E2 eliminations from esters. Moreover, cis-trans isomeric amines with rigid five-membered rings and with flexible sevenmembered rings, the exhaustive methylation of which has not been studied as yet, should also be used for comparison purposes.
VII. The Saytzeff and Hofmann RulesExamination of the E l and E2 eliminations described above in order to determine whether they obey one of the two rules shows that the relationships that have until now been regarded as generally valid [411 do not hold. E l eliminations do not all follow the Saytzeff rule, nor d o E2 eliminations from halides and esters of sulfonic acids, and the Hofmann rule in the E2 mechanism is not predominantly restricted to onium compounds. However, the behavior of the cycloalkane derivatives clearly shows that the steric structure of a molecule in the ground state can have a decisive effect on the mode of reaction.The Saytzeff rule follows from those El eliminations that proceed rapidly with participation of tertiary hydrogen. When the steric conditions for the participation of tertiary hydrogen are not satisfied, the Saytzeff rule may still be more or less strictly obeyed if a hydride shift in the initially formed ion leads to a tertiary carbonium ion. The stabilization of this ion by loss of a proton to give a trisubstituted or tetrasubstituted "ethylene" then competes t o a greater or lesser degree with stabilization of the initially formed primary or secondary carbonium ion by deprotonation, which leads to an olefin having a less highly alkylated double bond. For thermodynamic reasons, eliminations leading to equilibria obey the Saytzeff rule. The Hofmann rule is obeyed with statistical probability when all the hydrogen atoms available for elimination are equally suitable; in the least favorable case, i.e. when there is a choice between a CH3 and a CH2 group, 213 of the product is Hofmann hydrocarbon. The relative yield of the Hofmann hydrocarbon can be higher if the reactivity increases with the number of protons on a group. This is so e.g. in the case of tertiary hydrogen, which has an anionic character owing to the inductive effect ; this hydrogen remains intact in E2 eliminations, which are induced by a proton acceptor, and even in E l reactions it may be taken up by the solvent to a smaller extent than "primary" or "secondary" hydrogen. Thus vicinally trans-alkylated and vicinally cis-alkylated cycloalkyl p-toluenesulfonates undergo trans E2-elimination involving the secondary hydrogen, even when the tertiary trans-hydrogen occupies the sterically favorable coplanar positions (see Section V). If there is a choice between secondary and primary hydrogen, n o generalizations can be made in view of the small statistical superiority of the primary hydrogen. The attack on a given hydrogen atom may be facilitated or hindered by steric factors; the elimination will be accelerated or retarded, depending on whether the hydrogen and the leaving group are more or less coplanar. The importance of electronic effects as compared with steric effects is un...
