DETERMINATION OF E2/SN2 RATIO FOR THE REACTION OF AN ALKYL HALIDE WITH
POTASSIUM HYDROXIDE (Revised 10/20/2004)
Alkyl halides undergo competitive �-eliminations and nucleophilic substitutions via E2 and SN2
pathways, respectively, when heated with concentrated solutions of strong bases. The relative amounts
of elimination versus substitution products (E2/SN2 ratio) depends on a number of factors such as the
basicity of the base, the structure of the alkyl group, the reaction temperature, and the reaction solvent.
The attack of bases on �-hydrogens is less susceptible to steric effects than is the attack of bases on
carbon. An increase in the bulkiness of alkyl groups and/or bases causes a significant decrease in the
rates of SN2 reactions, but not in the rates of E2 reactions, which results in an increase in the E2/S N2
ratio. In general, substitutions dominate with primary alkyl halides unless a strong, bulky base like
potassium tert-butoxide is used; however, eliminations dominate with tertiary alkyl halides even when
pKa = 15.7).
RCH2CHR X
R ' O R ' OH
RCH
CHR
+
RCH2CHR OR '
REACTANT
Amount used (g)
H2O displaced (ml)
2-Bromobutane (2 pts)
1-Bromobutane (2 pts)
bases
as weak as cyanide ion (HCN pKa = 9) are used.E2/S
Eliminations
or substitutions may dominate with
REACTANT
N2 RATIO
secondary
alkyl halides,
2-Bromobutane
(3 pts) depending upon the basicity of the base. Eliminations dominate when the bases
are
as
basic
as
hydroxide
1-Bromobutane (3 pts) ion, but substitutions dominate when the bases are less basic than hydroxide
ion (H2O
In this experiment the effect of the alkyl group on the E2/SN2 ratio for the reaction of 1bromobutane and 2-bromobutane with potassium hydroxide in 95% ethanol will be examined
by holding all other variables constant. The percent of E2 reaction for an alkyl halide can be
calculated by comparing the experimentally determined volume of butene gas generated with
theoretical volume of butene gas that could be generated if 100% elimination occurred.
Assuming that all of the alkyl bromide reacts, the difference in the two percentages must be
the percent of SN2 reaction.
PROCEDURE
Working in pairs and referring to the diagram below (Fig. 1), assemble the apparatus for the
experiment. Dissolve 8 g of potassium hydroxide in 40 mL of 95% ethanol in the 100 mL
round-bottomed flask. Add a magnetic stirring bar, attach a reflux condenser via a Claisen adapter (be
sure to lightly grease the bottom joint of the adapter) that has the other opening closed off with a rubber
septum. Place a thermowell on top of a magnetic stirring plate under the flask. Do not connect the
separatory funnel to the reflux condenser until just before injecting the alkyl bromide. Start the magnetic
stirrer and heat the solution to reflux. Maintain a constant rate of reflux throughout the rest of the
experiment.
Fig. 1: Apparatus for the
Dehydrobromination
Fill
a
250
mL
separatory funnel
with water, attach a length
of rubber tubing to
the stopcock end, connect
the other opening to
the T-tube with another
length of rubber
tubing using a one-hole
rubber stopper, and
clamp it in the inverted
position. At all
times during the reaction,
the T-tube will act
as an overflow for water
displaced from the
separatory funnel. Open
the stopcock of the
funnel and adjust the
height of the T-tube
until the water in the funnel and the overflow point of the T-tube are at the same level. When these
levels are the same, the pressure of the gas in the system should be the same as the atmospheric pressure,
and any evolution of gas will result in the displacement of an equal volume of water. Fill a hypodermic
syringe with either ~0.7 mL (~0.86 g, ~6 mmol) of 2-bromobutane or ~1.6 mL (~2.0 g, ~14 mmol) of
1-bromobutane and weigh the filled syringe. Introduce the bromobutane into the reaction flask through
the rubber septum. Set the syringe aside so that you can reweigh it to determine the exact amount of the
bromobutane added. Continue the reaction until no significant change in the volume displaced water
occurs over a 5-minute period. To make a final reading of the volume of displaced water, adjust the
height of the T-tube until it and the water in the separatory funnel are at the same level. The total volume
of displaced water is now equal to the volume of butene gas generated at atmospheric pressure. Obtain
the final volume of displaced water from a team that used the other bromobutane, and use both sets of
data to calculate the E2/SN2 ratios for both bromobutanes by comparing the experimental final volumes
of butene gas with the theoretical final volumes.
% Each student needs to ultimately have the data recorded in his/her notebook!
% Each group (2 students) will perform experiment with either 1-bromobutane or 2bromobutane as assigned to you by the teaching assistant.
% Share data (weight of an alkyl halide used and amount of water displaced) obtained by other
group(s) using the alkyl halide that you did not perform experiment with.
% Keep the alkyl bromides capped and use a syringe to obtain directly from the container.
% The final reaction mixture is KBr and can be discarded in the sink with plenty of running water.
Name:_____________________________________ Section:__________
Date:________
POSTLAB EXERCISE
Due 1 DETERMINATION OF E2/SN2 RATIO FOR THE REACTION OF AN ALKYL
week
HALIDE WITH POTASSIUM HYDROXIDE (Revised 10/20/2004)
after
completio
n of the
experime
ntal
pKa = 15.7).
A.
portion of
R
'
O
R
'
OH
RCH
CHR
+
the
RCH2CHR X
RCH2CHR OR '
notebook
REACTANT
Amount used (g)
H2O displaced (ml)
(30 pts 2-Bromobutane (2 pts)
TOTAL).
1-Bromobutane (2 pts)
Please REACTANT
E2/SN2 RATIO
answer PRODUCT INFORMATION (12 pts)
2-Bromobutane (3 pts)
questions
1-Bromobutane (3 pts)
on this pKa = 15.7).
R ' O R ' OH
RCH CHR
+ RCH2CHR OR '
form.
RCH2CHR X
REACTANT
Amount used (g)
H2O displaced (ml)
2-Bromobutane (2 pts)
1-Bromobutane (2 pts)
CALCULATIONS
using
REACTANT(2 pts) Show one sample calculation E2/S
N2 either
RATIO2-bromonbutane or 1bromobutane.
2-Bromobutane (3 pts)
1-Bromobutane (3 pts)
B. SUMMARY QUESTIONS (18 pts)
In your summary include a discussion on reasons for the differences between the E2/S N2 ratios for the
two alkyl bromides.
1. (4 pts) What is the significance of an E2/SN2 ratio >1? …<1?
2. (4 pts) Explain the differences between the E2/SN2 ratios for the two alkyl bromides.
1
(5 pts) Would the E2/SN2 ratios for 1-bromobutane and 2-bromobutane increase, decrease, or
remain unchanged if potassium tert-butoxide (tert-butyl alcohol pKa = 19) were used as the base instead
of potassium hydroxide? Explain your answer.
2
(5 pts) Would the E2/SN2 ratios for 1-bromobutane and 2-bromobutane be greater than 1.0 or
less than 1.0 if the base used were (a) potassium tert-butoxide (tert-butyl alcohol pKa = 19) or
(b) potassium acetate (acetic acid pKa = 4.75)? Explain your answer.