Fall Organic Chemistry Experiment #7
SN1 and SN2 reactions
Suggested Reading:
Jones Chapter 7
Introduction:
This experiment is designed to illustrate some of the concepts introduced in Chapters 7 and 8 of
your text. These two chapters include two of the major reaction categories in organic chemistry,
substitution reactions and elimination reactions. We will employ these two experiments as a
means of investigating some of the critical features of substitution reactions. We will be
especially interested in learning about some of the basic criteria used to describe organic reactions
(e.g. solvent effects, nucleophilicity, basicity, thermodynamics, kinetics, catalysis, etc.) and how
these criteria may influence the course of a given reaction.
One thing that we can consider is how to predict the major reaction pathway (S N2, SN1,
E2, E1) based upon the given reaction conditions. That is, how do we know which reaction
mechanism provides the correct description of the actual reaction pathway? Is it a concerted SN2
reaction or the stepwise SN1 with a carbocation intermediate? Well, the "correct mechanism" is
the one that has been the most widely accepted in the primary chemical literature and accurately
describes experimental results. Many times we can make a reasonable prediction based upon a
complete analysis of the substrate and reagents involved. If you notice, the starting material
contains a leaving group attached to a primary -carbon. Therefore, a good starting point for
predicting the outcome of a reaction is the evaluation of the specific variables that can selectively
influence which reaction (SN2, SN1, E2, or E1) will predominate. Definitely spend some time
thinking about all of the variables responsible for optimizing each reaction category. It will
certainly help to read the material in Chapter 7. Some of the more important variables include:
(a) Structure of the substrate (e.g. primary, secondary, or tertiary alkyl halide)
(b) Type of solvent
(c) Strength of the nucleophile
(d) Basicity of the nucleophile
Today's experiment focuses on two major thematic variables: (a) nucleophilicty -- how the
type of nucleophile influences the course of a reaction and (b) kinetics (rate-determining step) -how we can use kinetic data to verify the mechanism of reaction. The first theme focuses on the
nature of reaction conditions in influencing the course of reaction and the second is purely a look
at how mechanisms are verified. In either case, it is the actual experimental findings that indicate
something about what is occurring at the molecular level in the reaction.
Part 1 (Competing Nucleophiles)
"Not all nucleophiles are created equal". Some nucleophiles (like the cyanide ion) are
quite strong and others (like methanol) are weaker -- see Table 7.3 in your text or the Table on
page 101 of the purple book. It should be noted that nucleophilic strength is not really this
straightforward. In fact, the actual strength of the nucleophile is dependent upon the reaction
solvent. The general trend is that the nucleophilicity of anions is enhanced in polar, aprotic
solvents rather than polar, protic ones. In other words, the nucleophile is more available in the
polar, aprotic solvent because of its decreased hydrogen bonding interaction with the solvent.
In this experiment, we will be using a polar protic solvent. This piece of information is
quite important because in polar, protic solvents, the nucleophilicity of anions is exactly the
opposite of the basicity (reversed). Therefore, the more basic the anion, the less nucleophilic
it will be. If the strength of one nucleophile is decreased and everything else remains equal, then
that nucleophile will not be able to compete effectively. Ineffective competition leads to a lower
percentage of the product distribution. If we can measure the actual distribution of products, then
we can determine whether one nucleophile is indeed stronger than the other.
In this experiment, our goal is to compare the relative nucleophilicities of the chloride
ion and the bromide ion in two different reactions. In the first reaction, chloride and bromide
will compete for 1-butanol (most likely an SN2 reaction -- why?). In the second reaction,
chloride and bromide will compete for 2-methyl-2-propanol (most likely an SN1 reaction -why?). Both nucleophiles are present simultaneously in the reaction mixture at equimolar
concentrations. The goal in this experiment is for you to analyze the products of the two
reactions by nuclear magnetic resonance spectroscopy (NMR) in order to determine the relative
amounts of each product. In addition, you will determine which ion is the stronger nucleophile
and for which of the two reactions this difference is important.
Procedure
This experiment has already been run and the NMR spectra have been obtained. They
will be distributed to each of you during the lab period. Your job is simply to use the data
collected to determine which product predominates in each reaction and why. You should also
suggest how these results reflect the relative nucleophilicities of the two attacking nucleophiles.
In addition, you should be able use this data to define the role of the nucleophile in both the S N1
and SN2 reactions.
Part 2 (Solvent Effects in Nucleophilic Substitution Reactions)
The information for the second part of today’s experiment can be found at the following web
addresses:
The experiment that we will be performing is:
What Role, If Any, Does Solvent Play in Nucleophilic Aliphatic Substitution Reactions?
Before lab you should read through the virtual pre-lab that can be found at:
http://creegan.washcoll.edu/gilabs/solvolysis_rx_%20folder/solvolysis_rx_vpl.html
You should also complete the 3 pre-lab questions (we will discuss at the beginning of the lab
session) at the following site: (Be sure NOT to submit online!!!! – simply answer in lab
notebook)
http://creegan.washcoll.edu/gilabs/solvolysis_rx_%20folder/solvolysis_rx_prelabqs.html
You might also check out the PowerPoint simulations (RBr simulation and RCl simulation)
that illustrate in more detail the procedure for today’s experiment at:
http://creegan.washcoll.edu/gilabs/
The team assignments for today’s experiment are as follows:
Names
Alkyl Halide
Solvent System
Brian and Victoria
2-chloro-2-methylpropane
2-propanol-water
Mark and Matt
2-chloro-2-methylpropane
acetone-water
Lindsey and Lindsey
2-bromo-2-methylpropane
2-propanol-water
Lynn and Cayleigh
2-bromo-2-methylpropane
acetone-water
Lauren, Jamie, and Kay
2-iodo-2-methylpropane
2-propanol-water
Brittany and Greg
2-iodo-2-methylpropane
acetone-water
Lauren, Jamie, and Kay
1-chlorobutane
2-propanol-water
Brittany and Greg
1-chlorobutane
acetone-water
Nick and Kevin
1-bromobutane
2-propanol-water
Nick and Kevin
1-bromobutane
acetone-water