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 need at least 10 teams).

Second, assess the reactivity of all of the compounds in ONE of the following groups (this may be extended to TWO groups if time permits). 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 any 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 variable(s) each group are 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 run in 1 dram vials. The vials should be clean and dry. You will test each starting halocarbon twice, i.e., two times with separate batches of reagent. If necessary, you will also test each substrate at two temperatures.

First, 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 40-50^oC. Monitor the temperature of this bath using a thermometer. Remove the beaker from the hot plate (and just heat setting on the hot plate) whenever its the water temperature reaches 50^oC, but return it to the hot plate whenever the temperature falls to 40^oC.

Second, test a substrate [NOTE 1]. 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 [NOTES 2 & 3]. 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 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 because you are to compare 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 has prepared their results, the results for all of the groups will be collected in a single data table. You should record the results obtained by the other groups before you leave the lab. Students will be given a chance to explain their results and comment on any anomalies. (2008 - Instead of making you hang around lab to look at the results of other groups, simply give your results to the laboratory assistant. The lab assts and I will collect results from the entire class and present them in lab lecture on Th, Nov 6. The results we need from you are: a) for each compound you test, the time to reaction and the temperature of reaction, b) the relative reactivity of compounds in each grouping.)