text: * In general, if more than one alkene can be formed during dehalogenation by an elimination reaction, the more stable alkene is the major product.  There are two types of elimination reactions,  E1 and E2.   An E2 reaction is a One step mechanism in which carbon-hydrogen and carbon-halogen bonds break to form a double bond. C=C Pi bond.   An E1 reaction is the Ionization of the carbon-halogen bond breaking to give a carbocation intermediate, then the Deprotonation of the carbocation.
  * For these two reactions, there are 3 possible products, 3-methyl-cyclohexene,1-methyl-cyclohexene, methylene-cyclohexane.  The production of each of these occurs at different rates and the ratios of these also change over time.  It is well known that the dehydration of the cis isomer is 30 times faster than the trans isomer. It then appears that the reaction proceeds mainly by a trans mechanism and, following the Zaitsev rule, 1-methylcyclohexene is preferentially formed in the early stages of the reaction. Indeed, if only about 10% of the total distillate is collected as the first fraction, one finds that the alkene is about 93% l-methylcyclohexene: at the end of the distillation one finds a value as low as 55% of l-methyl isomer.
  * From these results, the phenomenon of the Evelyn effect can be observed and  a conclusion can be drawn that a change of mechanism occurs somewhere during the synthesis.