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Lab VI: Nucleophilic Substitution Reactions: Relative Reaction Rates


Pre-lab Work

  1. Prepare a table 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. Using Mohrig, determine the relative polarity of these two organic solvents.

Reading Assignment:

Description of Experiment--see below & Appendix.

Example Table:

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.
  3. To test for the presence of halide using the Beilstein Test.
  4. To perform the Beilstein Test for Halides on the 2 unknowns. To perform the Classification Tests for Halides on the 2 unknowns.


Experimental Procedure
Do the reactions, using the following organic halides:

1-chlorobutane
1-bromobutane
1-iodobutane

2-chlorobutane
2-bromobutane
2-iodobutane
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.

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?)

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

Beilstein Test
Follow the procedure described in the Appendix.

Test 1-butanol, 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 positively in the 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
Flammable waste bottle: product from last week's Competing Nucleophiles experiment.
"Silver" waste bottle: AgNO3 in Ethanol experiments.


Post-lab Work

  1. Why do we use anhydrous acetone and not the wash acetone from the bottles in the waste hood?
  2. For each experiment, explain how the substrate structure affects the rate of a nucleophilic substitution reaction?
  3. For each experiment, how will the nature of the leaving group influence the rate of the nucleophilic substitution reaction?
  4. 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.
  5. Discuss any errors in your experiment and state what you would do differently if you were to do this lab again.