Ref. [10].
F. Cacace, A. Ricci / Chemical Physics Letters 253 (1996) 184-188
187
100
E RCsHs+ + XONO
80
\
0
/
RCeHs + (XONO)H+
©
60
~o
I%
O
40
RCeH~NO+ + XOH)
0 0
20
0
Fig. 4. Schematic diagram of reaction sequences (1) and (2).
0 0
) . . . .
@ i
. . . .
~
05
. . . .
1
@
•
•
i
. . . .
1.s
l
) . . . .
i
2
@ . . . .
2.5
i
3
4) . . . .
i
3.5
t(s) Fig. 3. Reaction o f C6H ~- with CH3ONO at 4.6X 10 -8 Tort.
Time profile of the ionic intensities: ( O ) C6H ~ m / z = 79; ( © ) C6H6NO +, m / z = 108; (4k) (CH3ONO)H +, m / z = 62.
tition by back dissociation of complex 1. Thus, the overall efficiency of route (2) reflects primarily the k 2 / k _ ~ branching ratio, affected in turn by two distinct factors, namely the RC6H ~-- - - X O N O binding energy (BE) in complex 1, and the APA difference between XONO and RC6H 5. The overall inefficiency of path (2) can reflect a low BE, which enhances k_ 1 by favoring back dissociation of 1, a n d / o r a small APA, which can make intracomplex proton transfer a slow process [18], depressing k 2. Significantly, the alternative nitrosation (2) is most efficient in the C6H~-/CH3ONO system, characterized by the highest BE, and the largest APA among the pairs investigated. It should be mentioned that an alternative mechanism is conceivable, involving preliminary formation of a structured, proton-bound INC, capable of evolving directly into the product via a concerted process, e.g.
t C.~HI+ + XONO
)
H
~
•
"n
CsHGNO*+ XOH
(2b)
Although process (2b) cannot be ruled out, it appears less likely than sequence (2), owing to the unfavorable activation entropy associated with its cyclic transition state. Moreover, one should assume that the nitrosation promoted by the arenium ions and the direct nitrosation (1) occur via two different mechanisms, since the latter process is most likely is simpe
ligand exchange, without any preliminary protonation of the arene. Finally, there is a strong evidence that strictly related reactions of arenium ions with other proelectrophiles such as olefines and alcohols do not proceed via a concerted mechanism [24,25]. The structure of the R6HsNO ÷ adducts from sequence (2) has been probed by reaction with strong gaseous bases/nucleophiles e.g. pyridine. In all cases, one observes only NO + transfer, whereas no protonated base could be detected, ! RC6HsNO+ + CsHsN
!
) RC6H5 + CsHsN-NO+
(5)
~ RC6H4NO + C6HsNH+
(6)
The evidence from these experiments points to the ~-complex structure I of the RC6HsNO + adduct from the reaction sequence (2), since isomeric structures such as the or-complexes II and III, and O-protonated nitrosobenzene IV would be expected to efficiently protonate pyridine, a strong gaseous base. Such expectation is born out by the results of control experiments, showing that the (C6HsNO)H + ionic population obtained from the unselective proton transfer from CnH ~- to nitrosobenzene undergoes efficient proton transfer, but not NO + transfer, to gaseous bases such as pyridine. Thus, it appears that the product from the alternative sequence (2) has the same structure as the adduct from direct nitrosation (1), convincingly characterized as a "rr complex [9,10l. 0 N+
R - - ~ I
R
NO
~ ~
H lI
NO / ~ H
~_/X R In
k
RC6H4NOH + H Iv
188
F. Cacace, A. Ricci / Chemical Physics" Letters 253 (1996) 184-188
4. Conclusions The present results demonstrate the operation of the alternative nitrosation process (2) occurring in the INC formed upon addition of a gaseous arenium ion to nitrous acid, or its methyl ester, leading to formation of the same RC6HsNO + rr-complexes obtained from direct nitrosation. The results have a direct bearing on an area of active current interest concerning the kinetic and mechanistic role of INCs in gas-phase ion chemistry [21-23], and extend to another aromatic substitution the 'inverse' route to aromatic alkylation, the so-called 'Crafts-Friedel reaction', previously demonstrated in the gas phase [24,25].
Acknowledgement Support of the Ministero per l'Universith e la Ricerca Scientifica e Tecnologica (MURST) and of Consiglio Nazionale delle Ricerche (CNR) is gratefully acknowledged.
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