This lab, like the one immediately following it, is a synthesis. You need to have mole tables in your notebook for each reaction. Mole tables include the usual physical properties and structures, as well as:
Background:
Acidification of an alcohol causes protonation of the oxygen. The C-O bond weakens, and water can be eliminated, leaving a carbocation. Recall that tertiary carbocations are stablest, followed by secondary, with primary a distant third.
If a nucleophile is present, the cation will add the nucleophile at the positive carbon. This is the SN1 reaction. The SN1 would happen if the acid were HCl or HBr.
<>But if the acid's conjugate base is a poor nucleophile, like hydrogen sulfate, substitution is much slower than elimination. This is the reaction we look at in this experiment.Preparation:
Be sure to look up the boiling points of the alcohol and the alkenes that are used in this lab. The reaction mixture must be heated for the elimination to occur. The procedure shows the use of a fractionating column in the "first" distillation. Keep in mind that this is only to ensure that only the alkenes distill, and that the alcohol and acid remain in the still pot while being heated. The large difference in boiling points of the starting material and the product require the presence of the fractionating column. [In the revised procedure below, you'll see that there is only one distillation of the alkenes. (Not for summer 2005)]
The low boiling points of the alkenes
makes
it necessary to keep the alkenes on ice as much as possible. They
should also be kept dry.
The second distillation, from NaOH and
Na2SO4 , is simply to remove any last traces of
acid and to make sure the final alkene mixture is dry. A GC
analysis will be performed on the alkene mixture to illustrate
"Saytzeff's rule" - the principle which says that more substituted
alkenes are stabler.
The procedure for doing the
dehydration and the subsequent hydrobromination in the same lab period
are found here.
[Not for Summer 2005}