P. B. D. De La Mare

Kinetics and mechanisms of aromatic halogen substitution. Part XXVIII. Environmental effects on rates and products of chlorination of some aromatic hydrocarbons

Chlorination of phenanthrene in acetic acid gives a mixture of products of substitution and addition, in which cis-9-acetoxy-10-chloro-9,10-dihydrophenanthrene has been found and estimated by n.m.r. spectroscopy, and 1-, 3-, and 4-chlorophenanthrene have been identified and estimated approximately by infrared spectroscopy. The effects of added electrolytes, and of change in solvent, on the rate of chlorination and on the proportions of adducts and products of substitution have been studied. The results suggest complexities analogous to those involved in the product-determining stages of other reactions involving carbonium ionic intermediates. The effects of added electrolytes on rates and products of the chlorination of biphenyl have also been investigated. Comparison of the effects of change in solvent on the rates and products of chlorination of naphthalene with the corresponding results for phenanthrene establish that the transition state leading to cis-addition can have considerable carbonium ionic character.

The alkaline dehydrochlorination of some naphthalene tetrachlorides and related compounds

The rates and products of alkaline dehydrochlorination of the four known naphthalene tetrachlorides have been investigated, one for the first time and the others in more detail than has previously been recorded. The results are compared with those reported for the benzene tetrachlorides; they throw light on the accessibility of the various possible modes of elimination in chloro-substituted cyclohexenes and derived systems.

Aromatic substitution with rearrangement. Part III. Isotope effects in the prototropic rearrangement of 4-bromo-2,6-di-t-butylcyclohexa-2,5-dienone

The prototropic rearrangement of 4-bromo-2,6-di-t-butylcyclohexa-2,5-dienone to 4-bromo-2,6-di-t-butylphenol has been studied in acetic acid and in aqueous acetic acid, with and without deuterium in the 4-position of the dienone and in the solvent. With perchloric acid as catalyst, the first-order rate-coefficient is linearly correlated with the protonating power of the medium (i.e. with Ho, as calculated from the extent of protonation of o-nitroaniline in the same solvent). The reaction is subject to a relatively large primary isotope effect k4–H(/k4–D= 4·2), which decreases slightly with increased water content of the solvent. The solvent isotope effect is nearly the same as that on the protonation of o-nitroaniline. The mechanistic implications of these results are discussed.

The kinetics and mechanisms of aromatic halogen substitution. Part XXIX. Chlorination of aromatic hydrocarbons in slightly aqueous (96%) dioxan

The kinetics of the chlorinations of p-xylene, m-xylene, naphthalene, and triphenylene by hypochlorous acid in slightly aqueous (96%) dioxan have been examined. The reactions are catalysed by acids, and for each substrate there is a good linear free-energy correlation between the rate of reaction and the extent of protonation of o-nitroaniline in a similar range (0·2–0·6M) of acidity. The reactions mainly give nuclear chlorination, with up to 10% of side-reactions producing chloride ions. Silver perchlorate further catalyses the acid-catalysed chlorinations; the kinetic results suggest that under these conditions two solvated forms of positive chlorine, probably ClOH2+ and ClAgCl+, can act as electrophilic chlorinating species, and that with some of the substrates investigated both these reagents can also give products of hydroxylation, probably by an addition–elimination sequence.

The kinetics and mechanisms of nucleophilic displacement in allylic systems. Part X. Bimolecular substitution with rearrangement: some comments on the SN2′ mechanism

Criticisms by F. G. Bordwell and his co-workers of the mechanisms assigned earlier in this series to some second-order allylic replacements with rearrangement are discussed and in part refuted.

The alkaline dehydrochlorination of some naphthalene tetrachlorides and related compounds. Part II. 1,1,2,3,4-Pentachlorotetralin

The rates and products of alkaline dehydrochlorination of 1,1,2,3,4-pentachlorotetralin and its 2- and 4-deuterio-derivatives have been examined; they establish that the primary kinetic isotope effect on both the first and second stage of the reaction is large (kH:kD > 7). Evidence for a small secondary deuterium isotope effect for hydrogen atoms α- to the displaced halogen (kH:kD= 1·12) is also obtained. Comparison with corresponding kinetic measurements on naphthalene α-tetrachloride and its octadeuterio-derivative throws light on the conformations and reaction paths involved in these reactions. The alternative paths available for dehydrochlorination, though not much affected by temperature, are substantially dependent on the solvent, in a way which indicates complex formation between certain aromatic solvents and the organic substrate, and can be correlated with solvent effects on the chemical shifts of the 1H n.m.r. signals from the alicyclic hydrogen atoms.

Aromatic replacement with rearrangement. Part II. Chlorodeacylation in the chlorination of acetoxynaphthalenes

The trichloro-product (m.p. 174°), obtained by chlorination of 1,5-diacetoxynaphthalene in acetic acid or in carbon disulphide, has been shown to be 5-acetoxy-2,4,4-trichloro-(4H)-naphthalen-1-one. Chlorination in chloroform as solvent, however, has now been shown to give 5-acetoxy-2,3,4-trichloro-3,4-dihydro-(2H)-napthalen-1-one as a major product, which can be dehydrochlorinated to give 5-acetoxy-2,4-dichloro-1-naphthol. This on chlorination in chloroform gives 5-acetoxy-2,2,4-trichloro-(2H)-naphthalen-1-one. These results are discussed with reference to other deacylations accompanying aromatic substitution in aryl esters, and to the halogenation of enol-esters.

The kinetics and mechanisms of aromatic halogen substitution. Part XXVII. Substitution products in the chlorination of triphenylene

Chlorination of triphenylene by chlorine in acetic acid at 25° gives 1- and 2-chlorotriphenylene in the ratio 73 : 27. A small proportion of addition accompanies substitution. These results confirm the qualitative prediction derived from certain theoretical parameters that the 1 - is intrinsically more reactive than the 2-position, and suggest that, for molecular chlorine and reagents no more demanding sterically, there is little steric hindrance to attack at this type of position.

Sulphuryl chloride as an electrophile for aromatic substitution

The reactions of sulphuryl chloride with anisole and some methyl-substituted anisoles have been studied kinetically in chlorobenzene as solvent. The reactions have the kinetic form –d[SO2Cl2]/dt=k2[ArH][SO2Cl2], and give almost exclusively the products of nuclear chlorination. The rate is powerfully facilitated by the electron-releasing effect of methyl groups in the nucleus. The results are interpreted as indicating that the reactions are heterolytic, and probably involve electrophilic attack by molecular sulphuryl chloride.

The kinetics and mechanisms of aromatic halogen substitution. Part XXV. The chlorination of 2-methylnaphthalene

The chlorination of 2-methylnaphthalene in acetic acid is generally similar to that of naphthalene, in that products of substitution are accompanied by those of addition. The main components of the reaction mixture have been separated, and the structures of a crystalline tetrachloride, m.p. 99–99·5°, and of an acetoxytrichloride, m. p. 130°, have been deduced from studies of their proton magnetic resonance spectra and of their behaviour on alkaline dehydrochlorination. Their properties are compared with those of a 1-chloro-2-methylnaphthalene tetrachloride, m. p. 153°, and of various related compounds. The synthesis of 1,4-dichloro-2-methylnaphthalene is described.