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Material Type: Lab; Class: Organic Chemistry; Subject: Chemistry; University: Adams State College; Term: Unknown 1990;
Typology: Lab Reports
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(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):
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
conical vial, add 2 mL of ether and 2 mL of water. Cap the vial and shake it to mix the contents, remembering to loosen the cap to vent the vial. Allow the layers to settle, then remove the aqueous phase (caution: which layer is it?) To the ether phase add 2 mL of 10% sodium bicarbonate solution (caution: there may be some fizzing!) and shake the mixture. (Why is this step needed?) Remove the aqueous phase. To the remaining ether phase, add anhydrous CaCl 2 , cap the vial, and allow the vial to stand for 10 minutes, until the ether phase is completely clear.
C. Analysis The products of this reaction will be analyzed by GC-MS. The first step is to look up the boiling points of both 1-bromooctane and 2-bromooctane. Then, go on to the analysis of the product. Transfer the ether sample into a small GC-MS sample vial (available from the instructor). Place the sample vial into the autosampler tray for the GC-MS instrument. Be certain to record the vial slot in your notebook. You will get a printout of the gas chromatograph and the mass spectrum for each major component. How many product peaks are observed from your solution? Use the boiling point information you looked up to help identify which peaks correspond to which product. Calculate the relative percentages of each product (1-bromooctane and 2-bromooctane) obtained. Do the mass spectra support you assignment of the two peaks from the GC? How are the two mass spectra similar? How are they different? Determine if the reaction exhibits regioselectivity and if so, whether it follows Markovnikov's rule or not. Do the results from the molecular modeling agree with your experimental results? If so, why? If not, why not?