SN 1 and SN 2 reactivity of 3 alkyl bromides
Willem Elbers
October 9, 2015
1
Abstract
In this experiment, we investigate the relative reactivities of three alkyl bromides with increasing steric bulk. We react the primary halide 1-bromobutane,
the secondary halide 2-bromobutane, and the tertiary halide 2-bromo-2-methylpropane with a solution of sodium iodide in acetone and a solution of silver
nitrate in ethanol. We wish to determine the mechanism behind these reactions. Results suggest that the reactions with NaI follow the SN 2 mechanism,
whereas the reactions with AgNO3 follow the SN 1 mechanism.
2
Theory
Due to the electronegativity of bromine, the alkyl bromides contain an electrophilic carbon atom. Nucleophilic compounds, such as iodide and ethanol,
may therefore attack the carbon atom, thereby replacing the bromine. The
formation of a white precipitate (NaBr or AgBr) indicates whether the reaction has occurred. This process of nucleophilic substitution can occur along
two pathways: unimolecular (SN 1) and bimolecular (SN 2) nucleophilic substitution.
In the SN 2 reaction, the nucleophilic attack and the breaking of the CBr bond occurs in a single step. See the transition state of the reaction of
2-bromobutane with NaI in figure 1. The process is therefore characterised
by second-order kinetics. [3, 223-228] The SN 1 reaction occurs in three steps.
First, the C-Br bond undergoes heterolytic cleavage, forming a carbocation.
Because opposite charges are separated, this step is slow and rate-determining.
Hence, the SN 1 process is characterised by first-order kinetics. In the second
step, the carbocation is attacked by the nucleophile, which is subsequently
deprotonated in the final step. [3, 251-256] See the reaction mechanism for
2-bromobutane with AgNO3 in figure 2.
1
A number of structural factors determines which of the two pathways dominates. First of all, the presence of alkyl groups around the electrophilic carbon
atom can hinder the nucleophile through steric repulsion. Because of this, the
SN 2 reaction occurs much faster for primary bromides than for secondary bromides and not at all for tertiary bromides. On the contrary, the presence
of alkyl groups stabilises the carbocation through hyperconjugacy, increasing
SN 1 reactivity. Indeed, primary halides do not undergo the SN 1 reaction because the energy of the carbocation, which features little hyperconjugacy, is
prohibitively high. A second factor is the polarity of the solvent. The first
step of the SN 1 process involves separating opposite charges, which is eased
by the presence of a polar solvent. In particular, SN 1 reactivity in protic solvents (ethanol) is significantly greater than in aprotic solvents (acetone). For
the SN 2 process, the effect is the opposite, because protic solvent molecules
encapsulate the nucleophile, weakening its attack on the electrophile. This is
not a factor in the SN 1 process, because the nucleophilic attack is not rate determining. This is also why the strength of the nucleophile does not influence
the kinetics of the SN 1 process, unlike that of the SN 2 process. [3, 235-243,
258-261]
These considerations imply that the SN 2 reaction will dominate the SN 1
reaction for the alkyl bromide/NaI/acetone system, because iodide is a good
nucleophile (which increases SN 2 reactivity), and acetone is aprotic (decreasing SN 1 reactivity). See figure 3 for the reactions. On the other hand, for the
alkyl bromide/AgNO3 /ethanol system, the SN 1 reaction will dominate, because NO−
3 is a poor nucleophile (decreasing SN 2 reactivity) and ethanol is a
protic solvent (increasing SN 1 reactivity, as well as decreasing SN 2 reactivity).
See figure 4 for the reactions. Finally, we observe that the tertiary bromide
will likely react very slowly with NaI, because the SN 2 reaction is significantly
hindered by steric effects. Likewise, unless additional energy is supplied, the
primary bromide will react very poorly in ethanol, because the energy of the
unstable carbocation formed in the SN 1 reaction is prohibitively high.
2
‡
Br
Br
CH2CH3
CH3C
Na I
CH2CH3
CH
acetone
Na+
CH3
CH3
I
acetone
CH2CH3
HC
Na Br
CH3
I
Figure 1: Reaction between 2-bromobutane and NaI/acetone. The SN 2 reaction is a one step process: in the transition state, the C-Br bond is broken as
the C-I bond is forming. Note the backside attack of the nucleophile.
Br
OH
CH
CH3
CH2CH3
NO3-
AgNO3
+
CH
CH2CH3
OH
CH3
Ag Br
H
O
NO3-
OH2
HC
OH
CH2CH3
O+
NO3-
HC
CH2CH3
CH3
CH3
Figure 2: Reaction between 2-bromobutane and AgNO3 /ethanol. In the first
step, the C-Br bond is cleaved in the presence of silver, which then coordinates
with the leaving group NO3 . In the second step, the nucleophile ethanol attacks
the carbocation. Finally, the product is deprotonated by ethanol.
3
acetone
Br
NaI
NaBr
I
Br
I
acetone
NaBr
NaI
I
Br
acetone
NaBr
NaI
Figure 3: Reaction between the alkyl bromides and NaI in acetone. The third
reaction is very slow.
Br
AgNO3
OH
O
Br
AgBr
NO3-
H+
O
OH
AgNO3
Br
AgBr
NO3-
H+
AgBr
NO3-
H+
O
OH
AgNO3
Figure 4: Reactions between the alkyl bromides and AgNO3 /ethanol. The
first reaction will go to completion only when energy is added.
4
3
Experimental
Initially, two solution were prepared: 3 g of NaI in 20 ml acetone (solution A)
and 0.34 g (0.1 M) AgNO3 in 20 ml ethanol (solution B). Test tubes labelled
A1, A2, and A3 were each filled with 3 ml of solution A. Subsequently, 3-4
drops of the alkyl bromides were added: 1-bromobutane (A1), 2-bromobutane
(A2), and 2-bromo-2-methylpropane (A3) and carefully mixed with the solution. If no white precipitate had formed after 5 minutes, a heat gun was used to
speed up the reaction. Next, test tubes labelled B1, B2, and B3 were each filled
with 3 ml of solution B. Again, 3-4 drops of the alkyl bromides were added: 1bromobutane (B1), 2-bromobutane (B2), and 2-bromo-2-methylpropane (B3)
and mixed with the solution. If no precipitate formed within 5 minutes, the
mixture was heated. Once the precipitate had formed, the pH of the solutions
in B1, B2, and B3 was measured, as well as that of the initial solution B. [1, 2]
4
Results and discussion
Within 3 minutes, a precipitate had formed in tube A1 containing the primary bromide 1-bromobutane. After 5 minutes, no precipitate had formed in
tubes A2 and A3, which had to be heated to induce the reaction. In tube
B3, containing 2-bromo-2-methylpropane, a white precipitate formed almost
instantaneously. Tube B2, containing 2-bromobutane, followed within the first
minute. No precipitate formed in tube B1, until it was heated after 5 minutes.
Due to time limitations, pH values were not measured, but another experimenter reported values of pH = 5 for solutions B and B1, pH = 4 for B2, and
pH = 3 for B3. See the table below for an overview.
Label
A1
A2
A3
B
B1
B2
B3
Substrate
1-bromobutane
2-bromobutane
2-bromo-2-methylpropane
1-bromobutane
2-bromobutane
2-bromo-2-methylpropane
Solution
Duration
pH
NaI/acetone
∼3 minutes
NaI/acetone
>5 minutes
NaI/acetone
>5 minutes
AgNO3 /ethanol
5
AgNO3 /ethanol
>5 minutes
5
AgNO3 /ethanol
< 1 minute
4
AgNO3 /ethanol instantaneously
3
Table 1: Overview of experimental results. Duration indicates the time until
a white precipitate (NaBr or AgBr) has formed.
The results clearly indicate that for the reactions with AgNO3 /ethanol
5
(solution B), the most reactive bromide is 2-bromo-2-methylpropane, followed
by 2-bromobutane and finally 1-bromobutane. This matches the theoretical
prediction for SN 1 reactions, where increasing steric bulk improves reactivity
due to hyperconjugacy. Moreover, the decrease in pH values in tubes B2 and
B3 suggests that the deprotonation step of the SN 1 process has occurred (see
figure 2). The negligible change in pH in tube B1 may be explained by the
fact that the reaction occurred only slightly. Alternatively, the heat may still
have induced an SN 2 reaction with the poor nucleophile NO3 .
For the reactions with NaI/acetone (solution A) the picture is less clear,
because only in tube A1 did precipitate form within the allotted 5 minutes. Although it is clear that 1-bromobutane is the most reactive, we cannot clearly distinguish between 2-bromobutane and 2-bromo-2-methylpropane.
Theory suggests that for the SN 2 process, where increasing steric hindrance
decreases reactivity, 1-bromobutane should be the most reactive, followed by
2-bromobutane and finally 2-bromo-2-methylpropane. This is consistent with
our results.
5
Conclusion
In this experiment, we have tested the reactivities of 3 alkyl bromides: 1bromobutane, 2-bromobutane and 2-bromo-2-methylpropane with a solution
of NaI in acetone and a solution of AgNO3 in ethanol. For the NaI/acetone
reactions, we found that 1-bromobutane was the most reactive, although we
could not distinguish between 2-bromobutane and 2-bromo-2-methylpropane.
This is consistent with the second-order nucleophilic substitution (SN 2) mechanism. For the AgNO3 /ethanol reactions, 2-bromo-2-methylpropane was the
most reactive, followed by 2-bromobutane, and finally 1-bromobutane. A measured decrease in pH values confirms that these reactions follow the first-order
SN 1 mechanism.
References
[1] Practical Manual, Molecules: Structure, Reactivity and Function, Faculty
of Mathematics and Natural Sciences, University of Groningen, 2015.
[2] J.W. Lehman. Multiscale Operational Organic Chemistry: A Problemsolving Approach to the Laboratory Course. Prentice Hall, 2002. 457-477.
[3] K.P.C. Vollhardt and N.E. Schore. Organic Chemistry: Structure and
Function. W. H. Freeman, New York, 6th edition, 2010.
6