Chem 21 Fall 2009
1
Experiment 7 —
Nucleophilic Substitution
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Pre-lab preparation (1) Textbook Ch 8 covers the SN2 and SN1 mechanisms. Read/review as
necessary. (2) Write the SN2 reaction of 1-bromobutane with NaI. Illustrate the electron flow
with curved arrows. Since this is a one-step reaction, you've just written the mechanism. (3)
Write a balanced equation representing the SN1 solvolysis of tert-butyl bromide in ethanol. The
product is t-Bu–O–Et. Write the reaction mechanism. (Refer to the introduction below or the
text if we haven't gotten to this mechanism in class before your lab.) (4) Draw the structures of
all the organo-halides you will be using in this lab. Don't bother looking up physical properties
of these. (5) You should, however, know the boiling points of the two solvents you'll be using.
(6) We'll be using two different solutions — NaI in acetone for SN2 reactions, and AgNO3 in
ethanol for SN1 reactions. Briefly explain why (a) the SN1 reaction pathway is disfavored with
NaI/acetone, and (b) why the SN2 pathway is disfavored with AgNO3/EtOH.
Nucleophilic substitution is one of the most useful and well studied class of organic
reactions. These reactions can occur by a range of mechanisms. SN2 and SN1 are the extremes.
The SN2 reaction occurs in a single step. The nucleophile enters as the leaving group —
usually a halide ion — departs. The reaction displays second-order kinetics; its rate is
proportional to the concentration of the organo-halide and the nucleophile.
In the SN1 reaction loss of the leaving group occurs first to generate a carbocation
intermediate. The carbocation then captures a nucleophile, often the solvent (followed by proton
transfer to produce the final neutral product). In this case the reaction is called a solvolysis.
Because the first step is rate-determining, the SN1 reaction displays first-order kinetics; its rate
depends only on the concentration of the organo-halide. It will be easier to remember which
label goes with which mechanism if you associate the "1" in SN1 with carbocation rather than
with the kinetic order of the reaction. (Perhaps this should be called SNCarbocat or SNC+.)
