The Dehydrohalogenationof 2-Brombutane:
A Simple Illustration of Anti-Saytzeff Elimination
as a Laboratory Experiment for Organic Chemistry
Stephen A. Leone and J. David Davis
Merrimack College
North Andover, MA 01845
The dehydrohalogenation (E2) reaction of 2-bromobutane with alcoholic potassium hydroxide has been studied
kinetically at the microscale level, and the results have
been reported previously in this Journal (1).In the present
paper we illustrate a quantitative microscale experiment
ofthis reaction, and we use it to explore how increasing the
base size affects the distribution of products.
CH3CH2CHBrCH3+ base +
cisltrans-CH3CH=CHCH3+ CH3CH2CH=CH2
Both the hydroxide-promoted dehydmhalogenation reaction cited here and the acid-catalyzed (El) reaction of
2-butanol demonstrate the Saytzeff rule, in which the
more highly substituted and stable alkene usually predominates (2,3). In this case, the trans-2-butene will predominate.
However, in the E2 reaction, as the base becomes bulkier
according to the following sequence
sodium methoxide
potassium isopropmide
potassium t-butoxide
potassium 3ethyl-3-pentoxide
the percentages of the three butanes change, showing a
pmgressive increase in the l-butene a t the expense of the
trans-2-butene. This trend, which represents a clear devican be observed readily
ation from the Saytzeff trend (4,5),
by gas chromatography. It is apparent in the chromatograms in the figure and in the data of the table.
As a n additional experiment, it would also be informative to perform the E2 reaction of l-bmmobutane using either of the two larger alkoxide bases as a comparative
study with the dehydration of l-butanol. This sequence not
only demonstrates the poorer nucleophilicity of the larger
Gas cnromatograms oJlene product 0 strcDLtlon from the E2 Reac!#onof 2-brornooutanew r h o~nercnta <oxoe bases (a, NaOCH,, rb)
XOCh(Ch,),. ,c, XOC(Crl,),. (01 XOC(Ch,Ch,j,
Butene Product Distribution from the E2 Reaction
of 2-Bromobutane with Different Alkoxide Bases
Base
'C
l-butene
trans-2.
cis-2-
butene
butene
bases but also reveals that, unlike the E l reaction, the E2
reactions proceed without rearrangement.
Experimental
Caution. Care should be used in handling potassium metal.
It is very actiue, and it reacts vigorously with oxygen. Keep it
under liquid that contains no oxygen.
Caution: Care should be used in handline sodium metal. It
1~1olon11)decornposcc uater, formmg sodlurn hydrox~drand
htdragen, whwh may rgmre spontaneou4y Sodlum rrans
wgnmusly wth oxygen, hurnrng w t h a yellow llnrnc Keep it
under liquid that contains no oxygen.
-
The estimated time to complete the experiment is two
and one-half hours for any student performing two reactions. A 3-4-mL portion of the anhydrous alcohol was
placed in a 5.0-mL conical vial containing a magnetic spin
vane. An air condenser was attached to the vial. Then a
60-mg piece of freshly cut potassium or sodium was added.
A drying tube was placed atop the condenser. The reaction
vial was placed in a sand bath on a magnetic stirrer and
then heated gently. (For the sodium and methanol, heat
was not necessary. A water-cooled condenser is recommended.) When all the metal was reacted, the assembly
was removed from the sand bath and cooled to nearly room
temperature.
With an Eppendorf pipet, 100 pl; of 2-bromobutane was
introduced to the alcohol-alkoxide mixture. Then the assembled reaction setup was placed in the preheated sand
bath. The drying tube was removed, and the gas delivery
tube was attached to the condenser, with the open end of
the tube beneath the water level of the reservoir. ARer
several air bubbles emerged, the delivery tube was inserted under the open end of the water-filled gas-collection
tube. The gas-collection apparatus was constructed as reported by Mayo, Pike, and Butcher (3)and by Pavia, Lampman, Kriz, and Engel (6).
After 3-4 mL of gas was collected, the gas-delivery tube
was disconnected only from the condenser to keep water
from backing into the reaction vial. A2-mL gas sample was
withdrawn using a hypodermic syringe. The gas sample
was analyzed a t mom temperature by direct injection into
the gas chromatograph. A Gow-Mac Series 150 Thermal
Conductivity Detector GC was used with a flow rate of
50mLlmin of helium gas and with a column (114 in. x 8 R)
packed with 20% silicone DC200 on Chromasorb P 801100
mesh.
The butenes eluted from the column in the following sequence: l-butene, trans-2-butene, and cis-2-butene. The
percentage ratio of the three components was calculated
by triangulation.
(Continuedon page A176)
Volume 69 Number 6 June 1992
A175