P-Nitrofluorobenzene

A mixture of 600 ml. of concentrated sulfuric acid and fuming nitric acid (sp. gr. 1.5) (2 parts of sulfuric and 1 part of nitric acid by 

The nitro compound may be reduced to p-fluoroaniline in 92-95% yield by hydrogenation [cf. Org. Syntheses Coll. Vol. 1, 61 (1943)] in the presence of Raney nickel catalyst [Org. Syntheses, 21, 45 (1941)] plus a small amount of platinic chloride as a promotor. The amine boils at 98-99°/33 mm. 

One hundred and forty-four grams (0.60 mole) of the dry monosodium salt of p-hydroxyphenylarsonic acid [Org. Syntheses Coll. Vol. 1, 490 (1941)] is added portionwise with stirring and cooling to 450 ml. of concentrated sulfuric acid. A mixture of 39 ml. of nitric acid (sp. gr. 1.4) and 39 ml. of concentrated sulfuric acid is added with continuous stirring and sufficient cooling to hold.the temperature of the reaction mixture below 0°. The temperature is then allowed to rise to 10° over a period of several hours. The reaction mixture is poured into 2250 ml. of water and allowed to stand for 24-48 hours. The precipitated 3-nitro-4-hydroxyphenylarsonic acid is filtered off and washed with saturated aqueous sodium chloride solution until free of sulfuric acid and then with water until free of salt. The dry product is obtained in a yield of 65-75%. 

---

The number of reactions that fit into this second class are manifold. Some typical examples are the amine-catalyzed reactions of 2,4-dinitrochlorobenzene with n-butylamine in chloroform (k"/k = 2.59 l.mole-1)23, with allylamine in chloroform (k"jk — 4.60 l.mole-1)24 and with allylamine in ethanol (k"fk =0.36 l.mole-1)25 and the amine-catalyzed reaction of p-nitrofluorobenzene with piperidine in polar solvents (k"/k < 3.2 l.mole-1)26. A typical example of a strongly catalyzed system is the reaction of 2,4-dinitrophenyl phenyl ether with piperidine in 60 % dioxan-40 % water27. 

Application of these methods has revealed the limitations of certain qualitative generalizations. Thus it had been common practice to attribute the rate enhancement resulting from a change of solvent from a protic one to DMSO primarily to desolvation of the anionic nucleophiles. However, it has been shown that in a number of cases increased solvation of the transition state is the predominant factor. For example, in the reaction of p-nitrofluorobenzene with azide ion in DMSO it is found that 5A//tr(Nj) = —1-0 kcal mol-1 whilst 8AHjt = —6-5 kcal mol-1 (Cox and Parker, 1973). Similar studies, using DMSO and other dipolar aprotic media, have been carried out by Haberfield (1971), Fuchs (1974), Abraham (1974), Jones et al. (1976), and their co-workers, and will be considered in later sections. 

Examples of nitration of nuclear and side-chain halogenated compounds are found in the preparation of p-nitrofluorobenzene (80%) and the isomeric o- and p-nitrophenylethyl bromides in 30% and 50% yields, respectively. 

Figure 13.1.1. Relationship between second-order rate constants at 50°C of the reaction of p-nitrofluorobenzene and piperidine and the solvent dielectric properties [from ref. 21].

The first solvent property applied to eorrelate reaetivity data was the statie dieleetrie eonstant s (also termed 8 ) in the form of di-electrie funetions as suggested from elementary eleetrostatie theories as those by Bom (1/s), Kirkwood (s-l)/(2e+l), Clausius-Mosotti (s-l)/(s+2), and (8-1 )/( + ). A sueeessful eorrela-tion is shown in Figure 13.1.1 for the rate of the Sn2 reaetion of p-nitrofluorobenzene with... 

The practical applicability of the Gutmann-Mayer concept was proved by calculation of the standard free energies of dissolution AG values) of sodium chloride and potassium chloride in various solvents (Tables 4.14 and 4.15), of the AG values of the coordination reaction C0CI3-hCP- CoClJ" in various organic solvents (T able 4.16) and of the rate constant of the reaction of p-nitrofluorobenzene with piperidine (Table 4.17). 

Figure 12.1.1. Relationship between second-order rate constants at 50°C of the reaction of p-nitrofluorobenzene and piperidine and the solvent dielectric properties. [Data from Y. Marcus, M. J. Kamlet, and R. W. Taft, J. Phys. Chem., 92, 3613 (1988).]...

An even more convincing experiment has been reported by Bolton et al. (62). p-Nitrofluorobenzene reacts with sodium azide in dry dimethylformamide in a second-order process, in which azide ion disappears but no fluoride ion is formed. The disappearance of the azide ion is paralleled by the formation of a substance showing maxi-... 

---