waves and the activated products are removed continuously from said reactor via an outlet orifice. 

[0111] In the case of actinic heating by microwave, it is recommended to use a power evolved by the microwaves of between 1 and 50 watts per milliequivalent of aromatic sub› strate. It is also desirable to accept the constraint according to which the power evolved by the microwaves is between 2 and 100 watts per gram of reaction mixture. 

[0112] The catalyst according to the invention may be used concomitantly with a catalyst acknowledged as being a phase-transfer catalyst, especially when this catalyst is a cata› lyst of cationic nature. 

[0113] Such a concomitant use is all the more appropriate since the mechanism of action appears to be different. 

[0114] The best phase-transfer catalysts that may be used are generally oniums, ie they are organic cations whose charge is borne by a metalloid. Among the oniums that may be mentioned are ammoniums, phosphoniums and sulfoniums. However, other phase-transfer catalysts may also be used provided that the phase-transfer catalysts are positively charged. They may also be cryptand cations, for example alkali-metal-cryptand crown ethers. 

[0115] These phase-transfer catalysts may be used in the presence or absence, preferably in the presence, of an alkali metal cation that is particularly heavy and thus from a high atomic row, such as cesium and rubidium. 

[0116] When the present invention is used to carry out a chlorine/fluorine exchange reaction, a dipolar aprotic solvent, a solid phase consisting at least partially of alkali metal fluo› rides and a reaction-promoting cation are generally used, said cation being a heavy alkali metal or an organic phase-transfer agent, this agent being of cationic nature. 

[0117] The content of alkali metal cation, when it is used as promoter, is advantageously between 1 mo! % and 5 mo! % and preferably between 2 mo!% and 3 mo!% of the nucleo› philic agent used. These ranges are closed ranges, ie they include their limits. 

[0118] The reagent may comprise, as promoter, phase› transfer agents that are oniums ( organic cations whose name ends with onium). The oniums in general represent 1 mo! % to 10 mo!% and preferably from 2 mo!% to 5 mo!% of the aromatic substrate, and the counterion may be of any nature but is usually halogenated. 

[0119] Among the oniums, the preferred reagents are tet› raalkylammoniums of 4 to 28 carbon atoms and preferably from 4 to 16 carbon atoms. The tetraalkylammonium is gen› erally tetramethylammonium. 

[0120] Phosphoniums and especially phenylphosphoniums should also be mentioned, which have the advantage of being stable and relatively sparingly hygroscopic; however, these reagents are relatively expensive. 

[0121] The aprotic solvent of halex type advantageously has a significant dipolar moment. Thus, its relative dielectric constant epsilon is advantageously at least equal to about 1 O; preferably, the epsilon is less than or equal to 100 and greater than or equal to 25. 

[0122] It has been possible to show that the best results were obtained when dipolar aprotic solvents with a donor index of between 1.0 and 50 were used, said donor index being the llH ( enthalpy variation) expressed in kilocalories of the combination of said dipolar aprotic solvent with antimony pentachloride. 

[0123] The oniums are chosen from the group of cations formed by colunms VB and VIB as defined in the Periodic Table of the Elements published in the supplement to the Bulletin de la Societe Chimique de France in January 1966, with four or three hydrocarbon-based chains, respectively. 

[0124] In general, it is known that a fine particle size has an influence on the kinetics. Thus, it is desirable for said solid in suspension to have a particle size such that its d,$_{0 }$ ( defined as the mesh that allows 90% by mass of the solid to pass through) is not more than 100 µm, advantageously not more than 50 µm and preferably not more than 200 µm. The lower limit is advantageously characterized in that the d$_{10 }$of said solid in suspension is not less than 0.1 µm and preferably not less than 1 µm. 

[0125] In general, the ratio between said nucleophilic agent, preferably the alkali metal fluoride, and said substrate is between 1 and 1.5 and preferably in the region of 5/4 relative to the exchange stoichiometry. 

[0126] The mass content of solids present in the reaction medium is advantageously not less than $^{1}$/s, advantageously … and preferably 1 A 

[0127] The stirring is advantageously performed such that at least 80% and preferably at least 90% of the solids are maintained in suspension by the stirring. 

[0128] According to the present invention, the reaction is advantageously performed at a temperature ranging from about 150 to about 250° C. In the present description, the term "about" is used to illustrate the fact that the values that follow it correspond to mathematic round-ups and especially that, in the absence of a decimal point, when the figure( s) the furthest to the right in a number are zeros, these zeros are positional zeros rather than significant figures, except, of course, if otherwise specified. 

[0129] However, it should be pointed out that when the temperature increases, the kinetics increase but the selectivity decreases. 

[0130] Another aim of the present invention is to provide a composition capable of serving as a reagent for nucleophilic substitution, especially aromatic nucleophilic substitution. 

[0131] This aim is achieved by means of a composition comprising: 

[0132] a polar aprotic solvent; 

[0133] a nucleophile; 

[0134] a compound of formula (I). 

[0135] It should be noted that the compounds of formulae (I) and (II) are particularly suitable for the usual recycling techniques. 

[0136] Another aim of the invention is to provide, besides that of having provided a novel family of novel compounds that is useful as nucleophilic substitution catalysts having a pronounced catalytic nature. 

[0137] Another aim of the present invention is to provide a process for synthesizing compounds that are used or that may be used as catalysts for second order nucleophilic substitution and especially for SN Ar nucleophilic substitution. 

[0138] These aims have been achieved with compounds of formula (I) in which the number of alkyl substituents is not more than 2 and in which the total number of carbons is not less than 14 and preferably 16 per positive charge borne by the molecule. It should also be pointed out that it is particularly advantageous to have molecules that are not completely sym› metrical around one of the atoms Z, but also around one of the atoms Pn. 

[0139] Thus, it may be indicated that the sum of the carbons in the radicals R$_{1}$ , R$_{2}$ , R$_{3}$ , R$_{4}$ , Rs and R$_{6 }$ is greater than 12, preferably not less than 14 and advantageously not less than 16. 

[0140] The condition regarding the alkyls may also be expressed by indicating that not more than 2 and preferably not more than one of the radicals R$_{1}$ , R$_{2 }$ and R$_{3}$ , on the one hand, and/or R$_{4}$ , Rs and R$_{6}$ , on the other hand, represent an alkyl group. 

