Mechanism and orientation

Comparisons between typical carbonium-ion processes in solution and dehydration over alumina have been made . On alumina, ethyl alcohol undergoes exchange with labelled water, viz. 

The majority of investigations have been concerned with the dehydration of ethyl alcohol. Controversy has raged for many years over the possibility that diethyl ether is a precursor in the formation of ethylene. The inconsistency in the conclusions may arise from a failure of the earlier workers to appreciate the importance of the intrinsic acidity of the alumina. Whereas Balaceanu and Jungers regard diethyl ether as the precursor of ethylene (169), Brey and KriegeT prefer simultaneous reactions (170), a mechanism favoured by Topchieva et al. who further suggested a covalently bonded intermediate (171). 

The reversibility of ether formation has been demonstrated , but it is not known whether the ether is formed from the collision of gaseous and absorbed alcohol or from adsorbed alcohol molecules only. On alumina, ethyl alcohol 

The low stability of primary carbonium ions makes their intermediacy highly improbable and any rearrangements have to be accounted for usually in terms of concerted mechanisms. The primary dehydration products of n-butanol are 1-butene (97.3%) and 2-butene in which the cis isomer predominates by a two-fold excess. Whereas 1-butene possibly arises from an fl/i/i-elimination, the 2-butenes are best explained as arising by removal of the y-hydrogen with a concerted migration of a beta hydrogen, viz- 

Steric interactions between the alkyl substituents and the catalyst surface cause trans-o tfin formation to be less favourable than cis. Dehydration of 2-methyl-1-propanol over a variety of alumina catalysts gives isobutylene (77-88%), similar amounts of 1-butene and r/i-2-butene (4-10%) and smaller yields of tra 5-2-butene (2-4%). Non-classical rather than classical intermediates were suggested, as the least acidic alumina, which possessed the lowest dehydration activity, caused the greatest skeletal isomerisation (173). 

---

MECHANISMS AND ORIENTATION IN PYROLYTIC ELIMINATIONS Meehan isms ... 

---