KINETICS OF DEHYDROBROMINATION AND E2/SN2 RATIOS (10/30/01)
Alkyl bromides undergo competitive β-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 depends on (1) the basicity of the
base, (2) the structure of the alkyl group, (3) the reaction solvent, and (4) the reaction
temperature.
R'O
RCH 2CHR
R 'OH
Br
RCH
CHR
+
RCH 2CHR
OR'
In this experiment the effect of the alkyl group on the dehydrobromination of 1-bromobutane and
2-bromobutane with potassium hydroxide in 95% ethanol will be examined. Kinetic data on the
eliminations can be collected by monitoring the evolution of butene gas. From this data the rate
constants for the two alkyl bromides can be determined and compared. In addition to the
determination of the rate constants, the E2/SN2 ratios for the two alkyl bromides can be
calculated by comparing the theoretical volume of butene that could be generated (assuming
100% elimination) with the actual volume generated.
The eliminations are second order reactions with the rate law given by equation 1. The kinetics
-
d[RBr]
dt
=
(1)
k[RBr][KOH]
can be simplified by using a large excess of hydroxide so that its concentration remains
essentially constant throughout the reaction. Thus, equation 1 reduces to equation 2. The
integrated form of equation 2 is equation 3.
-
d[RBr]
dt
=
k'[RBr]
or
log
C0
C
dC
dt
=
=
k'C
k' t
2.303
where
where
k ' = k[KOH]
C = [RBr]
C 0 = initial [RBr]
C = [RBr] at time t
(2)
(3)
One does not have to monitor the change in the actual concentration of a reactant, or a product.
All that is required is to monitor some physical quantity that is related to concentration. In this
reaction, the change in volume of butene gas evolved will be followed. If the final volume, Vf,
and the final volume minus the volume at time t, Vf - Vt, are substituted for C0 and C,
respectively, equation 3 becomes equation 4. The latter form of equation 4 is readily seen as the
equation for a
log
Vf
(Vf -Vt)
=
or
log (Vf -Vt)
=
k' t
2.303
k'
- 2.303 t
(4)
+
log Vf
straight line. By plotting log(Vf - Vt) versus t, a straight line with a slope of -k'/2.303 is obtained.
The rate constant k' is then obtained by multiplying the slope of the line by -2.303.
The experiment will be much easier if done in pairs with one member making volume readings
and controlling the apparatus while the other member keeps the time and records the data. Each
pair will collect kinetic data for both 1-bromobutane and 2-bromobutane.
Procedure:
Refer to the diagram below when assembling the apparatus for the experiment. Dissolve 15g of
potassium hydroxide in 56mL of 95% ethanol in the 100mL round-bottomed flask. Add a
boiling stone, 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, and heat the
solution to reflux using a thermowell. Maintain a constant rate of reflux throughout the rest of
the experiment.
Fig. 1: Apparatus for Kinetics of Dehydrobromination
Fill a 250mL 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. Do not connect the separatory funnel to the
reflux condenser until just before injecting the alkyl bromide. 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. Maintain this control level carefully during
the reaction. 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 ~2.4mL (~3.0g) of 1-bromobutane and weigh it. Introduce the 1bromobutane into the reaction flask through the rubber septum. Set the syringe aside so that you
can reweigh it to determine the exact amount of 1-bromobutane added. As the reaction proceeds,
keep a record of the volume of gas formed as a function of time. It is sufficient to take readings
after each 10mL of water is collected. Continue the reaction until no significant change in
volume occurs over a 5-min. period. Refill the separatory funnel with water and repeat the
procedure using ~0.8mL (~1.0g) of 2-bromobutane, re-using the ethanol/KOH mixture for the 2nd
alkyl bromide.
Plot the log of the volume change (Vf - Vt) versus time for both alkyl bromides and determine
the values of the respective rate constants, k’ where:
Vf is the final volume
Vt is the volume (total) at time, t.
Also, calculate the E2/SN2 ratios for both alkyl bromides by comparing the experimental final
volumes with the theoretical final volumes.
Keep the alkyl bromides capped and use a syringe to obtain directly from the container.Reuse the ethanol/KOH mixture for the second alkyl bromide.
One of the alkyl bromides (which one?) will eliminate a good deal faster and may generate
more than 100mL of butene. Be prepared!
The final reaction mixture is KBr and can be discarded in the sink with plenty of running
water.
POSTLAB EXERCISE
KINETICS OF DEHYDROBROMINATION AND E2/SN2 RATIOS
Ø Due 1 week after completion of the experimental portion of the notebook (30pts
TOTAL). Please answer questions on this form.
A. PRODUCT INFORMATION (12pts)
REACTANT
E2/SN2 RATIO
2-Bromobutane (3pts)e
1-Bromobutane (3pts)
(6pts) Attach a plot of log (Vf – Vt) vs. t for both bromobutanes.
B. SUMMARY QUESTIONS (18pts)
1. (3pts) Which compound below has a larger value for the rate constant k’?
a) 1-Bromobutane
b) 2-Bromobutane.
2. (3pts) Explain why the above compound (a or b) has a larger rate constant than the other
compound.
3. (3pts) What is the significance of an E2/SN2 ratio >1? …<1?
4. (3pts) Explain the differences between the E2/SN2 ratios for the two alkyl bromides.
5. (3pts) How could you determine the value of the rate constant, k, in equation 1 from the
value of the rate constant, k’?
6. (3pts) Would the E2/SN2 ratios increase, decrease, or remain unchanged if potassium tbutoxide were used as the base instead of potassium hydroxide? Explain your answer.