LAB 9: NUCLEOPHILIC SUSTITUTION REACTIONS
PRE-LAB:
1. Complete the Safety section for Lab 10.
2. a) Draw the following compounds: 1-chlorobutane, 2-chlorobutane, 2-methyl-2chloropropane, and benzyl chloride. Classify them as primary, secondary, tertiary halides.
b) Draw the carbocations that would form if the halide was removed. Rank the stability of the
carbocations from least stable to most stable.
3. Set a table up in your notebook with the following headings: compound, predicted reaction for
test SN1, predicted reaction for SN2, experimental reaction for test SN1, experimental reaction for
test SN2. Use + or – to denote reaction or no reation, respectively. Now make predictions for
each reactant under SN1 and SN2 conditions.
PURPOSE: You will learn and use the techniques of classical qualitative organic analysis to
study the chemical differences between Sn1, Sn2, E1, and E2 reactions of organic halide
compounds.
BACKGROUND INFORMATION:
Substituted alkanes can undergo displacement or elimination reactions to give new
compounds. These reactions can be useful in helping to identify the nature of the compound or
to synthesize new classes of compounds. Next semester, you will synthesize an alkene by an E1
elimination reaction of an alcohol. In this experiment you will study the differences among the
displacement reactions of alkyl halides.
Nucleophilic Substitution Reaction:
R
X
+
Nu-
R
Nu
+
X-
As a background for this experiment, you should read the relevant chapter in the lecture course
textbook. There are many facets of these reactions that you should be aware of. Hopefully, this
experiment will help you better understand these classes of reactions.
The four types of reactions we will consider include two substitutions (Sn1 and Sn2) and
two eliminations (E1 and E2). There are four different mechanisms (reaction pathways) and the
mechanism that will predominately occur is dependent on many factors such as: (1) the nature of
the alkyl group, (2) the nature of the leaving group, (3) the nature of the solvent, and (4) the
temperature of the reaction. It is possible to have mixtures of products depending on how the
reaction is carried out. Recall before we start that substituted alkanes can undergo these
reactions readily but substituted alkenes or alkynes generally do not undergo these reactions.
Further, substituted aromatic compounds are not reactive.
In this experiment we will use reactions of alkyl and aryl halides. We will vary the halide
between chloride, bromide and iodide. We will use various primary, secondary, and tertiary
alkyl compounds. We will use a reaction (silver nitrate in aqueous alcohol) that favors the Sn1
mechanism and a reaction (sodium iodide in acetone) that favors the Sn2 mechanism. A positive
test for the silver nitrate reaction will be the precipitation of silver chloride, silver bromide or
silver iodide depending on the halide being tested. Silver halides darken on exposure to light.
Also, silver nitrate will cause a black stain if it touches your skin. This stain will go away in a
few days. A positive test for the sodium iodide reaction will be the precipitation of sodium
chloride or sodium bromide depending on the alkyl halide being tested. Note that sodium iodide
has reasonable solubility in acetone while sodium chloride and sodium bromide are insoluble in
acetone. We will not directly study elimination reactions in the experiment but elimination may
occur as an unwanted side reaction you should be aware of. You should address and discuss this
complication in your report.
SN1 Mechanism: Test #1:
R–X
R
+
X
Ag
ethanol
EtOH
ppt
H
R
O
–H
Et
R
O
Et
Silver nitrate is soluble in ethanol, but silver chloride is not.
SN2 Mechanism: Test #2:
R-X
+
Na
+
I
R-I
+
NaX
Sodium iodide is soluble in acetone. However, sodium chloride is not.
Primary alkyl groups favor the Sn2 mechanism and tertiary alkyl groups favor the Sn1
mechanism. Both mechanisms may apply in the case of a secondary alkyl group. Alkyl groups
that can give a stable carbocation (carbenium ion) such as the allyl or benzyl group tend to show
an Sn1 mechanism even though they are strictly primary groups.
In your tests, you should note whether or not you observe formation of a precipitate and,
if you do, how many minutes were needed at room temperature and whether heat needed to be
applied. No test should take more than 20 minutes and many tests can be done concurrently if
you carefully label your test tubes.
EXPERIMENTAL PROCEDURE:
Halogenated organic waste is usually kept separate from non-halogenated waste.
The disposal of halogenated organic wastes is more costly. Be sure to use the proper waste
containers in this experiment.
You will test the following substances with silver nitrate in aqueous alcohol in one test tube and
sodium iodide in acetone in a second test tube: 1-chlorobutane, 2-chlorobutane, 2-methyl-2chloropropane (t-butyl chloride), benzyl chloride.
1) Carefully label a number of test tubes and begin heating a beaker of water on a hotplate in the
hood for use as a hot water bath.
2) The test is done by placing 1.0 ml of the test solution (either silver nitrate solution or sodium
iodide solution) in a labeled test tube and adding 10 drops of the halide to be tested. Be sure to
shake the tube to mix the reagents. Let the tube sit at room temperature and observe the time
needed for precipitation to occur and the approximate amount obtained.
3) If no precipitation is noted in 5 minutes place the tube in the hot water bath and observe the
time needed for precipitation to occur. If no precipitation is noted in 10 more minutes, record the
alkyl halide as unreactive. Be careful to note any evaporation of solvents which may give you
erroneous results.
Question: If water were present in the test tube in which you carried out the reaction with NaI,
how might this change the results? Is acetone soluble in water? Is sodium chloride?
4) Record your results noting the formation of any precipitate and whether time or heat was
needed for any precipitation to occur.
5) You must dispose of halogenated waste in a different container than other organic waste.
IMPORTANT INFORMATION ABOUT THE REPORT:
Post-Lab questions:
1. Carefully tabulate the results of all your tests in a nice table format. Discuss your findings and
note how your results support the facts you have learned about substitution reactions. Note any
discrepancies you have found and give reasons for these anomalies. For any reactions that gave
a positive result predict the product and show the reaction that took place.
2. How can you predict if a particular organochloride compound will react under the different
conditions?
3. If water were present in the test tube, how might this change the results? Is ethanol soluble in
water? Is silver chloride?
4. What other reactions could compete with the substitution reactions to give false positives?
Give at least one example and show the chemical reaction.
END OF EXPERIMENT.
© 2010 Carberry and Carreon