and 2-Bromoheptanes

Organic Chemistry 341-2014
LABNOTES-6
OCT 28-OCT 30, 2014
Dehydrobromination of 1- and 2-Bromoheptanes: Base Comparison
Experiment 24B: Dehydrobromination of 1-Bromobutane and 2-Bromobutane, pp 204-210 and
suggested readings within
Read Technique 5.1, Automatic Pipets, pp 582
NOTE: We will be using 1- and 2-bromoheptanes in place of 1- and 2-bromobutanes. This will allow
liquid extraction of the reaction mixtures since the product alkenes are liquids at room temperature,
rather than collecting the gaseous alkene products as described in the lab textbook. Be certain to
preserve the molar ratios of the reactants as described in the experiment, while accounting for this
change in the experiment (i.e. the base to bromoalkane ratio should be the same for your bromoalkane as
is described for the analogous bromobutane). You will be using the same amount (in mL) of base as
described in the book. You can approximate that the 15 g of KOH corresponds to 15 mL (see footnote
in lab book) if necessary. As always (and should now be part of your weekly prelab routine), read
the MSDS profiles for all chemicals used in the lab today.
Odd numbered lab drawer: 1-bromoheptane
Even numbered lab drawer: 2-bromoheptane
Tuesday and Wednesday Lab’s Base: KOH in 95% ethanol
Thursday’s Lab’s Base: 1.0 M Potassium tert-Butoxide (t-BuOK) in 2-Methyl-2-propanol
Reaction temperature for 1-bromoheptane: 85oC
Reaction temperature for 2-bromoheptane: 90oC
PRELAB: Prepare your lab notebooks for this synthesis experiment in the standard format for synthesis
experiments, based upon the information above. You and your partner will only conduct one experiment
(based upon which bromoalkane and base combination applies to you), but the combined class data from
all of the four experiments will be shared. Be certain that you have calculated how much of each
reactant to use and have it on a notecard for reference. Before lab starts, have the notebook ready for the
TA to sign. You will be working in pairs, following a microscale procedure. Don’t forget to fill in the
pertinent data (molecular weight, boiling point, melting point, density, etc.) that you will need during
the lab. While the reference books available to you in the lab are good, much of this information can be
found (for this lab and those remaining) in the library and on the internet. If your reference comes from
the internet, be certain it is from a reliable source.
INTRODUCTION: Alkyl halides are excellent precursors to alkenes. The dehydrohalogenation
reaction is carried out using a strong alkoxide base with elevated temperatures. The yields are usually
good (60-90%), but normally more than one alkene is produced. The percent ratio of the possible
alkenes is explained by Zaitsev’s rule which predicts that the most thermodynamically stable alkene that
can form by 1,2-elimination will form in the highest yield. This rule is generally accepted, but
exceptions are noted (especially if a sterically hindered base is used). In these cases, the base cannot
access the more sterically demanding proton required to provide the internal alkene. Today you will be
conducting an experiment to test Zaitsev’s rule. Both 1 and 2-bromoalkanes will be subjected to strong
bases (CH3CH2O /OH and t-BuO ). You will compare the ratios of alkenes produced in these four
experiments, but will only conduct one of them for the combined class data (see above). This is the
current topic in class (see Carey, Organic Chemistry)
AMENDMENTS TO PROCEDURE: Work in groups of two. Choose one of your assigned micro
kits to obtain the glassware. Prepare a drying tube as described in your laboratory textbook, the reaction
is sensitive to the presence of water (especially the t-BuOK reaction). Because we will not be collecting
a gas, but refluxing liquids, place a condenser between the 5-mL conical vial and the drying tube (and
do not use a thermometer adapter). Lightly grease the male joint of the condenser before inserting into
the 5-mL conical vial. Have your hot plate and heat transfer block pre-heated to the proper temperature
before placing your reaction into the aluminum block. Use a plastic graduated pipet to transfer the
required amount of potassium hydroxide solution, or a 5-mL plastic syringe to transfer the required
amount of potassium t-butoxide solution base solution, to the 5-mL conical vial. Do not get the base
solution on the ground glass joint. Use an automatic pipet (Eppendorf) to obtain the required volume of
your bromoalkane (NOTE: THIS WILL NOT BE 0.32 mL or 0.16 mL, you must have calculated the
required amount prior to lab). Use a magnetic stirring vane. Keep the reaction at 85oC (or 90oC if using
2-bromoheptane) for 30 minutes. If you heat the reaction higher than 110oC or lower than 65oC for
more than 5 minutes during the reaction time, you must start over. When you place your conical vial
into the heat transfer block, a dip in temperature will be observed as the glass heats. Start the 30-minute
reaction time when the temperature has returned to desired value (you may need to slightly increase the
heat setting to return to the desired temperature). After the 30 minute reaction period, remove the entire
apparatus from the heat source and allow the vial and it’s contents to cool to room temperature. This is
important since the extracting solvent (pentane) boils at 36oC and will boil away very quickly (or ignite)
if added to a warm vial. Remove the spin vane with a pair of tweezers. Add 1.3 mL of pentane and 1.0
mL of water to the reaction vial. Cap the vial tightly and shake the mixture gently, but thoroughly, until
the white solid dissolves. A few additional drops of water may be necessary to completely dissolve the
solid. Remove the lower aqueous phase with a Pasteur pipet and transfer it to a 50-mL Erlenmeyer
flask, leaving the organic phase in the conical vial. Repeat the above extraction procedure with two
1.0-mL portions of water, removing the lower aqueous phase before adding the next portion of water; all
the aqueous phases should be combined in the Erlenmeyer flask. After removing the last portion of
water, add about 200 mg of anhydrous sodium sulfate to the pentane solution remaining in the vial. Cap
the vial and allow the mixture to dry for at least 10 min. Transfer the dried solution, without any
crystals of drying agent, to a clean dry 4-mL sample vial, using a Pasteur pipet.
GC/MS ANALYSIS OF ALKENE RATIOS: Remove one drop of your sample and place in an
autoinjector vial (2-mL), followed by 1.5 mL of pentane. Cap the vial until ready for analysis. Using
the GC/MS, determine your product ratios (C7.M method file and Heptenes.S method file). Make sure
that there is PENTANE in the autoinjector turret (Solvent A).
USING HEXANE WILL
CONTAMINATE FUTURE INJECTIONS. Identify the peaks in the spectrum and integrate them to
determine the amounts of each product. The mass spectrums of the heptene products are very similar,
so the mass spectral library-matching algorithm will have difficulty differentiating between them. Use
boiling points to assign the identity of each peak. Once you have obtained the ratios of your products,
write your results on the chalkboard. Each day at the end of lab, these results will be added to the
webpage for everyone to use to answer the questions below.
QUESTIONS:
1. Discuss the ratios of your reaction. Do they make sense?
2. Compare the results of the four reactions (using combined class data provided on the web) and
discuss how the use of different bases and different bromine substitutions has affected the
product composition.
3. Which factor predominates in each reaction: the size of the base, the bromine substitution or the
stability of the alkenes formed? Explain.
4. Because alcohol protons are reasonably acidic and easily undergo exchange, it is important that
the base be matched to the appropriate solvent. Explain this, telling why it would be virtually
impossible to understand what factors in Question 3 predominate in a reaction carried out in
methanol solvent and using potassium tert-butoxide as the base.
5. Simple internal alkenes are usually found to be somewhat more stable when trans (the E form)
rather than cis the Z form). Explain why.
6. In contrast to Question 5, cyclohexene normally occurs as a cis-cycloalkene and cannot be
isolated in a trans-form. Explain.

Download Report