Nucleophilic Substitution Reactions: Relative Reaction Rates
- Prepare a table
indicating the substrate structure for each of the halides listed
the beginning of the Experimental Procedure section below. Label them
as aliphatic, benzylic, or aromatic, and indicate whether they are
secondary, or tertiary. (See Hornback, pages 153-154 and 466-468)
- Note the structures
and boiling points of ethanol and acetone. Using
Mohrig, determine the relative polarity of these two organic solvents.
Description of Experiment--see
below & Appendix.
Name of Halide
Type of Halide
The following experiments are often used to identify when a halide is
present or how it is attached on an unknown compound. We will use these
experiments to continue our investigation of nucleophilic substitution
reactions. Read this experiment through from start to finish before coming
There are four main
components to a nucleophilic substitution reaction: nucleophile, substrate
structure, leaving group and solvent. In the previous lab, we varied only
one component of the nucleophilic reaction---the nucleophile. In these
experiments, we will be looking for trends in the three other aspects
of a nucleophilic reaction. To do this we will perform two experiments,
each based on a different solvent system, varying the substrate structure
and leaving group within each of the two experiments.
The two experiments are as follows:
Notice that in both
of these experiments the halide (X) is now acting as the leaving group
and not the nucleophile. By using several different leaving groups
and substrate structures in a series of related examples, we may be able
to determine a trend in the relative reactivities for these two aspects
of a nucleophilic substitution reaction. If the relative reactivities
for these two aspects of the nucleophilic reaction vary between these
two experiments then we can also look at the only other variable as
cause for this change---the solvent. The solvent for the silver nitrate
reaction is ethanol and the solvent for the sodium iodide reaction
acetone. The main distinction between these solvents is their polarity.
The protic solvent ethanol is more polar than aprotic acetone.
Even though different
nucleophiles (ethanol and iodide) are used in each experiment they do
not change the relative reactivities of each alkyl halide within each
- To determine
which organic halides react in two different nucleophilic substitution
reactions, and to rank the reactivity of those halides which react.
- To determine
how the substrate structure of the organic halide, the type of halide,
and the differences in reaction solvent polarities affect the reactivity
of the organic halide.
- To test for the
presence of halide using the Beilstein
- To perform the
Beilstein Test for Halides on the 2 unknowns. To perform the Classification
Tests for Halides on the 2 unknowns.
Do the reactions, using the following organic halides:
Label a series of ten clean test tubes from 1 to 10 and follow the procedure
given in the Appendix.
Label a series of eight clean dry test tubes from 1 to 8 and follow the
procedure as given in the Appendix.
list of 10 alkyl halides above, which two do you leave out in performing
this procedure? Why?)
either of the experiments above, see you instructor if any of your
reactions are difficult to rank.
Follow the procedure described in the Appendix.
one of the chlorides or bromides from this week's experiment, and your unknowns from your Dual Unknown lab. Record
your results in your notebook. If either of your dual unknowns tests
Beilstein Test, perform both the silver nitrate in ethanol and the sodium
iodide in acetone tests.
Which halogens give
a positive Beilstein Test?
What causes the green flame in a positive Beilstein Test?
Special Waste Disposal
Non-flammable waste bottle: NaI in Acetone experiments
waste bottle: product
from last week's Competing Nucleophiles experiment.
"Silver" waste bottle: AgNO3 in Ethanol experiments.
- Why do we use
anhydrous acetone and not the wash acetone from the bottles in the
- For each experiment,
explain how the substrate structure affects the rate of a nucleophilic
- For each experiment,
how will the nature of the leaving group influence the rate of the nucleophilic
- Do you think that
the two reactions (halide with NaI in acetone and halide with AgNO3 in ethanol) are similar or different? Justify your answer.
- Discuss any errors
in your experiment and state what you would do differently if you were
to do this lab again.