Materials Included In Kit

Additional Materials Required

Safety Precautions

This activity requires the use of hazardous components and/or has the potential for hazardous reactions. Hydrochloric acid is highly toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Hydrochloric acid should be handled with great care. Spills and drips are common around the hydrochloric acid bottle. Keep the acid bottle in a fume hood during dispensing and wash the area thoroughly afterward. Maleic acid is moderately toxic by ingestion and a body tissue irritant. Fumaric acid is an eye irritant. Always place the immersion heater in the water before plugging it in. Always wear chemical splash goggles, chemical-resistant gloves and a chemical resistant apron. Review all Safety Data Sheets and safety procedures before starting.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. The fumaric acid product produced in this lab is of such a small quantity that it probably can be disposed of in the trash according to Flinn Disposal Method #26a. Fumaric acid is a common ingredient in beverages and some baking powders. Larger quantities should be disposed of using Flinn Disposal Method #24a. The hydrochloric acid solutions can be disposed of by neutralization and then flushing down the drain according to Flinn Disposal Method #24b. 

Teacher Tips

• The formation of fumaric acid from maleic acid is an example of a cis-to-trans isomerization reaction. The isomerization reaction is straightforward and the isolation of pure fumaric acid is simplified by the differences in solubility of the two isomers. The fumaric acid product is virtually insoluble in water and precipitates as beautiful, white, needle-like crystals. Yields around 0.28 to 0.32 g (70–80% yields) are possible using any of the three isolation procedures. The vacuum filtration procedure is the quickest and provides the highest yields. Gravity filtration or using a microtip pipet will give good yields but does take slightly longer to isolate and dry the product.
• The reaction procedure is almost foolproof and students will get good results. Do not worry about dissolving all the maleic acid in the hydrochloric acid, it will dissolve as it begins to heat up. The longer the reaction time, the greater the yield. Heating the reaction mixture for 20 minutes will give 60–70% yields while 25–30 minutes should give yields over 75%. Make sure the fumaric acid is dry before massing the product to avoid “inflated” product yields.
• The copper wire handle is a nice option for handling the vials. Simply cut a piece of copper wire (any gauge wire will do) about 10–12 cm long. Place the cap on the vial and then wrap one end of the copper wire around the vial just below the cap. One to two revolutions are sufficient. The remaining copper wire makes a nice handle to lift the vial in and out of the hot water bath. The copper wire is reusable.
• A good melting point can be obtained for maleic acid. Do not determine the melting point of fumaric acid since its melting point is above the range of the usual laboratory thermometer found in high schools.
• The mechanism proposed in the background material is a very plausable mechanism but experiment results will disprove it. Performing the reaction using either sulfuric acid or ammonium chloride instead of hydrochloric acid results in no fumaric acid. Heating chlorosuccinic acid does not yield fumaric acid. This eliminates any simple nucleophic, electrophilic or H–Cl addition across the double bond mechanism. Using a combination of both sulfuric acid and ammonium chloride does provide almost identical yields of fumaric acid as the hydrochloric acid procedure. This indicates a more complex mechanism (see Figure 5). An excellent article (J. Chem. Ed. 1975, 52, 541) outlines a series of reactions to determine the mechanism of this reaction. The author of the paper, however, has asked that the mechanism not be included in text books or lab manuals because it does make an excellent lab exercise for undergraduate organic chemistry students.
  {12912_Tips_Figure_5}

  Glass Reaction Vial

• Glass reaction vials used in this kit are the small, or 1-dram size. The vials are made from borosilicate glass and are extremely durable. When dropped, they will usually resist breaking, and can be reused if cleaned (see Figure 6).
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• The most important contributor to the success of Vial Organic^™ labs is the vial cap which contains a Teflon^™ seal. The vials and Teflon seals are designed to resist the organic solvents and contain the pressure generated as a reaction progresses.
• It is important that you tighten the cap of a reaction vial very well. If the seal fails, the reactants will leak out into the water bath. To obtain a tight seal, hold the cap and vial in opposite hands and tighten as hard as possible. Never use pliers to tighten the cap. Rubber or latex gloves or a wet towel are sometimes useful to provide extra grip for tightening the cap. After a little practice you will be able to “feel” the seal set without using too much force.
• When the vial is heated, the glass vial and plastic cap expand differently and the cap may no longer be sealed tightly. This will lead to a leak and may contaminate the reaction. This is evidenced by a small stream of tiny bubbles coming from the vial cap. If this happens, simply remove the vial from the water bath with tongs, allow it to cool for a few seconds and carefully tighten the vial cap. The vials cool down rapidly and the plastic caps do not retain heat so they will not be excessively hot. However, care must be taken not to burn oneself; use rubber gloves and carefully test the temperature of the vial before attempting to handle it.

  Hot Water Bath and Immersion Heating Coil

• This Vial Organic laboratory procedure requires heating and this is safely performed in a hot water bath utilizing an immersion heating coil. The hot water bath is simply made by adding 300 mL of deionized water to a 400-mL or 600-mL Pyrex^® beaker (using deionized water greatly extends the life of your heater) and placing the immersion heater into the water (see Figure 7).
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• The size of the water bath is not critical to Vial Organic. If 400-mL beakers are not available, 250-mL or a beaker larger than 400-mL will also work. Make sure the immersion heater coil is always submerged. Always make sure the beaker is Pyrex or borosilicate glass. The immersion heater does not have to be attached to the side of the beaker; it can rest on the bottom. There are five simple rules that must be strictly observed whenever the immersion heater is used:
  1. Place the coil into the water before plugging it in.
  2. Unplug the coil before removing it from the water.
  3. Always unplug the immersion heater and remove it from the water bath as soon as the reaction is complete.
  4. Only heat water with the coil. Do not heat oils or organic solvents.
  5. Never drink water heated by the immersion heater.
• A hot plate can also be used to heat the hot water bath but it will take longer and is considerably more expensive. It is a good practice to start the hot water bath as soon as the laboratory begins so the bath is boiling by the time the reaction vial is ready. If the above rules are followed and deionized water is used, the immersion heater will last a long time.

Sample Data

*A good reference for acid dissociation constants and other physical property data is the CRC Handbook of Chemistry and Physics.

Answers to Questions

1. Would the isomerization of maleic acid occur if it were to be performed in neutral water?

   The concentration of hydrogen ions (H⁺) would not be great enough to cause the isomerization of maleic acid. The formation of fumaric acid requires a high concentration of H⁺ for a complete reaction.

2. What is the function of the hydrochloric acid in the isomerization of maleic acid? 

   The hydrochloric acid provides a good source of hydrogen ions, protons, for the isomerization reaction. The hydrogen ion undergoes a electrophilic addition to the carbon–carbon double bond.

3. What are some possible sources of error in the preparation and isolation of fumaric acid?

   Lower Yield: Not heating long enough to convert all the maleic to fumaric acid. A small amount of fumaric acid is soluble in the water. This loss will occur both in the reaction vial and during the washing steps in the filtration.
   Loss of product during isolation. Either not removing all the product from the vial or losing some during filtrations, product is almost always lost.
   The cis-trans isomerization is reversible. Since the majority of the fumaric acid is in the solid state and does not react with the acid, the losses are minimal.
   
   Higher Yield: Incomplete drying of product. A small amount of water on the fumaric acid product will lead to an inflation of yield.

4. Explain how the results from the analysis show that the product is indeed fumaric acid and not maleic acid.

   Answers will vary.

Introduction

Isomerize the cis double bond in maleic acid to fumaric acid and study the difference in properties of the two isomers.

Background

Many organic compounds have similar molecular formulas but different physical and chemical properties. These differences are primarily due to the structure of the molecule. When two or more compounds have exactly the same molecular formula, but different properties, they are called isomers. Isomers have different properties because the arrangement, or precise placement of specific atoms within the molecule, differs. Understanding the placement of atoms within a molecule will sometimes lead to a better understanding of its properties and reactivity.

There are two main classes of isomers, structural isomers and stereoisomers. Structural isomers contain the same number and types of atoms but one or more bonds differ. Many structural isomers cannot be converted into one another because bonds have to be broken and reformed which requires a great deal of energy. For example, there are three structural isomers for the molecular formula C₅H₁₂: n-pentane, isopentane and neopentane (see Figure 1). These structural isomers are not easily interconverted to one another because a carbon–carbon bond would have to be broken and then reformed.

Stereoisomers have the same number and types of atoms, the same bonding arrangement, but the spatial arrangement of the individual atoms differ. One type of stereoisomers is called geometric isomers because the atoms or groups of atoms assume different geometric positions around a rigid bond or ring of atoms. Carbon–carbon double bonds a re very rigid bonds and are common in organic compounds. There are three different arrangements that two different atoms or groups of atoms can take around a carbon–carbon double bond (see Figure 2). [In many organic structural drawings, R and R′ represent an atom or a group of atoms (e.g., OH, CH₃, C₆H₅).] The isomer with the R and R′ bonded to the same carbon is a structural isomer of the cis and trans isomers because a carbon–carbon bond would have to be broken to convert it into one of the other two isomers. The cis and trans isomers are stereoisomers because the atoms are identical, are bonded to the same atoms, but their geometry is different. Cis and trans isomers always have a hydrogen and a non-hydrogen atom bonded to each carbon of the double bond. The cis isomer is the isomer where both hydrogens are on one side of the double bond and the trans isomer has the hydrogen atoms on opposite sides.

In general, rotation about a carbon–carbon single bond occurs readily at room temperature, while rotation about carbon–carbon double bonds does not occur. Cis and trans isomers can be interconverted or isomerized under a variety of conditions depending on the molecule. Carbon–carbon double bonds are isomerized using heat, photolysis or a catalyst. Common catalysts include enzymes, transition metal catalysts and simple protic acids. Most carbon–carbon double bond isomerization processes involve a carbon–carbon single bond intermediate that can undergo a bond rotation to give either the cis or trans isomer (see Figure 3). Trans isomers are generally more stable than the corresponding cis isomer because the large “R” groups are farther apart and steric hinderance is minimized. Steric hinderance is due to the atoms in the “R” groups being too close to one another. Since the trans isomer is usually more stable, it is often the preferred product in an isomerization reaction. However, intramolecular interactions such as hydrogen bonding can sometimes favor the cis isomer. Most isomerization processes give some mixture of cis and trans isomers.

A simple example of a cis-to-trans isomerization is the conversion of maleic acid to fumaric acid. Maleic acid is cis-butendioic acid and fumaric acid is trans-butendioic acid. A proposed mechanism for the cis-to-trans isomerization reaction is an electrophilic addition of a hydrogen ion to form the carbonium cation followed by rotation about the carbon–carbon single bond to move the two acid groups as far away from each other as possible. Elimination of a hydrogen ion gives the trans isomer (see Figure 3).

Cis and trans isomers usually differ in properties (see Figure 4). Trans isomers generally have more symmetry, a smaller dipole moment, a higher melting point and lower solubility. Cis isomers are not as stable and normally have higher heats of formation (about 1–2 kcal/mol higher). For this reason, cis isomers can often be transformed into the trans isomer by heating. At higher temperatures, enough energy is available to break the carbon–carbon double bond. Rotation about the carbon–carbon single bond can occur and the molecule will prefer to be in a lower energy conformation so when the double bond reforms, the trans isomer is the predominate product.

Materials

Safety Precautions

Hydrochloric acid is highly toxic by ingestion or inhalation and is severely corrosive to skin and eyes. Avoid all body tissue contact. Maleic acid is moderately toxic by ingestion and a body tissue irritant. Fumaric acid is an eye irritant. Always place the immersion heater in the water before plugging it in. Always wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron.

Procedure

Setup

1. Add approximately 300 mL of water to a 400- or 600-mL beaker. Place an immersion heater in the water, plug it in, and allow the water to come to a boil. Do not plug in the immersion heater until after it is placed in the water.
2. Place 0.4 g of maleic acid in a small reaction vial.
3. Add 3 mL of 6 M hydrochloric acid to the reaction vial.
4. Seal the reaction vial and shake vigorously for 1–2 minutes to dissolve the solid. If the solid does not completely dissolve, wait 1–2 minutes and shake again. Any remaining solid will dissolve when the solution is heated. (Optional) Wrap a piece of copper wire under the cap to form a handle.
5. Using the copper wire handle or tongs, place the reaction vial in the beaker containing boiling water and the immersion heater. If at any time during the reaction a small stream of bubbles begins to flow out of the reaction vial cap, remove the vial from the boiling water, allow it to cool and tighten the cap.
6. After heating for about 10 minutes, a white solid will begin to appear. Observe how the crystals grow and then slowly fall to the bottom of the vial.
7. After a total of 25 minutes, remove the vial from the hot water using tongs or the copper wire handle and place it on the table top. Allow the vial to cool for a minute and then place it in an ice water bath for 2–3 minutes.

Isolate the product using a pipet, vacuum filtration or gravity filtration.

Isolation of Product—Pipetting

8. Allow the product to settle. Remove the copper wire handle and the cap.
9. Use a microtip pipet to remove the solution above the crystals. Be careful, the solution is still 6 M hydrochloric acid and is very corrosive. Dispose of the hydrochloric acid according to your teacher’s instruction. When most of the solution is removed from above the solids, place the tip of the pipet on the bottom of the vial and slowly withdraw the remaining solution from the solids. Only a small amount of solids will be drawn into the pipet.
10. Add 2 mL of cold deionized or distilled water to the product. Place the cap back on the vial and shake the vial to wash the solid product. Allow the solid to settle. Uncap the vial and withdraw the solution again with a microtip pipet.
11. Rinse the solid twice more with 2 mL of cold distilled water, repeating the procedure in step 10.
12. After the final rinse, uncap the vial, remove the water, and scoop the solid product onto a piece of pre-weighed filter paper to dry. Use a small spatula or a piece of copper wire to remove the fumaric acid from the vial.
13. Allow the product to dry overnight. Drying can be hastened by placing the product under an incandescent lamp.

Isolation of Product—Filtration

8. Remove the copper wire handle. Shake the reaction vial to create a suspension. Quickly pour the suspension into a gravity filtration or vacuum filtration setup.
9. Add 2 mL of cold deionized or distilled water to the reaction vial. Place the cap back on the vial and shake the vial to dislodge any solid product. Pour the water and any solids into the filtration setup.
10. Rinse the solid twice more with 1–2 mL of cold water.
11. If using a vacuum filtration setup, allow the product to dry for a few minutes before transferring it to a watch glass or weighing dish to dry overnight.
12. If using a gravity filtration setup, remove the filter paper when all the filtrate has flowed through the filter paper. Open up the filter paper and place it on a paper towel or larger filter paper to help wick away as much water as possible from the product.
13. Allow the product to dry overnight. Drying can be hastened by placing the product under an incandescent lamp.

Purification and Analysis

14. After the product has dried, determine the mass of the product and calculate percent yield. Record in the data sheet.
15. pH test: Place a small sample (a few particles) of maleic acid and your product in two small test tubes (or vials). Add 2 mL of distilled water. Cap the test tubes with cork or rubber stoppers and shake vigorously. Add 2–3 drops of 0.02% cresol red indicator to each tube. Shake again to mix the indicator in the solution. Record your observations in Table 1. Discard the mixtures down the drain. Rinse the test tubes.
16. Solubility test: Place approximately equal amounts (about 0.1 g) of maleic and your product in two small test tubes (or vials). Add 2 mL of distilled water. Cap the test tubes with cork or rubber stoppers and shake vigorously. Record your observations in Table 1. Discard the mixtures down the drain. Rinse out the vials.
17. Melting point (optional): Place approximately equal amounts of maleic and your product in two capillary tubes. Attach the capillary tubes to a thermometer with a rubber band. Place the capillary tube/thermometer setup in an oil bath and heat using a hot plate. Heat until one of the acids melts. Record your observations in the data table. Allow the tubes to cool. Discard the materials as directed by your teacher.

Student Worksheet PDF

12912_Student1.pdf