KINETICS ISOTOPE RATE EFFECT AND MECHANISM OF DEHYDROBROMINATION OF CIS-1,2-DIBROMOETHYLENE WITH TRIETHYLAMINE IN DIMETHYLFORMAMIDE

The reaction between cis-dibromoethylene and triethylamine in dimethylformamide (DMF) is one of elimination rather than displacement. The probable first product is monobromoacetylene. The activation parameters for this second-order reaction are ΔH‡ = 18.1 kcal/mol and ΔS‡ = – 32 eu. The isotope effect for elimination is kH/kD = 1.00, and there is neither rate retardation nor hydrogen exchange with cis-dibromoethylene, when triethylammonium or triethylammoniun-dbromide is present during reaction. The reactivities of a few other compounds relative to cis-dibromoethylene with triethylamine in DMF were also estimated: k(HCBr═CHN(CH3)3+ Br-)≄ 2500 (40°); k(trans-C2H2Br2) ≄ 0.003 (101.8°); k(cis-C2H2CI2)≄ 0.0064 (101°)(HC≡CBr) ≄ 100 (100°). To account for the observations on the cis-dibromoethylene, a novel (ElcB)ip or preequilibrium ion-pair mechanism is proposed, in which (HCBr═CBr−HN(C2H5)3+) is formed. Analogy for this elimination mechanism is found in the ion-pair processes of electrophilic substitution (SE) reactions. The (ElcB)ip mechanism may provide a rationale for several “low” isotope effects, kH/kD. A reactivity scale, sp3 ~ sp ˃ sp2 ˃ sp2,5, is also suggested for the Sn reactions of triethylamine with various organic halides. © 1969, American Chemical Society. All rights reserved.