Lab 5 - S_N2 Reactions of Alkyl Halides

Objective

To examine factors affecting the relative rates of the

S_N2 

reaction of alkyl halides such as:

• •
  alkyl halide structure
• •
  nature of the leaving group
• •
  properties of the nucleophile
• •
  steric hindrance

Introduction

The alkyl halides play an important role in organic synthesis. They can be easily prepared from alcohols or alkenes, among other starting materials. They in turn can be used in the synthesis of a large number of functional groups. These syntheses are often carried out by nucleophilic substitution reactions in which the halide is replaced by some nucleophile. These substitution reactions can occur in one smooth step, or in two discrete steps, depending primarily on the structure of the alkyl group. These mechanisms are known as

S_N2 

and

S_N1. 

In this laboratory period we will spend some time looking at

S_N2 

reactions. To get started, let's propose a mechanism for the reaction:

• •
  The rate of this reaction is directly proportional to the concentration of both methyl bromide and sodium hydroxide.

• •
  The reaction proceeds with inversion of configuration.
• Any ideas?

This mechanism was initially elucidated by Sir Christopher Ingold and Edward Hughes, in the 1930s. They interpreted the second-order kinetic behavior to mean that the rate-determining step is a bimolecular, concerted process. Therefore, the following mechanism was proposed — the backside attack of the nucleophile, with the leaving group leaving in a concerted step. This backside attack causes the inversion of stereochemistry (known as Walden inversion).

Pre-Lab

Answer all assigned WebAssign questions.

Procedure

Waste disposal

The reaction mixtures from this experiment should be collected in the labeled waste container. Do not put them down the drain. Ask your teaching assistant if you have any questions concerning the proper procedures for waste disposal.

Part 1: Effect of Structure of the Alkyl Halide on the Relative Rates of

S_N2 

Reactions

Measure 2 mL of 15% sodium iodide in acetone into each of three clean, dry 10-cm test tubes. Add 2 drops of 1-bromobutane (butyl bromide) to the first test tube; add 2 drops of 2-bromobutane (sec-butyl bromide) to the second test tube; and add 2 drops of 2-methyl-2-bromopropane (t-butyl bromide) to the third test tube. Stopper the tubes with corks. (Do not use rubber stoppers.) Shake the tubes to mix. Observe closely during the first 15-20 minutes, then at intervals throughout the lab period. Look for any sign of cloudiness or precipitation. Record your observations and the time at which the initial cloudiness was observed for each reaction.

Part 2: Steric Effects and the Relative Rates of

S_N2 

Reactions

Measure 1 mL of 15% sodium iodide in acetone into each of two clean, dry 10-cm test tubes. Into one tube, add 2 drops of 1-bromobutane; into the other test tube, add 2 drops of 1-bromo-2,2-dimethylpropane (neopentyl bromide). Stopper the tubes and shake. Observe closely. Record your observations.

Part 3: Effect of the Leaving Group on the Relative Rates of

S_N2 

Reactions

Measure 1 mL of 15% sodium iodide in acetone into each of two clean, dry 10-cm test tubes. Add 2 drops of 1-bromobutane into one test tube and add 2 drops of 1-chlorobutane into the other test tube. Stopper and shake the tubes. Observe closely. Record your observations.

In-Lab Questions

Please print the worksheet for this lab. You will need this sheet to record your data.

Questions

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