Transition state geometry in the scission and formation of cyclopentane and cyclohexane oxiranes. Use of the dilatometer in studying mixed reactions of different orders

Two methods, one more accurate, the other more convenient, were developed by which the dilatometer could be used with concurrent reactions of the second and third order. The more accurate method, employing a dilatometer of high specific surface area and equipped with a stable and responsive thermometer, was used in studying as many as six simultaneous oxirane scissions in acid solution. Interlocking double comparisons were made between cyclohexane and cyclopentane ring systems, between systems possessing or lacking a methoxyl substituent and between others having the substituent in varying orientations, between charged and uncharged nucleophiles, and between transition states of different degrees of protonation. Searching experimental checks were therefore possible in four main areas: (i) a quantitative dissection of the inductive effect from steric and conformational effects, and from dipole–ion or dipole–dipole interactions with the nucleophile, (ii) a quantitative demonstration of three main differences between transition states based on 5- and 6-membered rings, (iii) a proof that syn-1,3-effects in the transition states are very charge dependent, being large and positive with unfavourably arranged dipoles, and small or perhaps negative with favourable arrangements, and (iv) a rejection of certain proposed ion-pair alternatives to the A2 mechanism.