Microscale Organic Experiments

Microscale Organic Experiments

Fourth Edition

A Laboratory Manual for

Organic Chemistry II

Chemistry 2040

Ray A. Gross, Jr., Ph.D.

Prince George’s Community College

Fall 2012

Preface

The experiments in this manual overlap the topics covered in the second edition of “Organic Chemistry” by Janice G. Smith. They can be conducted in any sequence, but the order of experiments parallels, as closely as possible, the order in which the chapters of Smith’s text are covered in CHM 202, Organic Chemistry II. This manual is the second-semester companion to “Techniques, Spectra and Reactions—A Laboratory Manual for CHM 2010, Organic Chemistry I, To Accompany Smith’s, Organic Chemistry, 3d Edition.”

The experiments are designed to minimize the use of chemicals and the subsequent generation of waste while optimizing the use of microscale glassware. This is now known as “Green Chemistry.” Instructors have the option of adding molecular modeling or instrumental analysis of products to any of the experiments.

Like the earlier editions of this manual, this edition focuses on well-known and important reactions and syntheses, such as the aldol condensation and Diels-Alder reaction that students are likely to encounter in their follow-on studies in the biological sciences. The discussions have been expanded to include mechanisms of the reactions and related reactions. Some questions accompanying each experiment have been changed. These questions are designed to test a student’s knowledge of the fundamental principles covered during the second-semester organic course. Students who master this content are well prepared for biochemistry, advanced chemistry and related courses. This edition of this manual is not available in a printed form.

ORGANIC LAB REPORT FORMAT

Serious students will keep a lab notebook and write up their results in the following format.

Name R. Gross Lab Partner S. Sinex Expt # 2 Date9/7/11

1. Purpose. To conduct a Diels-Alder reaction

A short, concise statement—not a complete sentence—of what you intend to accomplish.

2. Theory.Certain dienes react with double bonds (dieneophiles) to form new 6-membered rings containing one double bond (cyclohexenes)—see Equation 1 below—or triple bonds (the dieneophiles) to form new 6-membered rings containing two non-conjugated double bonds (1,4-cyclohexadienes)—see Equation 2 below. The reaction is concerted and gives high yields. Diels and Alder shared the Nobel Prize for their discovery of the reaction and the demonstration of its utility. The key features of the reaction are: 1) it makes two new carbon-to-carbon single bonds, and 2) it makes 6-membered rings from two acyclic compounds. In this experiment, both reactants are part of ring systems—see Equation 3 below. The diene is cyclopentadiene and the double bond reactant is maleic anhydride, a cyclic anhydride, containing a double bond. The product of our reaction contains three rings—the cyclohexene—expected from a Diels-Alder reaction—and a 5-membered ring made-up of the same 5 carbon atoms that were present in the cyclopentadiene and a 5-membered ring made up of the same 5 atoms that were present in maleic anhydride. Thus, we make a tricyclo (3 rings in one structure) compound. The Diels-Alder reaction belongs to a class of reactions called cycloaddition reactions because the product always contains at least one ring (cyclo-) and results from the addition of an -ene to a diene. The reaction proceeds by a cis-addition--substituents that are cis to each other in the original -ene reactant are also cis in the adduct. In tricyclo compounds such as the one made in this experiment, the principle of maximum overlap of π bonds leads to an endo-product. The endo-product is produced even though the exo-product is thermodynamically more stable than the endo-product. Thus, the reaction is the result of kinetic and not thermodynamic control.

In the Theory section, include principles that apply to the experiment. Include information you can study for the final exam to refresh your memory about the nature, scope and generalities of the experiment. Include enough information so that it will be unnecessary to refer to a lab manual or text—use these references to write the Theory section.

3. Chemical Equations.[Note: Show the mechanism of the reaction!

Do not cut and paste (do your own work).

4. Table of Reactants and Reagents.

Name or
Formula / Molecular
Weight (g/mol) / Mass (g) or
Volume (mL) / Moles* / Melting or
Boiling pt / Other
Cyclopenta-
diene / 66.10 / 0.20 mL =
0.16 g / 0.0024 / bp 41oC / Dry
den 0.80
Maleic
anhydride / 98.06 / 0.20 g
(powdered) / 0.0020 / mp 53oC / Limiting
reagent
Ethyl
acetate / 1 mL / Solvent
Ligroin / 1 mL + wash / Solvent

* Moles = mass (g)/mol wt (g/mol). Since we need grams, if a liquid is used, we find grams by using the density: vol (mL) x den (g/mL) = mass (g).

[This table clearly identifies what will be used and how much. The limiting reagent must be specified in the “Other” box. The limiting reagent governs how much product (i.e., the maximum amount) that can be made. This amount is the theoretical yield in moles. (Consider two people, Jack and Jill. Jack has 24 half-dollar coins and Jill has 20. The two are allowed to contribute only equal amounts of half-dollars to make whole dollars. How many dollars can they make? Answer 20 because of the constraint that both must contribute half-dollars. Note that the number of dollars that can be made depends on Jill who has the fewest number.) By analogy, cyclopentadiene and maleic anhydride react according to Equation 3, which shows that one molecule of cyclopentadiene reacts with one molecule of maleic anhydride. Thus, the number of molecules of product that can be made depends on which reactant has the fewest number of molecules. We call this reactant the limiting reagent because it limits the maximum amount of product to the number of molecules it contributes to the reaction. We measure the number of molecules in moles (one mole is 6.02 x 1023 molecules). The weight (mass) of 6.02 x 1023 molecules is the molecular weight of the compound in grams of the compound per mole. Therefore, the compound, which contributes the fewest moles to the reaction, is the limiting reagent. Thus, from the table, we see immediately that maleic anhydride is the limiting reagent because it contributes only 0.0020 moles and cyclopentadiene contributes 0.0024 moles. The number of moles of the limiting reagent is the maximum number of moles of product that can be formed in this reaction.

In some reactions, the coefficients in the equation representing the reaction are not all one. Consider 4A + B  2C. In a case such as this, A must contribute 4 moles for every mole contributed by B, so finding the limiting reagent requires an additional step. That is, you cannot see directly from the table which reactant is the limiting reagent. If 0.24 moles of A are used and 0.2 moles of B are used, it would require 4 x 0.2 moles of A to react with 0.2 moles of B, and 4 x 0.24 = 0.96. Since only 0.24 moles of A are available, A is the limiting reagent because it will be all used up before all of B reacts.] Most of our reactions are 1:1 but be alert to reactions that are not.]

5. Diagram of the equipment setup or list of equipment.

1. Small test tube

2. Pipette

3. Hirsch funnel

Normally, this section will include a free hand drawing of the equipment setup. For example, if the reaction involved a distillation, you would include a drawing of the apparatus. In several experiments, it is only necessary to list the equipment. Do NOT draw pictures of commonly used items such as test tubes and pipettes.

6. Procedure.

1. Maleic anhydride and cyclopentadiene were allowed to react in ethyl acetate/ligroin--after several minutes, a white ppt formed.

2. The ppt was collected on a Hirsch funnel and allowed to dry on the filter paper.

3. The white solid was weighed (0.295 g) and turned in.

As you and your partner conduct the experiment, you should record exactly what you do in your notebook as you do it. This is not an essay on the procedure. It is a short, concise summary of each step of the experiment, consolidated to the extent possible.

7. Calculations.

1. Theoretical yield in grams. (Theoretical moles of product times its MW)

Because maleic anhydride is the limiting agent, 0.0020 moles of product are possible because Equation 3 shows that 1 mol + 1 mol  1 mol.

0.0020 mol x 164.16 g/mol = 0.32832 g  0.33 g (only 2 significant figures)

This calculation tells you how many grams of product you could make if every molecule of the limiting reagent reacted to give product.

2. Percent yield

Actual yield (grams)x 100 = % Yield

Theoretical yield (grams)

0.295 gramsx 100 = 89.3939%  89% (only 2 significant figures)

0.33 grams

This calculation is a measure of the overall efficiency of the reaction.

8. Table of Product(s).

Name or
Formula / Molecular
Weight
(g/mol) / Mass (g)
or
Volume (mL) / Melting
or
Boiling
point / Properties / Other
cis-norbornene-
5,6-endo-
dicarboxylic
anhydride / 164.16 / 0.295 g / mp 165oC / White
plate-like
crystals

9. Observations. The reaction occurred quickly and produced a high yield.

In the Observations section, you are free to express any thoughts that you have about the experiment.

10. Conclusions. A diene and dieneophile reacted according to the Diels-Alder reaction to produce a new 6-membered ring. The starting diene is locked in the s-cis conformation by the fact it is part of a 5-membered ring, so the reaction occurred at room temperature. It was unnecessary to heat the reaction, because the diene is already in the conformation to make the reaction go (i.e., the energy of activation must be small). The three π bonds that took part in the reaction were converted into two sigma (single bonds) and one new π bond (double bond). All of the new bonds are part of the new 6-membered ring. No other bonds were altered, thus a tricyclic product was made. The product is a derivative of the common bicyclic compound, norbornene. Norbornene is a simple double bond derivative of norbornane (bicyclo[2.2.1]heptane).

The high yield and stereospecificity of the reaction make it very valuable in the synthesis of compounds containing 6-membered rings. The product of our reaction contains an acid anhydride functional group, thus it will display two carbonyl absorptions in its IR spectrum. The double bond will show an absorption to the left of 3000 cm-1 in the IR. The endo-product is thermodynamically less stable than the exo-product. See the structures above.

The completed report contains ten sections as numbered above and two attachments, the data sheet and the answers to the 10 problems. Please number your entries 1-10 as shown above. The entire report is worth 20 points—each completed section is worth 1 point, and each question is worth 1 point.

Attachments

1. Original Data Sheet

2. Answers to the 10-Problem Set

1