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Experiment 8
The SN2 Reaction: Factors affecting SN2 Reaction
Done by: Mammar Al-Kalbani
TA: Abiola Azeez
03/17/2017
Introduction:
Chemical reactions are significant in producing everyday products. Every reaction has
its factors and reasons to happen. An example of these reactions is substitution reaction in which
a functional group or an atom is substituted by another (Meislich, 2001). In addition, there are
two types of mechanisms involved in substitution reactions: SN1 and SN2. The SN1 mechanism
occurs when “a tertiary alkyl halide reacts with a good nucleophile and weakly basic solvent, and
the secondary alkyl halide reacts with a poor nucleophile and protic solvent.” On the other hand,
when a primary alkyl halide reacts with a good nucleophile with a strong unhindered base or
with a weak basic solvent it promotes SN2. Also, “when a secondary alkyl halide is being reacted
with a poor nucleophile in a polar aprotic solvent” (Earley, 2009). There are factors controlling
the rate at which the reaction happens (both speed and time). The main three factors that affect
SN2 mechanism “are steric hindrance, the nucleophilicity (concentration and reactivity) of the
reactant that substitutes the alkyl halide (nucleophile), and the nature of the leaving group
(physical and chemical properties)” (Earley, 2009). Other minor factors can be the solvent that is
used in a reaction (Weldegirma, 2017). In this experiment, an “SN2 reaction will be performed to
examine how steric hindrance, nucleophilicity and nature of the leaving group affect the rate of
an SN2 reaction” (Weldegirma, 2017). The figures below show the mechanism of SN2 reaction
in trimethylamine and the possible side reaction of it.
Reactions:
Figure 1: SN2 reaction between trimethylamine and HC3I.
Figure 2: Possible side-reaction from the first part
Experimental Procedure:
A mixture of
the provided
solvent was
prepared in the
ratio of 1:5:4 of
Acetone: ether:
pentane.
In test tube #2,
the same
procedure was
repeated as in
#1.
In test tube #3,
15 drops of 2bromopropane
were poured in.
RECORD
observations.
Melting point
test was
conducted to
the collected
product and
compared to
the list
provided in the
manual.
3 medium test
tubes and 3
small ones
were prepared.
Another test
tube with the
unknown was
prepared.
Each set was
labeled 1-3
(two sets total)
20 drops of
trimethylamine
were poured in
test tube #1.
Then 10 drops
were added of
methyl iodide.
20 drops of
ethyldiisoprop
ylamine and
10 drops of
diethyl iodide
were poured
in test tube #3.
Observe!
20 drops of
triethylamine
were added to
the second set
consisting of
small test
tubes.
In test tubes #1
& #2 15 drops
of iodoethane
and another 15
drops of 1bromopropane
were mixed.
40 drops of
the unknown
amine were
poured in the
seventh test
tube plus 2 ml
of the ‘1:5:4’
solvent.
“10 drops of
methyl iodide
was added to
the unknown
test tube. The
tube was shaken
constantly.”
Using vacuum
filtration, the
precipitate was
collected and
dried on a filter
paper.
Table of chemicals:
Table 1 shows the chemical and physical properties of the chemicals used in this experiment
Chemical used
Chemical’s Structure
Chemical
Molecular
M.P. and
’s
weight
B.P. (ºC)
Formula
(grams/mol
e)
1-Bromopropane
C3H7Br
123
Bp: 71
Mp: 70
Iodoethane
C2H5I
155.97
Bp: 42.51
Mp: -111.11
2-Bromopropane
C3H7Br
123
Bp: 59.37
Mp: -110.6
Triethylamine
C6H15N
101.2
Bp: 89.5
Mp: -114.72
Tripropylamine
C9H21N
143.267
Bp: 156
Mp: -94
Pentane
C5H12
72.15
Bp: 36
Mp: -129.83
Ethyldiisopropylamine
C8H19N
129.247
Bp: 127
Mp: -46
Acetone
C3H6O
58.07
Bp: 56.03
Mp: -95
Diethyl ether
C4H10O
74.12
Bp: 34.6
Mp: -116.31
Iodomethane
CH3I
141.94
Bp: 42.43
Mp: 71.5-73
Results
Tube
Nucleophile
n.o.
Present
Substrate used
Solvent used
Reaction
Precipitati
Color of
on
solution/
precipitate
1
Trimethylamine
Iodomethane
1:5:4 of
Yes
Small to
Yellow –
(Acetone:Diethyl-
(few
no
white
ether:pentane)
seconds)
precipitati
on
2
Tripropylaminne
Iodomethane
1:5:4 of
Yes
(Acetone:Diethyl-
(5 min)
Yes
Cloudy
(white)
ether:pentane)
3
Ethyl-
Idomethane
diisopropylamine
1:5:4 of
_____
No
Cloudy
Yes
Yes
White
(Acetone:Diethylether:pentane)
1
Trimethylamine
Iodoethane
_____
(60 sec)
2
Trimethylamine
1-bromopropane
_____
_____
(cloudy)
No
Yellowish
3
Trimethylamine
2-bromopropane
_____
_____
No
clear
7
Unknown
Methyl iodide
1:5:4 of
Yes
Yes
cloudy
(Acetone:Diethyl-
rapidly
ether:pentane)
Melting Point of the unknown amine: 150-160˚C
Unknown Salt: (C6H5)CH2N(CH3)3+ IDiscussion:
In this experiment, both sets of test tubes used showed different results showing different
factors affecting the substitution reaction (SN2). The first couple of test tubes showed rapid
precipitations within short amount of time. This was because of the chemicals containing strong
nucleophiles reacting with aprotic solvents and performed an SN2 reaction. On the other hand,
there was neither reaction nor precipitation in test tube #3. “This was because of the steric
hindrance on the substrate which caused the reaction SN2 to slow” (Earley, 2009). The alkyl
halide is a strong leaving group as the first part of the experiment showed. Furthermore, in the
second sets of small test tubes, test tubes #1 and #2 had fast results in which some coloring
change and precipitations occurred. The reason behind this is that iodine is a better leaving group
the bromine in SN2 reaction and the steric hindrance is “strong since it has a primary substrate,
which is the best case for SN2 reaction to happen.” However, the third small test tube (reaction
with 2-bromopropane) didn’t show any results and might needed more time to show any results.
Moreover, for the unknown amine, the reaction happened immediately showing that the substrate
is primary and the nucleophile was a strong one. In addition, the solvent (Acetone: Diethyl ether
pentane) used in this experiment was aprotic and increased the rate of the SN2 reaction.
The melting point test helped in identifying the product from the precipitate of the
unknown amine and compare it to the quaternary ammonium salts list in the lab manual. The
unknown was found to have a melting point about 150-160˚C that is near to (C6H5)CH2N(CH3)3+
I- that has a range of melting point of 178-179˚C. This concludes that the salt was
(C6H5)CH2N(CH3)3+ I-. Although the melting point was little bit lower than the theoretical value,
it was close enough to prove the identity of the unknown.
Conclusion
The experiment proved that SN2 reaction with the presence of strong nucleophile
replacing a primary substrate and suitable solvent, the reaction would happen in a very fast rate.
However, if the nucleophile is weak and is attacking a secondary leaving group, the reaction will
not happen. The experiment was a success in accomplishing the objective, which is “performing
SN2 reaction and evaluate the effect of the steric hindrance, nucleophilicity of the amine, and the
nature of the leaving group on the reaction.” (Weldegrima, 2017). Such substitution reactions
have many industrial uses. For instance, fluid isolations are done in fluids industries. PCB’s, for
example, is “an insulating fluid that was used for a long time but it was banned later due to its
affect to the environment” (Klein, 2013).
References
Earley, C. (2009). Reactions of Alkyl Halides: Substitution vs. Elimination. Web. Mar 12, 2017
Klein, D. (2013). Organic Chemistry, 2nd edition. Web. Mar 12, 2017
Meislich, H., Nechamkin, H., & Sharefkin, J. (2001). Schaum's Easy Outlines : Based on
Schaum's Outline of Organic Chemistry by Herbert Meislich, Howard Nechamkin, and
Jacob Sharefkin. New York: McGraw-Hill Professional. Web. Mar 13. 2017
Weldegirma, Solomon, Experimental Organic Chemistry Laboratory Manual, CHM 2210L.
Department of Chemistry, University of South Florida Mason Ohio, 2017, pp. 46-48.