Chemical Communications (The Journal of The Chemical Society, Section D) NUMBER 2/1971 20 JANUARY The ortho :para Ratio in Aromatic Nucleophilic Substitution By T. 0. BAMKOLEand J. HIRST* (Departmentof Chemistry, University of Ibadan, Nigeria) Summary p-Nitrofluorobenzene reacts slightly faster than o-nitrofluorobenzene with methoxide ions in methanol, but under the same conditions the 2-fluorine atom in 2,4-difluoronitrobenzene is replaced faster than the 4-fluorine atom; an explanation is given to reconcile these observations. THE reaction of 2,4-difluoronitrobenzene with methoxide ions takes place preferentially at the 2-position,lY2 i.e. ,ortho-activation by the nitro-group predominates, but 0-and p-nitrofluorobenzenes react with methoxide ions to give an o :p ratio of less than indicating that a p-nitro-group is more activating than an o-nitro-group.We have rein-vestigated these apparently contradictory reactions. We measured the rates of reaction of 0-and p-nitro- fluorobenzenes with methoxide ions in methanol and obtained an o :p ratio of 0.69 at 49.15" in good agreement with Bevan and Bye's3 value of 0.68 at this temperature (0.70 at 25"). Kinetic runs were carried out with 2,4-difluoronitro-benzene and methoxide ions in methanol at 25". In all cases when the methoxide :substrate ratio was greater than 2 :1, the amount of methoxide consumed at infinity corresponded to the theoretical value calculated for the replacement of both fluorine atoms, and the expected product, 2,4-dimethoxynitrobenzene was isolated from these solutions.When the initial concentration ratio was 1:1, the x9Fn.m.r. spectra of the infinity solutions consisted of two peaks A and B, A being 12.3 p.p.m. downfield from B. Addition of 2,4-difluoronitrobenzene to these solutions resulted in two additional peaks, hence A and B are due to the products of the reaction. Also, when the runs were carried out with 2,4-difluorobenzene present in excess, the 19F spectrum consisted of four peaks, two identifiable as belonging to the substrate, the other two being in the same relative position as A and B. The position of absorption of B coincided with that of fluorobenzene. In all cases when the substrate was in excess, no 2,4-dimethoxynitrobenzene could be detected in the products by t.1.c.When the initial concentrations of methoxide ion and substrate were 1:1 the ratio of the intensities of A and B was 1:0.815 ; when the methoxide :substrate ratio was 1:1.5 the corres- ponding intensity ratio was 1:0.823. 3-Fluoro-4-nitroanisole (I) and 3-fluoro-6-nitroanisole (11),the products of replacement at the 4-and 2-positions in 2,4-difluoronitrobenzene were synthesised by the method of Hodgson and his co-~orkers.~ (I)had a 19Fn.m.r. absorp- tion which coincided with that of fluorobenzene and (11) absorbed 12.2 p.p.m. downfield from fluorobenzene. Hence the above intensity ratios indicate a value of 1.22 for the o :p ratio in 2,4-difluoronitrobenzene. The apparent inconsistency of this result when compared with that obtained by individual measurements on 0-and p-nitrofluorobenzenes arises from the fact that the value of the o :p ratio derived from measurements on 2,4-difluoro- nitrobenzene is not a true measure of the relative activating powers of o-and 9-nitro-groups.In aromatic nucleophilic substitution reactions the activating power of a group X, as measured by the relative rate ratio k, :K,, where k, and k, are the rate constants of the substituted and unsubsti- tuted compounds, depends on the initial degree of activa- tion of the parent system.5~~ In 2,4-difluoronitrobenzene the rate of replacement of the 2-fluorine atom reflects the influence of a m-fluoro-substituent in an o-nitro-activated series, while replacement at the 4-position includes the effect of m-fluorine in a p-nitro-activated series, and the magnitude of 1.n-fluorine activation cannot be assumed to be the same in both series.Using initial methoxide :substrate concentrations in the ratio of 1:1-33 we have obtained a value of 5.88 x 10-3 1 mol-1 s-1 for the second-order rate constant for the overall reaction of 2,4-difluoronitrobenzene with methoxide ions at 25". Combined with the above product ratios, this gives rate constants of 3.23 x and 2.65 x 10-3 1 mol-1 s-1 69 70 CHEMICAL COMMUNICATIONS, 1971 for replacement at the 2-and .i-positions, respectively. substituent (k, :k,) is 24.5 in the o-nitrofluorobenzene Rate constants for rnethoxydefluorination calculated from series and 13.9 in the $-nitrofluorobenzene series.These Bevan and Bye's data are, at 25O, 1.32 x lo4 1 mol-l s-l figures can be compared with the value of 93.3t at 25" for o-and 1.91 x 10-4 1mol-1 s-1 for P-nitrofluorobenzene. found in the m-nitrofluorobenzene series. Hence the relative activating power of a m-fluorine (Received, November 3vd, 1970; Corn. 1907.) t Calculated at 25' from the data in ref. 6. 1 J. F. Bunnett and R. J. Morath, J. Amer. Chew. SOC.,1955,77, 5051. 2 F. Swarts,Rec. Trav. chim., 1915,35, 154. 3 C. W. L. Bevan and G. C. Bve, J. Chem. SOC.,1954, 3091. * H. H. Hodgson and J. Nixon, J. Chem. Soc., 1928,1879; H. H. Hodgson and D. E. Nicholson, ibid., 1940, 1268. B. A. Bolto, M. Liveris, and J. Miller, J. Chem. Soc., 1956, 750. 6 C. W. L. Bevan, J. Hirst, and S. J. Una, Nigerian J. Sci., 1966, 1, 27.