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Relative Reactivity of Alkyl Halides
Organic Chemistry I Lab (University of Alabama at Birmingham)
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Relative Reactivity of Alkyl Halides
CH 236-A8 Group 7
Writer: Jama Messner
Reviewer: Jacob Foncea
Editor: Kenny Simmons
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Introduction
After over seventy years of study, two mechanisms for nucleophilic substitution have become very
useful and important: SN1 (unimolecular reaction) and SN2 (bimolecular reaction).2 The two mechanisms are
primarily distinguished by their rate-limiting steps and the timing of the bonds forming and breaking.3
In SN1 reactions, a leaving group departs from the substrate and leaves behind a carbocation
intermediate, then a nucleophile attacks and bonds to the substrate.3 This mechanism occurs in two steps, and
the first step, that of the leaving group leaving, is the rate-limiting step.3 Therefore, only the substrate
concentration affects the rate of the reaction.
In some cases, such as when the reaction takes place in water, the solvent may act as the nucleophile.
This type of SN1 reaction is called a solvolysis reaction, or, in water’s specific case, hydrolysis.1
In SN2 reactions, the nucleophile attacks from the backside of a carbon that is bonded to a leaving
group; the leaving group departs at the same time the nucleophile bonds.1 This mechanism happens in one
concerted step, meaning the entire reaction occurs during the rate-limiting step.3 Thus, both the substrate and
nucleophile concentrations affect the rate of the reaction.
Understanding these mechanisms allows chemists to predict which products will form from reactions
between different substrates and nucleophiles, depending on a short list of factors. These include the
substrate’s level of substitution (the substrate is generally an alkyl halide), the stability of the leaving group, the
strength of the nucleophile, and the type of solvent in which the reaction occurs.1,3
The dominating factor in determining which substitution mechanism occurs is the structure of the alkyl
halide. Less substituted alkyl halides prefer the SN2 mechanism as there is less steric strain, which allows
backside attack.1 Furthermore, less substituted alkyl halides are less likely to form a carbocation intermediate
because of how unstable it would be, so SN1 reactions simply couldn’t occur.1 More substituted alkyl halides,
on the other hand, prefer the SN1 mechanism, as it is easier for them to form a stable carbocation intermediate,
and the steric strain prevents SN2 from occurring.1
Both mechanisms prefer leaving groups that are stable and therefore able to depart from the substrate,
but they differ again in the type of solvents. Polar protic solvents are better for SN1, and polar aprotic solvents
are better for SN2 reactions.1
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The general mechanisms for each reaction are depicted below in Figures 1 and 2.
Figure 1. Mechanism for a general SN1 reaction between a substrate and a nucleophile.
Figure 2: Mechanism for a general SN2 reaction between a substrate and a
nucleophile.
Table 1. Table of Reagents4
Name
2-Bromo-2-Methylpropane
2-Bromobutane
1-Bromobutane
1-Chlorobutane
Methanol
Ethanol
1-proponal
Acetone
Water
NaI solution
AgNO3
NaOH
MW (g/mol)
137.02
137.02
137.02
92.566
32.042
46.069
60.896
58.08
18.015
149.894
169.87
39.999
MP (°C)
-16.2
-169.4
-112.4
-123.1
-97.6
-114.1
-126.1
-96.55
0
660
212
323
BP (°C)
73.3
91.2
101.3
78.6
64.7
78.2
97.2
58.75
100
1204
440
1388
Density (g/cm3)
1.21
1.26
1.28
0.886
0.81
0.79
0.0853
0.78
1
3.67
5.35
2.13
Experimental
5 drops of each of the following alkyl halides were pipetted into a different test tube: 2-bromo-2methylpropane, 2-bromobutane, 1-bromobutane, and 1-chlorobutane. 20 drops of 15% NaI solution in acetone
was then added to each test tube. The time of the first drop in each test tube was recorded exactly, then the
test tubes were shaken to stir the contents, and the time it took for the formation of a precipitate was recorded.
Whenever precipitates did not form within 5 minutes, the tubes were placed in warm water baths between 5060°C.
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The test tubes were emptied and cleaned with ethanol or acetone. The procedure was repeated, except
that instead of adding 15% NaI solution in acetone to the 5 drops of each alkyl halide, 20 drops of 1% AgNO3
solution in ethanol was added. The time that the first drop was pipetted into each solution was recorded
exactly, then the contents of each tube were shaken, and the time it took to form a precipitate was recorded for
each test tube.
In the third part of the experiment, four clean, dry test tubes were placed in a test tube rack. The four
test tubes were each filled with one of the four solutions: 1:1 methanol/water, 1:1 ethanol/water, 1:1 1propanol/water, and 1:1 acetone/water. 5 drops of 0.5 M NaOH and 3 drops of 1% phenolphthalein were added
to each test tube. Each solution appeared pink in color. Then, 3 drops of 2-bromo-2-methylpropane were
added to each test tube, with the exact time of the first drop in each tube recorded. The solutions were shaken,
and the exact time that the pink color of the solution disappeared was recorded exactly.
Results
The structural effects on a SN2 reaction were studied in Part A of the experiment. For each of the alkyl
halides, the exact time (hh:mm:ss) of the initial addition of the NaI in acetone solution to the test tube was
recorded, as well as the exact time (hh:mm:ss) cloudiness was observed and/or a precipitate was formed.
Additionally, if the reaction was not progressing, the test tube was placed in a warm water bath, as indicated in
Table 2 below. The total time elapsed (hh:mm:ss) for each of the 4 reactions was then calculated and recorded,
as shown in Table 2.
It was observed that 2-bromo-2-methylpropane showed no signs of cloudiness, rather it formed a white,
flaky precipitate after about 10 minutes elapsed. Additionally, it was noted that 1-bromobutane completed the
reaction at the fastest rate and did not require placement in the warm water bath.
Table 2. Part A Results- Structural Effects on SN2 Reaction
Time of
Warm Water
Time for
Time of Solid
Total Time
Addition
Bath?
Cloudiness
Formation
Elapsed
(hh:mm:ss)
Y/N
(hh:mm:ss)
(hh:mm:ss)
(hh:mm:ss)
1
2-bromo-2-methylpropane
08:51:05
Y
N/A
09:01:50
00:10:45
2
2-bromobutane
08:37:12
Y
08:57:15
08:59:52
00:22:40
3
1-bromobutane
08:41:28
N
08:42:56
08:44:01
00:02:33
4
1-chlorobutane
09:08:00
Y
09:11:43
09:19:01
00:11:01
Table 2 shows the time of the addition of NaI in acetone solution to each test tube, the time signs of cloudiness were
observed (if any), the time a solid / precipitate formed, and the total elapsed time for the reaction to occur. All times were
recorded by hh:mm:ss. Additionally, if a warm water bath was required, it was denoted.
Test
Tube #
Compound / Alkyl Halide
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The structural effects on a SN1 reaction were studied in Part B of the experiment. For each of the alkyl
halides, the exact time (hh:mm:ss) of the initial addition of the AgNO3 in ethanol solution to the test tube was
recorded, as well as the exact time (hh:mm:ss) cloudiness was observed and/or a precipitate was formed. Like
Part A, if the reaction was not progressing, the test tube was placed in a warm water bath, as indicated in Table
3 below. The total time elapsed (hh:mm:ss) for each of the 4 reactions was then calculated and recorded, as
shown in Table 3.
In Part B, it was observed that neither 2-bromo-2-methylpropane nor 2-bromobutane required
placement in the warm water bath, as both reactions completed in less than a minute. Also, it was denoted that
even after placement in the warm water bath, no precipitate formed for 1-chlorobutane.
Table 3. Part B Results- Structural Effects on SN1 Reaction
Test
Time of
Warm Water
Time for
Time of Solid
Total Time
Tube
Compound / Alkyl Halide
Addition
Bath?
Cloudiness
Formation
Elapsed
#
(hh:mm:ss)
Y/N
(hh:mm:ss)
(hh:mm:ss)
(hh:mm:ss)
1
2-bromo-2-methylpropane
09:27:06
N
09:27:13
09:27:40
00:00:36
2
2-bromobutane
09:12:16
N
09:12:57
09:13:02
00:00:46
3
1-bromobutane
09:15:09
Y
09:21:45
09:24:32
00:09:23
4
1-chlorobutane
09:16:36
Y
09:17:09
no solid formed
00:00:33
Table 3 shows the time of the addition of AgNO3 in ethanol solution to each test tube, the time signs of cloudiness were
observed, the time a solid / precipitate formed (if any), and the total elapsed time for the reaction to occur. All times were
recorded by hh:mm:ss. Additionally, if a warm water bath was required, it was denoted.
The solvent effects on a SN1 reaction were studied in Part C of the experiment. For each of the
compounds, the exact time (hh:mm:ss) of the initial addition of the alkyl halide, 2-bromo-2-methylpropane, to
the test tube was recorded, as well as the exact time (hh:mm:ss) the pink color, attributed to the indicator
phenolphthalein, disappeared. The total time elapsed (hh:mm:ss) for each of the 4 reactions was then
calculated and recorded, as shown in Table 4.
It was observed that the 1:1 propanol / water mixture reaction occurred the fastest, and the mixture of
1:1 methanol / water occurred relatively quickly, as well. The mixture of 1:1 Acetone / Water reacted third,
followed by the 1:1 Methanol / Water mixture reaction occurring last, which turned from pink to a light-yellow
color. This was unlike the other mixtures, as the other three mixtures turned from pink to clear.
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Table 4. Part C Results- Solvent Effects on SN1 Reaction
Test
Time of Addition
Time of Disappearance
Total Time
Tube
Solvent Mixture
of Alkyl Halide
of Pink Color
Elapsed
#
(hh:mm:ss)
(hh:mm:ss)
(hh:mm:ss)
1
1:1 Methanol / Water
09:39:50
09:48:36
00:08:46
2
1:1 Ethanol / Water
09:39:52
09:41:50
00:01:58
3
1:1 Propanol / Water
09:39:54
09:40:50
00:00:56
4
1:1 Acetone / Water
09:39:55
09:45:15
00:05:20
Table 4 shows the time of the addition of alkyl halide 2-bromo-2-methylpropane to each test tube, the time the pink color
disappeared, and the total elapsed time for the reaction to occur. All times were recorded by hh:mm:ss.
Discussion
In Part A of the experiment, an SN2 the preferred reaction order was 1° > 2° > 3° carbocation stability,
with 1° being the most preferred. This was consistent with a SN2 reaction because the major hindrance
experienced is steric strain5. The clutter in tertiary carbocations prevents the nucleophile from back-attacking
the carbocation. There is very little steric strain in primary carbocations, which allows for faster formations. In
Part A there was a large time difference in precipitate formation between 1-chlorobutane and 1-bromobutane,
1-bromobutane formed a precipitate faster than 1-chlorobutane. Both alkyl halides formed primary
carbocations. In both SN1 and SN2 reactions, a stable leaving group is preferred. The leaving group of 1bromobutane, bromine, was better than 1-chlorobutane, chlorine, because its larger size could better stabilize
the carbocation. In Part B the preferred reaction order for an SN1 reaction was 3° > 2° > 1° carbocation
stability, with 3° being most preferred. This was consistent with SN1 reactions because the major hindrance
was carbocation stability5. Tertiary carbocations are the most stable and can be made the fastest. A precipitate
was not formed with 1-chlorobutane in a reasonable amount of time due to chlorine being a very poor leaving
group. In Part C the best combination of solvents was 1-propanol/water > ethanol/water > acetone/water >,
and methanol/water. This was mostly consistent with solvents and SN1 reactions. All the solvents used were
polar protic, meaning they had the ability to form a hydrogen bond. Polar protic solvents work well with SN1
reactions because they can act as a nucleophile5. Dielectric constants, regarding a solvent, is a measure of
polarity. The higher the number, the more polar the solvent6. When the very polar water was combined with
the slightly polar methanol, the overall polarity of the mixture is decreased. This is because like dissolves like.
The reduced polarity decreases the mixture’s ability to act as a nucleophile. Possible sources of error could
have been stopping and starting the times too early or too late.
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Conclusion
SN1 reactions prefer tertiary carbocations in alkyl halides while SN2 reactions prefer primary carbocation
in alkyl halides. The stability of the leaving group can also affect the formation of a precipitate. The larger the
difference in dielectric constant, the better ability for the mixture to act as a nucleophile in a SN1 reaction.
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References
1. Casselman, B.L. Kinetic Study of the Hydrolysis of t-Butyl Chloride.pdf
http://uab.instructure.com (accessed Mar 24, 2019).
2. Collison, Christina G., Cody, Jeremy A., and Stanford, Courtney. An SN1-SN2 Lesson in an Organic
Chemistry Lab Using a Studio-based Approach. Journal of Chemical Education 89.6 (2012): 750-54. Web.
3. Moore, J.W.; Stanitski, C.L. Chemistry: The Molecular Science, 3rd ed.; Cengage Learning: Stamford, 2015.
4. Royal Society of Chemistry. Chemspider.
http://www.chemspider.com/Chemical-Structure.175.html (accessed Mar 24, 2019).
5. Ashenhurst, J. Master Organic Chemistry. Home.
https://www.masterorganicchemistry.com/2012/08/08/comparing-the-sn1-and-sn2-reactions/ (accessed Mar
24, 2019).
6. Libretexts. Dielectric Constant.
https://chem.libretexts.org/Ancillary_Materials/Reference/Organic_Chemistry_Glossary/Dielectric_Constant
(accessed Mar 24, 2019).
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Lab Report Questions
1. What would be the major product if 1-bromo-3-chloropropane was allowed to
react with one equivalent of NaI in acetone? Explain.
 The major product of the reaction between 1-bromo-3-chloropropane and one equivalent of Nal in
acetone is 1-iodo-3-chloropropane because the bromide ion is a better leaving group than the chloride
ion.
2. In the reaction of 1-chlorobutane with cyanide ion, the reaction rate is increased
by the addition of a catalytic amount of sodium iodide. Explain this result.
 Iodine is a strong nucleophile and leaving group. The I- ion replaces the Cl to form
1-iodobutane. Since the iodo group is a better leaving group than the chloro group, the reaction goes
faster. As the reaction reproduces NaI, it is added back into the first equation.
R-Cl + NaI
R-I + NaCl
R-I + NaCN
R-CN + NaI
3. What would be the major product if 1,4-dibromo-4-methylpentane was allowed to
react with:
a. One equivalent of NaI in acetone?
 1-iodo-4-bromo-4-methylpentane
b. One equivalent of silver nitrate in ethanol?
 2-bromo-2-methylpentane + silver bromide precipitate
4. What causes the color change in Part C?
 The color change is caused by a titration between hydrogens from the solvents with the NaOH pipetted
into the solution. It occurs in the presence of an indicator, phenolphthalein (pink). The more protic
solvents completed the titration and exhibited a faster color change.
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