RESEARCH be changed from o n e in which the above intermediate is formed to the one in which it is decomposed to give the final products, or to both steps being Kinetic and spectroscopic d a t a show existence of rate-controlling steps of comparable magnitude. The results farther indiintermediates in aromatic substitutions cate, savs Schubert, that agreement with the Hammett equation for a uniE v i d e n c e is molecular mechanism over a limited rapidly accumuacid range is not a sufficient criterion lating that the for a unimolecular mechanism. mechanism by >The Element Effect. A new conwhich aromatic cept i n the determination of reaction substitution reacmechanisms is the "element effect." Br SOr tions occur inJoseph F. Bunnett a n d coworkers at volves formation of metastable interthe University of North Carolina have mediate complexes. After Melander's Upon mixing an aqueous solution of conducted experiments pointing t o (Sweden) isotope effect experiments sodium 3,5-dibromo-4-hyclroxybenzene wards the conclusion that activated in 1949 were widely hailed as proof with an equimolar amount of bromine, nucleophilic substitution reactions prothat such intermediate complexes Cannell observed immediaLte disappear- ceed i n two steps via intermediate comexisted, Hammond (Iowa State) ance of the bromine color and an ab- plexes. For nucleophilic substitutions, pointed out that those experiments did sorption maximum i n the u v at 279 m/u. this n o w has the same sort of signifinot uniquely prove existence of the and extinction coefficient: of 14,000. cance as Melander's bad i n 1949 for metastable complexes. More recently, (Also, potassium iodide solution lib- electrophihc substitutions. however, a number of workers in severated iodine quantitatively equal t o Bunnett and coworkers allowed nine eral universities obtained evidence from initial bromine.) different 2,4-dinitrobenzenes with varikinetic studies that such complexes That the n e w compoond was ac- ous substituents in t h e 1-position to must intervene in two important types tually an intermediate w a s indicated react with piperidine. T h e 1-substitof electrophilic aromatic substitution by (1) the absorption naaximum de- uents (SOC 6 H 5 , Br, CI, S 0 2 C 6 H 5 , reactions. creases over a several hour period until O C 6 H 4 N 0 2 - p , and I ) were displaced spectrum closely resembles that of 2,- at nearly the same rate. Last week, Lawrence G. Cannell of Displaced Harvard University and W. M. Schu- 4,6-tribromophenol, the final reaction much faster were F , N 0 2 , and O S 0 2 product; (2) the immediate liberation C 6 H 4 CH 3 -p. The former six groups bert and Philip C. Myhre of University of I 2 . of Washington told die symposium on have first atoms representing five difaromatic substitution, conducted b y the Cannell assigned the intermediate a ferent elements; Bunnett has concluded Division of Organic Chemistry, of 2,5-cyclbhexadienone structure on basis that since the rate of heterolysis of a strong evidence supporting the same of the u v spectra data. Investigation bond involving the ring carbon with conclusions. of the disappearance of tHe intermedi- the various 1-substituents varies greatly ate showed that i t followed a first as the substituents are changed, indiBy the bromodesulfonation of Bve order rate law in agreement with a cations are that this bond i s not broken aromatic sulfonate salts, Cannell finds mechanism involving rapid interme- in a rate-determining step. An interstrong indication of the existence of a quinonoid intermediate i n the bromina- diate formation followed by its uni- mediate of the type molecular decomposition to products tion process. In brominating two comtribromophenol and H 2 S 0 4 . pounds, sodium p-methoxybenzenesulY X fonate and sodium 3,5-dibromo-4Schubert and Myhre bave deduced aminobenzenesulfonate, Cannell obfrom a study of the rates o f decarboxyl•N02 served that b y varying the Br- concenation of 2,4,6^trialkylbenzaldehydes i n tration from 0.0025 t o 0.1500M at 60 to 100% sulfuric acid that the reacconstant ionic strength and acidity, the tion proceeds via bi- or termolecular rates of bromination were decreased steps. Deuterium isotope studies (usby factors of 16 and 230, respectively, ing deuteroaldehyde, &rCDO, and oindicating that a reverse reaction was D 2 S 0 4 - D 2 0 mixtures) ^gave results taking place. consistent only with a ro^chanism of is thus indicated. successive bimolecular proton transfer A decrease in rate caused by part • New Garb for Hammett Equation. of the bromine's being converted to steps proceeding via an Intermediate: What may b e a major advance in helpBr 3 - (inert as brominating agent) would ing to correlate the rates of electrophilic account for a decrease by a factor of aromatic substitution reactions is a n e w about four, says Cannell. However, set of sigma values (
