home
site map appendix about course read first lab manual
read first
site map read first about course lab manual appendix

Lab I

Lab II

Lab III

Lab IV

Lab V

Lab VI

Lab VII

Lab VIII

Lab IX

Lab X

Lab XI

Lab XII

Lab VII: Nucleophilic Substitution Reactions: Relative Reaction Rates


Pre-lab Work

Reading Assignment:

Description of Experiment--see below & Appendix.

Pre-lab Questions:
(Please turn in the answers to these questions when you arrive to lab.)

  1. Prepare a table, similar to the Table 1 below, indicating the substrate structure for each of the halides listed at the beginning of the Experimental Procedure section below. Label them as aliphatic, benzylic, or aromatic, and indicate whether they are primary, secondary, or tertiary. (See Hornback, pages 153-154 and 466-468)
  2. Note the structures and boiling points of ethanol and acetone. Determine the relative polarity of these two organic solvents based on their dielectric constants.
  3. What is the definition of a "protic" solvent?
  4. Why don't we use any of the iodobutanes in the Sodium Iodide/Acetone experiment?
  5. In the Sodium Iodide/Acetone experiment, why is it important that the solutions never go above 50 degrees Celsius?
  6. How will you know when a reaction has taken place in each experiment?
  7. What is the nucleophile used in each experiment?


Table 1: Sample Table for Substrate Structure

Name of Halide

Structural formulas

Type of Halide

1-fluorobutane

Primary Aliphatic

2-fluorobutane

Secondary Aliphatic

fluorobenzene

Aromatic

benzyl fluoride

Primary Benzylic


Introduction

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 to lab.

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:

reaction

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 (X) 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 the cause for this change---the solvent. The solvent for the silver nitrate reaction is ethanol and the solvent for the sodium iodide reaction is 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 experiment.


Experimental Work


Objectives:

  1. To determine which organic halides react in two different nucleophilic substitution reactions, and to rank the reactivity of those halides which react.
  2. 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.

Safety Note:Organic halides are toxic and volatile. All work should be done in the hood. The use of gloves when handling these compounds is recommended.

Experimental Procedure

Working with a partner, do the reactions below using the following organic halides:

1-chlorobutane 1-bromobutane 1-iodobutane
2-chlorobutane 2-bromobutane 2-iodbutane
2-chloro-2-methylpropane 2-bromo-2-methylpropane  
benzyl chloride bromobenzene  


Silver Nitrate in Ethanol
Label a series of ten clean test tubes from 1 to 10 and follow the procedure as given in the Appendix.
Rank all 10 reactions from fastest to slowest.

Sodium Iodide in Acetone
Label a series of eight clean dry test tubes from 1 to 8 and follow the procedure as given in the Appendix. (From the list of 10 alkyl halides above, which two do you leave out in performing this procedure? Why?)
Rank all 8 halides.

NOTE: For either of the experiments above, see you instructor if any of your reactions are difficult to rank.

Waste Disposal and Clean-up

  1. Dispose of the products from the silver nitrate in ethanol experiments in the silver halides waste container. Rinse the test tubes with acetone and dispose of the acetone rinse in the same container.
  2. There is a separate waste container for test tubes containing residual silver halide waste. Do not put them in the glass disposal box.
  3. Dispose of the products from the sodium iodide in acetone experiments, and the acetone rinse after cleaning, in the flammables waste container. Used test tubes from this reaction can go in the glass disposal box.


Post-lab Questions

(Please turn in the answers to these questions when you arrive at your NEXT lab.)

1. Using a data table similar to the one shown in lab, explain how the substrate structure/electrophile affects the rate of the nucleophilic substitution reaction. Determine any trends for the various substrate structures within each experiment (NaI/acetone & AgNO3/ethanol) based on your data table.  [For example, compare the relative reactivity of a primary aliphatic with a secondary aliphatic where both compounds have the same leaving group.  Here’s the key: if their relative reactivity is the same no matter what leaving group is attached, than you have a TREND!  Wahoo!]  If you spy a trend in substrate structure reactivity in either of the experiments, are the trends the same or different between the two experiments?

2. Create a new data table, similar to the one above, yet sorted by relative reactivity within similar substrate structures showing various leaving groups.   Use this data table to show the relative reactivity in each experiment for the various leaving groups.  Determine any trends for the various leaving groups within each experiment based on your data table.  Again, if you spy a trend in either of the experiments, are the trends the same or different between the two experiments?

3. In general, how do you think benzylics compare to aliphatics in each experiment?  Due to the lack of benzylic compounds, please infer what you think would be the reactivity differences between these two types of substrates.  Justify your answer with lab data and specific compounds from our lab. 

4. Discuss any unexpected results in your experiments. What you would do differently if you were to do this lab again?