Chemistry 321 Laboratory
ADDITION OF HBr TO 1-OCTENE
(Adapted from Brown, T. M.; Dronsfield, A. T.; Ellis, R. J. Chem. Educ. 1990, 67, 518)
In this experiment, you will investigate the addition of HBr to 1-octene and use gas
chromatography-mass spectrometry (GC-MS) to determine whether the addition exhibits any
regioselectivity.
BACKGROUND
The normal addition of HBr to alkenes involves the reaction shown below (illustrated for 1-
butene):
+ HBr
Br
+Br
Typically, the addition follows Markovnikov's rule, and the major product is that derived
from the most stable carbocation intermediate in the mechanism. However, if free-radical initiators
are present, the addition leads to the so-called anti-Markovnikov product. The major product in
this case is derived from the most stable radical intermediate in the mechanism.
In today's experiment, you will add HBr to 1-octene to produce a mixture of 1- and 2-
bromooctane. You will use GC-MS to analyze the mixture and determine if the reaction follows
Markovnikov's rule or not.
CAUTION: Ether is a volatile chemical. Keep it closed up or under the hood. HBr is a strong
acid. Take care not to spill it on your hand.
PROCEDURE
A. Molecular Modeling
Using CAChe to calculate heats of formation, determine which of the two possible
carbocations from this experiment (see your notes for the mechanism of the reaction) is more
thermodynamically stable. Be certain when you construct the carbocations to give a formal
positive charge to the correct carbon atom. You must build each species and perform the
calculation separately.
B. Synthesis
Add 0.25 mL of 1-octene to a dry 5 mL conical vial. To the alkene, add 1.0 mL (an excess)
of the 30% HBr in acetic acid solution. Cap the vial and shake it frequently to mix the contents for
10 minutes. Occasionally loosen the cap to vent the vial. Allow the phases to separate, then
remove the lower acetic acid layer with a Pasteur pipette. To the remaining organic phase in the
