J . Phys. Chem. 1990, 94, 7 18 1-7 184
7181
Reactivity of Bromine Atom Complexes with Organic Compounds Lian C. T. Shoute and P. Neta* s
Chemical Kinetics Division, National Institute of Standards and Technology, Gaithersburg, Maryland 20899 (Received: March 13, 1990; In Final Form: May I , 1990)
Complexes of bromine atoms with bromo compounds (C2H!Br.Br, CH2Br2.Br,CHBr,.Br, and CBr4.Br) were produced by pulse radiolysis of the corresponding bromoalkane dissolved in cyclohexane. With benzene as solvent, the complex C6H6.Br was produced. These complexes oxidize phenols to phenoxy1 radicals and abstract reactive hydrogens from various compounds. Absolute rate constants for these reactions were determined by following the decay of the Br atom complexes (at 370, 390, 425, 480 and 540 nm, respectively) as a function of substrate concentration. The rate constants varied from IO5 to 1OIo M-' s-I and were strongly dependent on the nature of the Br atom complex and of the organic substrate. Hammett's plots of the rate constants ( k ) for substituted phenols vs the substituent constants (up), according to the equation log k = pup, yielded p values of -4.2 for the reactions of C6H6.Br and -3.1, -1.9, and -1.3 for C2H5Br.Br,CH2Br2.Br, and CHBr3-Br, respectively. On the other hand, substituents on the benzene ring in XC6H5.k exerted only minor effects on the reactivity of this complex with phenol and with cumene. The rate constants for reaction of C&&I, C6H6-Br,and C6H6.1with phenol were found to be 9 X IO8, 3.5 X IO', and IO5 M-' s-I, respectively.
-
Introduction
I n a recent study' we found that radiolysis of bromoalkanes leads to production of bromine atoms, which form complexes with the parent compounds, RBr.Br. Such complexes were studied with C2H5Br,CH2BrZ,CHBr3, and CBr4, and their absorption peaks were found to be at 370,390,425, and 480 nm, respectively. These species were produced by radiolysis of the neat bromoalkanes as well as by radiolysis of their cyclohexane solutions. Radiolysis of bromoalkanes leads to ionization and C-Br bond scission, e.g. CH2Br2 [CH2Br2]'+ + e-
-
CH2Br2-,'CH2Br
+ Br
(2) and radiolysis of cyclohexane solutions forms Br atoms by energy transfer from excited solvent molecules, e.g.
--+
+ c-Hx'+ + ec-Hx* CBr4 c-Hx + CBr4* CBr4* 'CBr3 + Br' c-Hx
c-Hx*
-,
-,
(3)
(4) (5)
In both experiments the Br atoms bind to the parent bromo compound:
+
Br' RBr -, RBr'Br (6) The same complexes may be produced also by reaction of RBr'+, formed by direct ionization or by charge transfer from c-Hx'+, with Br-, formed by reaction of e- with RBr. However, in the radiolysis of bromoalkanes in benzene the Br atoms form com-' plexes with the aromatic compound: Br' + C6H6 C6H6'BT (7) +
Such complexes of halogen atoms with various arenes have been characterized in recent photochemical studies.2 Since many organic reactions of Br2 take place via Br atom intermediates and since these atoms may form complexes with the solvent or other solutes, it may be possible to detect solvent effects3 that are much greater than those due to polarity or hydrogen bonding4 Our preliminary experiments' indicated that CH2Br2.Brand CBr4.Br exhibit different reactivities toward several compounds. In the present study we examine the reactivities of ( I ) Shoute, L. C. T.; Neta, P. J . Phys. Chem. 1990, 94, 2447. (2) Bunce, N. J.; Ingold, K. U.; Landers, J. P.; Lusztyk, J.; Scaiano, J. C. J . Am. Chem. SOC.1985,107,5464. McGimpsey, W.G.; Scaiano, J. C. Can. J. Chem. 1988. 66, 1474. Raner, K. D.; Lusztyk, J.; Ingold, K. U. J. Phys. Chem. 1989, 93, 564.
(3) Johnson, M. D. In Bromine and Its Compounds; Jolles, Z. E., Ed.; Academic Press: New York, 1966; p 255. (4) For discussion of solvent effects on CI atom reactions see the recent paper by: Raner, K. D.; Lusztyk, J.; Ingold, K. U. J . Am. Chem. Soc., 1989, 111, 3652, and references therein.
0022-3654/90/2094-7 18 1$02.50/0
these two complexes, as well as other aliphatic and aromatic complexes of Br atoms, with a variety of organic reactants, including phenols, benzyl alcohol, benzhydrol, durene, and cumene. The various complexes were found to exhibit widely differing reactivities. In preliminary experiments we found that the same Br atom complexes can be produced by excimer laser photolysis (at 193 or 248 nm) of the corresponding cyclohexane solutions. However, this technique does not permit the desired kinetic measurements with aromatic compounds due to light absorption by these compounds. Experimental SectionS
Dibromomethane (DBM) was a Fluka puriss reagent and was used as received, bromoform was from Fisher and was purified by vacuum distillation and passing through activated alumina, carbon tetrabromide was from Eastman and was recrystallized from acetone/water, and ethyl bromide was from Fisher and was purified by passing through a column of activated alumina. Cyclohexane was analytical grade from Mallinckrodt, cumene and chlorobenzene were from Eastman, bromobenzene was from Fisher, cyclohexene, fluorobenzene, trifluorotoluene, trichlorotoluene, benzonitrile, methyl iodide, and the bromochloromethane-s were from Aldrich and were all purified by passing through activated alumina. Tetrahydrofuran was a Baker analyzed reagent and was distilled before use. Benzyl alcohol was from Fisher, hexamethylbenzene was from Eastman, durene and mesitylene were from Aldrich, and benzene, toluene, and 2-propanol were analytical grade reagents from Mallinckrodt. Benzhydrol was obtained from Aldrich and was recrystallized and sublimed. Phenol and p-methoxyphenol (PMP) were from Sigma, and the other substituted phenols were from Aldrich. Aniline was from Fisher and was purified by vacuum distillation. Fresh solutions were prepared before each experiment, and samples were transferred with a syringe to the irradiation cell and irradiated under air. The pulse radiolysis apparatus was described before.6 It utilizes 50-11spulses of 2-MeV electrons from a Febetron 705 accelerator. The dose per pulse was between IO and 30 Gy in most experiments, as determined by dosimetry with N,O-saturated aqueous KSCN solutions.' The kinetic spectrophotometric detection system consisted of a Varian 300-W xenon lamp, a 2-cm optical path length irradiation cell, a Kratos high( 5 ) The mention of commercial equipment or material does not imply recognition or endorsement by the National Institute of Standards and Technology nor does it imply that the material or equipment identified are necessarily the best available for the purpose. (6) Neta, P.; Huie, R. E. J . Phys. Chem. 1985, 89, 1783. (7) Schuler, R. H.; Patterson, L. K.; Janata, E. J . Phys. Chem. 1980.84, 2088.
0 1990 American Chemical Society
7182 The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
Shoute and Neta
TABLE I: Rate Constants for Reactions of Br Atom Complexes witb Phenols and Aniline" k , M-I s-' reactant C6H6.Br C2H5Br.Br CH2Br2-Br UP 4-methoxyphenol -0.268 (6.3 f 0.9) X IO9 (1.1 f 0.2) x 10'0 (6.3 f 1.0) x 109 (6.5 f 1.0) X IO8 (2.1 f 0.4) x 109 -0.197 (3.7 f 0.6) x 109 4-tert-butylphenol (8.9 f 1.3) X IO8 -0.170 4-methyl phenol (3.6 f 0.6) X lo9 (4.1 f 0.6) X IO9 0.00 (3.5 f 0.5) x 107 (1.9 f 0.4) X lo8 (2.3 f 0.3) x 109 phenol +0.062 (6.9 f 1.0) x 107 (3.1 f 0.5) X lo8 (8.2 f 1.2) X IO8 4-fluorophenol (3.4 f 0.6) x 107 (5.9 f 0.9) X lo8 (2.6 f 0.4) X IO8 +0.226 4-chlorophenol +0.232 (2.4 f 0.5) X lo7 (2.7 f 0.4) X I O 8 4- bromophenol (1.0 f 0.2) x 109 + O S 16 4-acet ylphenol (1.5 f 0.5) X IO6 4-cyanophenol +0.628 (3 f 1) x 105 (1.1 f 0.2) x 10'0 aniline (5.3 f 0.8) x 109
CHBr,.Br (5.1 (1.6 (2.1 (8.4 (1.1 (8.0 (7.8
f 0.8) X IO9 f 0.3) X IO9
A 0.3) x 109 f 1.3) X IO8 A 0.2) x 109 f 1.2) X IO8 f 1.2) X IO8
'The rate constants for C6H,.Br were measured in benzene as solvent and for C2H5Br.Br, CH2Br2.Br, and CHBr,.Br were measured in cyclohexane.
intensity monochromator, an RCA 4840 photomultiplier, and the appropriate shutters, lenses, and optical filters. The signals were digitized with a Tektronix 761 2 transient recorder and analyzed with a PC. All experiments were carried out at room temperature, 22 f 2 T.
Results and Discussion In the previous study] we measured the rate constants for several reactions of Br atom complexes with organic compounds by monitoring the rate of decay of the complex and the rate of formation of products. For compounds such as p-methoxyphenol, triphenylamine, and trimethoxybenzene the products were found to result from an overall one-electron oxidation of the organic substrate. In this study we determined the rate constants for such reactions by following the rate of decay of the Br complex, at the peak of its optical absorption spectrum, as a function of concentration of the organic compound. The second-order rate constants were calculated from linear plots of the first-order rates versus substrate concentration. These rate constants, for each compound, were found to vary by up to 2 orders of magnitude upon varying the reacting Br complex, as observed in our preliminary results.' The rate constants for reaction of each complex with substituted phenols (Table 1) are strongly dependent on the substituent. Electron-withdrawing groups decrease the reactivity of phenol, whereas electron-donating groups increase the reactivity, as expected for reaction with an electrophilic species. To quantitate these effects, we correlated the rate constants with Hammett's substituent constants, according to the equation log k = pa,. Reasonable fits are obtained for several Br atom complexes (Figure I). The most extensive set of data were obtained for C6H6.Br, and it demonstrates the electrophilic nature of this species. A reaction constant of p = -4.2 is derived from the slope of the line. The set of data obtained for CH2Br2.Br is less extensive because it was not possible to measure the rate constants for the less reactive phenols, those with acetyl and cyano substituents, due to their limited solubility in cyclohexane. Nevertheless, the correlation gives a reasonable line (Figure 1) whose slope gives p = -1.9. This lower slope for the aliphatic complex is in accord with its higher reactivity. The correlations for CHBr3.Br and C2H5Br.Br gave more scattered results with approximate slopes of -1.3 and -3. I , respectively. It was not possible to obtain reliable results with CBr4.Br because a slow thermal reaction between CBr, and the phenols hampered some of these experiments. In the series of RBr.Br complexes, the absolute value of p (slope) decreases with increasing number of bromine atoms or with increasing electron affinity of RBr. Complexation of Br with these compounds and with benzene imparts some negative charge on the bromine atom because the electron affinity of free Br is higher than that of the compounds to which it is bound. As a result, the reactivity of Br complexes as electrophiles is much lower than that of free Br. Increasing the electron affinity of the complexing molecule (in the order benzene, C2H5Br,CH2Br2,CHBr3, CBr4, assuming that electron affinity increases with the number of (8) Wiberg, K. B. Physical Orgonic Chemistry; Wiley: New York. 1964.
10 9
l
a 0,
0 -
7
6 53
0
.3
.6
OP
Figure 1. Rate constants for reaction of substituted phenols, 4XC6H50H, with C6H6.Br ( 0 )and with CH2Br2.Br (A)as a function of the substituent constant for X, up, taken from ref 8.
bromine atoms) results in decreased negative charge on the complexed Br atom, and thus it may behave as a stronger electrophile. On the other hand, as suggested before, increasing the number of Br atoms on the molecule increases the strength of the complexation with the free Br, probably by increased delocalization of electrons from the bound bromine to the complexed bromine, which may decrease its reactivity. These two opposing effects result in the observation that the general reactivity of the complexes, C6H6&, C2H5Br.Br,CH2Br2-Br,CHBr3-Br,CBr4.Br, does not follow the same order, Le., the order based on electron affinity only, but rather increases and then decreases, with the second or third complex generally showing the highest reactivity. The reactions of the Br atom complexes with phenols and aniline result in an overall one-electron oxidation of the organic compound, as demonstrated for similar cases in our previous study.' The mechanism of oxidation is not likely to be an outer-sphere electron transfer in these nonpolar solvents. Direct hydrogen abstraction from phenol and aniline by the RBr-Br complex also is unlikely because the bond dissociation energies of the 0-H and N-H in these compounds are nearly identical with that of H-Br.9 Since the substrates can form complexes with Br atoms, by binding to the heteroatom or to the aromatic mystem, it is possible that the initial step involves transfer of Br from the RBr.Br complex to the substrate. If a *-complex is formed between Br and the phenol ring, the process will be reversible and the equilibrium concentration of this complex will be relatively small, due to the low concentration of phenol. If the complex is formed on the heteroatom, a rapid intramolecular electron transfer will ensue to give the ion pair of the substrate radical cation with Br- (reaction 8), and finally HBr may be eliminated to give the neutral radical RBr-Br + ArOH z RBr
+ ArOHSBr (+ArOH+Br-)
ArOH'Br- .= ArO'
+ HBr
(8) (9)
(reaction 9). The final step (eq 9) can be reversed by addition of excess HBr.] (9) Handbook of Chemistry and Physics, 67th ed.; CRC Press: Boca
Raton, FL, 1986.
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990 7183
Reactivity of Bromine Atom Complexes
TABLE 11: Rate Constants for Reactions of Br Atom Complexes with Organic Compoundsa k , M-' s-I
reactant benzhydrol benzyl alcohol hexamethylbenzene durene mesitylene
cumene toluene cyclohexene ethvlene allil alcohol tetrahydrofuran 2-propanol rerr-butyl alcohol rerr-butylamine "he
C6H6'BT (1.2 f 0.2) X IO8 (1.2 f 0.2) X IO8 (4.2 f 0.8) X IO' (6.3 f 0.9) X IO7 (2.1 f 0.3) X IO6 (4.6 f 0.7) X IO7 (9.8 f 2.0) X IO5 (1.3 f 0.2) x 109 ( 6 f 2) X IO6 (1.8 f ' 0 . 4 ) X 10' (9.7 f 2.0) X IO5 (4 f I ) x 105
