SN1 leaving group and have an overall result

SN1 & SN2 Substitution Reaction Analyses

 

Diana Zermeno

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Partner: Yoselin Acosta

TA: Muneeb Mohiuddin

Lab: Thursday, 12:00pm Room # PSE 307

 

Abstract:
The goal of this experiment was to perform a series of reactions with the same
seven halides; once with 15% sodium iodide and another with 1% silver nitrate.
The purpose was for the halide in the substrate structures to become a good
leaving group and have an overall result of a precipitation under two
temperatures, which depended on whether it was an SN1 or SN2 reaction. The
group results had the majority of the SN2 results in a 50°C reaction condition;
while the SN1 reactions occurred in a room temperature conditions. Compared to
the lab results, there was roughly the same amount of reactions between room
temperature and the designated temperature for both SN2 and SN1, which was due
to the ambiguity of the experiment.

 

Substitution
Analysis of SN1 & SN2 Reactions

 

Introduction.

The
purpose of the experiment was to observe the different conditions that SN1 and
SN2 reactions occur in. SN1 and SN2 are substitution reactions that require
breaking a bond and creating a new one. The reactant is a Lewis Base/
nucleophile that does the substituting. The aim is to see how and why the
various halides react the way they do and how the structures affect it. Bromine
and chlorine substituents are chosen because of how easily they can be removed.
The easiness in removal is due their size and the need to have a full octet.
Hence, it is expected that the chlorine is better leaving group because it is
larger. Since, chlorine is a better leaving group, it will also have a higher
reactivity.

SN2
reactions, as shown in Figure 1, are more likely to occur with primary
substrates, compared to tertiary. This is because it wants the maximum amount
of surrounding space to continue the substitution reaction. On the other hand,
SN1 reactions favor tertiary substrates because the reaction is performed in
steps, with intermediates.

Le Chatelier’s principle is when the precipitation of the
by-product moves the reaction from the left to the right. This is important to
this experiment because it signals whether the substitution reaction occurred
and under which conditions. Furthermore, the results that will be gathered in
this experiment, will not be completely reliable. The uncertainty is because there
is no distinguishing factor between substitution and elimination reactions, at
this level.

 

Experimental.1

 

 

 

 

Determine the
Mechanism of Substitution. To begin, 14 clean, dry test tubes
were gathered. Each bromide and chloride structure had 2 labelled test tubes.
Four drops of each halide were added to the corresponding test tubes. Before
continuing the experiment, a water bath was set up and warmed to 50°C.

Test
for SN2 Reaction. One set of the labelled test tubes was
used for this part of the experiment. Two mL of 15% sodium iodide in acetone
solutions was added to each of the tubes. The solutions were left for 5 minutes
to react. The solutions that produced precipitate was recorded, and the others
were places in the water bath for another 5 minutes. After, the solutions with
precipitate were recorded. The results were given to the TA and the water bath
was heated to 80°C.

Test
for SN1 Reaction. Two mL of 1% silver nitrate in ethanol
solution was added to the remaining 7 test tubes. After they were left for 5
minutes to react, the precipitated solutions were recorded. The unreacted
solutions were placed in the heated water bath for 5 minutes. The solutions
with precipitate were recorded and given to the TA.

 

 

Results.

As
shown below, most of the SN2 reactions occurred under 50°C conditions.          

Table
1.
Results for SN2 reaction

Name

Group Results

Lab Results

Room Temp

50°C

Room Temp

50°C

1-bromobutane

x

12

1-chlorobutane

x

8

4

2-bromobutane

x

2

4

2-chlorobutane

x

1

3

t-butylbutane

x

1

t-butylchloride

x

bromobenzene

           

 

            As seen in Table 2, the majority of the SN1 reactions
were completed in room temperature conditions.

Table
2.
Results for SN1 reaction

Name

Group Results

Lab Results

Room Temp

50°C

Room Temp

50°C

1-bromobutane

x

1

1-chlorobutane

1

2-bromobutane

x

2

4

2-chlorobutane

x

1

3

t-butylbutane

x

12

t-butylchloride

x

7

4

bromobenzene

 

Discussion.

As
mentioned before, Le Chatelier’s principle is used to determine if the halide
structures reacted in SN1/SN2 and in which temperature conditions. The
precipitate formed by the by-product is that determinant. For SN1 reactions, a
weak nucleophile was used, and a strong nucleophile was used for SN2 reactions.
As demonstrated in Figure 2, SN2 is a fast reaction because there are not any
intermediates involved. Therefore, since SN2 is a fast reaction, bromine is a
strong nucleophile. While SN1 is slower, therefore chlorine is a weak
nucleophile. SN2 reactions occurred mostly in 50°C conditions because it has a
higher activation energy it needs to reach. Compared to SN1, room temperature
conditions were ideal because of the lower activation energy.

As seen in the results, SN2 reactions
occurred more in 50°C and SN1 reactions were in room temperature. The better
leaving group, chlorine, had a higher chemical reactivity at room temperature.
In Table 2, it is shown that SN1 reactions mostly occurred with tertiary
substrates; while in Table 1, SN2 reactions were with primary substrates. This
aligns with the hypothesis because of the size trends for each reaction.  SN2 reactions prefer easily accessible center
atoms and SN1 do not. Since SN2 is a fast reaction, the activation energy
required is higher than SN1, which is why it occurs in higher temperatures.

 

Comparing Group and Lab results, there were a few discrepancies.
This was due to the confusion as to what was considered precipitation.
Additionally, there could have been competing reactions and possibly confusing
an elimination reaction with substitution.

 

Conclusion.

The objective of this
experiment was to see why SN2 and SN1 reactions occurred in the conditions set
and certain halide structures. It was observed and concluded that SN1 reactions
occur in low temperatures with tertiary substrates, since it has a low
activation energy. Also, SN2 reactions occur in high temperatures with primary
substrates, because it has a high activation energy.

Work
Cited.

1.     Arizona
State University (2017a, November 5). Substitution Synthesis Lab Package.

Retrieved
from https://myasucourses.asu.edu/