SNAr mechanism reactivity

The fact that the order of halide reactivity is Br > I > Cl > F (when the reaction is performed with KNHi in liquid NH3) shows that the SNAr mechanism is not operating here.31... 

The most important experimental data about NMR, IR, and UV spectroscopy have been reported in CHEC-I. In addition, an AMI SCF-MO study has been published <88JOC3900>. The relaxed reaction profile for aromatic nucleophilic substitution of some chloropyrimido[4,5-J]pyridazine has been investigated using the MNDO procedure <90JST(63)45>. Kinetic measurements and MNDO calculations show that the C-8 position of the pyridazine ring is more reactive than C-5 in nucleophilic substitution reactions, and these follow a two-step SNAr mechanism <89T4485>. 

The reaction between aryl halides and cuprous cyanide is called the Rosenmund-von Braun reaction Reactivity is in the order I > Br > Cl > F, indicating that the SNAr mechanism does not apply. Other cyanides (e.g., KCN and NaCN) do not react with aryl halides, even activated ones. This reaction has been done in ionic liquids using CuCN. ° The reaction has also been done in water using CuCN, a phase transfer catalyst, and microwave irradiation. 

The most widely used approach to the preparation of PESs in both academic research and technical production is a polycondensation process involving a nucleophilic substitution of an aromatic chloro- or fluorosulfone by a phenoxide ion (Eq. (3)). Prior to the review of new PESs prepared by nucleophilic substitution publications should be mentioned which were concerned with the evaluation and comparison of the electrophilic reactivity of various mono- and difunctional fluoro-aromats [7-10]. The nucleophilic substitution of aromatic compounds may in general proceed via four different mechanism. Firstly, the Sni mechanism which is, for instance, characteristic for most diazonium salts. Secondly, the elimination-addition mechanism involving arines as intermediates which is typical for the treatment of haloaromats with strong bases at high temperature. Thirdly, the addition-elimination mechanism which is typical for fluorosulfones as illustrated in equations (3) and (4). Fourthly, the Snar mechanism which may occur when poorly electrophilic chloroaromats are used as reaction partners will be discussed below in connection with polycondensations of chlorobenzophenones. 

PROBLEM 14.75 2,4-Dinitrofluorobenzene is a better electrophile for SNAr reactions than the corresponding chloride (see Fig. 14.95). 2,4-Dinitrofluorobenzene reacts with amines at room temperature to give orange-colored adducts. Show the mechanism of the reaction between 2,4-dinitrofluorobenzene and dimefhylamine. Predict the product. Why is the fluoride more reactive than the chloride Why is the product highly colored ... 

In SnAr reactions, thiophene reacts more slowly than furan the SnAr reactivity of thiophene corresponds to that of anisole. The mechanism of electrophilic substitution is the same as described for furan (cf. p. 63) monosubstitution occurs predominantly in the 2-position, disubstitution in the 2,5-positions. 

---