Rate of Nucleophilic Substitution

Procedure

Organization

We have collected together a large number of bromo- and chlorocarbons for you to check as substitution substrates. Rather than ask every student to assess the reactivity of every substrate with every reagent, which would be very time-consuming and use up lots of substrate and reagent, we will abbreviate some of the activities as follows.

First, work with a partner. This rule will be relaxed if we have trouble creating enough teams (we would like to have at least 10 teams).

Second, assess the reactivity of all of the compounds in ONE of the following groups. The groups are listed below.

Group 1

  • 1-chlorobutane v. 1-bromobutane
  • 2-chlorobutane v. 2-bromobutane
  • 2-chloro-2-methylpropane v. 2-bromo-2-methylpropane

Group 2

  • 1-chlorobutane v. 2-chlorobutane v. 2-chloro-2-methylpropane
  • 1-bromobutane v. 2-bromobutane v. 2-bromo-2-methylpropane

Group 3

  • 1-bromobutane v. 3-bromo-1-propene v. 1-bromo-1-propene
  • bromocyclopentane v. 3-bromocyclohexene v. bromobenzene

Group 4

  • 2-bromobutane v. bromocyclopentane v. bromocyclohexane v. bromocycloheptane

Group 5

  • 1-bromobutane v. 1-bromo-2-methylpropane v. (bromomethyl)cyclohexane v. 1-bromo-2,2-dimethylpropane

Third, test only one reagent, either NaI/acetone or AgNO3/ethanol.

Fourth, share your results with the rest of the class. The combined results will be discussed during the lab.

You will be assigned a substrate group and reagent in class, so come to class prepared to work with all of the groups and reagents listed above.

Pre-lab preparation

Draw the molecular structures of each compound in each group in your lab notebook. Determine what structural variable(s) each group is designed to highlight and write these variable(s) in your notebook.

Pick one starting material and draw a balanced chemical equation for the reaction between this starting material and NaI. Draw a second balanced equation for the reaction between this starting material and AgNO3.

Make a table of physical properties, but enter data for acetone and ethanol only. Do not enter physical properties of the bromo- and chlorocarbons.

Enter the following hazard and disposal information in your notebook:

  • Halocarbons – Volatile, reactive electrophiles. Toxic. Avoid fumes and skin exposure. Dispose in organic waste.
  • Acetone – volatile, flammable, mild irritant.
  • Silver nitrate – toxic, discolors skin upon contact.
  • Sodium iodide test mixtures – dispose in organic waste.
  • Silver nitrate test mixtures – dispose in organic waste.

Create a data table in your notebook to record your data.

  • Title that includes spaces for your name, your partner’s name, reagent, group #, and variable(s) being tested
  • Column headings: substrate, test # (1 or 2), start time at room temperature, start time for heating, precipitate time, additional observations

You will enter one row for each test, but do not enter any information in these rows until you come to lab and are assigned a reagent and group #. You will do two tests on each substrate, so plan accordingly.

Procedure

The reactions are typically run in 1 dram vials, but we may provide you with different glassware on the day of lab. Vials should be clean and dry. Plan to test each starting halocarbon twice, i.e., two tests with separate batches of reagent. If necessary, you will also test each substrate at two temperatures.

To begin, make a warm water bath by placing about 20 mL of water in a 100 mL beaker and heating the beaker on a hot plate to 40-50oC. Monitor the temperature of this bath using a thermometer. Remove the beaker from the hot plate (and adjust the heat setting on the hot plate) whenever its the water temperature reaches 50oC, but return it to the hot plate whenever the temperature falls to 40oC.

Second, test a substrate [NOTE]Instructions are given for testing a single substrate, but you may test multiple substrates simultaneously. Do not test more samples than you and your partner can comfortably handle and monitor. Do not start a test unless you are sure you can give the test mixture immediate and adequate attention.. To conduct a test, place about 1 mL of test reagent in a vial. Add 4 drops of substrate to the vial and record to the second when you make this addition. Swirl the vial to make sure the reagent and substrate have mixed completely.

Watch for the appearance of a precipitate and record to the second when a precipitate first appears [NOTE]If a precipitate results from a substitution reaction, the amount should increase with time. The appearance of a trace of precipitate and the failure to see increased precipitate over time, or after heating, is a suspicious result. Be sure to record all observations that will help you detect reactivity patterns. The time for initial precipitate formation may not be enough. For example, suppose you test two compounds. The first test solution becomes cloudy immediately and becomes more cloudy so that bits of precipitate eventually settle to the bottom of the tube after 5 minutes, while the second solution forms a precipitate immediately that settles in a layer in the bottom of tube in less than 30 seconds. While the time for initial formation of precipitate (0 seconds) is identical in both tests, the second test clearly involves a faster reaction (faster buildup of precipitate) and should be described as such.. If no precipitate appears after 5 minutes, carefully place the vial in the warm water bath (record the time to the second) and watch for formation of a precipitate for a period of 10 minutes.

After you test a substrate, repeat your test with a new clean dry vial and a new portions of reagent and substrate.

Preparing your results

After you have completed all of your tests, create a summary results table with the following components:

  • Title that includes your names, reagent, group #, and variable(s) being tested
  • Column headings: substrate, precipitate formed?, and reactivity ranking

List your substrates in the order given in the group listings at the top of this page. Make sure you list compounds that are separated by “v.” next to each other; the idea is to compare the reactivity of these compounds.

Under “precipitate formed?”, enter Yes, No, or Y*es with heat.

Use your reaction times and other observations to determine to rank the reactivity of your substrates (1= fastest, 2 = slower, 3 = slower still, etc.). Notice that Group 1 requires three pairwise comparisons, i.e., rank substrates 1 and 2 in each set. Groups 2 and 3 both require two three-way comparisons, i.e., rank substrates 1, 2, and 3 in each set. Groups 4 and 5 require a single four-way comparison, i.e., rank substrates 1, 2, 3, and 4.

Sharing results

After everyone has completed their tests and prepared their summary results table, the results for all of the groups will be collected and discussed in class. (Note: your lab instructor will give you a ‘discussion’ time and you should return to lab at that time.)