Experiment #2: Stereochemistry of Alkenes and Molecular Modeling By: Nouhad Mokdad Section: 53 TA’s: Sheida and Ryan Mills Lab Partner: David Wilson Date of Experiment: October 7th, 2008 Purpose: The purpose is to study the difference between the stereochemistry of the two isomers, maleic and fumaric acid. The first part of the experiment called for the conversion of maleic acid into fumaric acid with the addition of a proton using the acid-catalyzed isomerisation, vacuum filtration, and reflux processes.
The second part of the experiment was intended for using the molecular modelling kits as a way to visualize the differences between such isomers. Discussion: Organic compounds and their three-dimensional shapes can be represented through diagrams. Two particular methods of illustrations are the sawhorse representation and the Newman projection. An example of both these methods is seen in figure 1. (1) | | | Sawhorse projection| Newman projectionFig. 1| The sawhorse representation examines a carbon-carbon bond from a certain angle, so that one is able to visualize the molecule as a whole.
The bonds in the plane of the main chain are demonstrated by solid lines, bonds away from the viewer are represented by hatched lines, and bonds coming straight towards the viewer are drawn as solid wedges. The Newman projection views the carbon-carbon bond from front to back; the front carbon being represented by a dot and the back carbon represented by a circle. The three bonds in the front carbon are arranged 120° from each other while the same applies to the three bonds in the back carbon. The fourth bond is positioned in the middle of both the front and back carbons and is therefore not visible.
The carbon-carbon bonds can be positioned in either the staggered or eclipsed formation (see Fig. 2 ). Staggered formation occurs when the substituents off of the front carbon are aligned with the substituent off of the back carbon. This is less favourable, as repulsion between these aligned substituents exists, causing for an unstable compound. The staggered conformation occurs once the substituents off of both front and back carbons are not aligned. This is more favourable as there is now less tension between the substituents; this allows for a more stable carbon-carbon bond. (2) Fig. 2 Eclipsed Staggered
Such depictions of carbon-carbon bonds are especially important for isomers. Isomers are compounds with the same molecular formula but different structural formula. A molecular formula is “the formula for the molecule which gives the ratio of the contained elements” while the structural formula is “the formula which indicates specifically to which atoms each atom is bonded”. (3) Constitutional (positional) isomers have one atom placed on a different position on the chain. Alternatively, stereoisomers match each other with respect to how the atoms are joined together, but differ on how such atoms are arranged in space.
Geometric isomers, a sub-class of stereoisomers, exist because of the restricted rotation about the carbon-carbon double bond. In order for rotation to occur, the pi bond in the double bond must be broken. Since that requires much energy, it is highly unfavourable. Due to this restriction, isomers of two kinds can form. These two isomers are cis/trans (E/Z); cis occurring when substituents are on the same side, and trans when substituents are on opposite sides. The E/Z system usually replaces the cis/trans, as it follows a set of clear rules and is not as vague.
This experiment deals with geometric isomers of maleic and fumaric acid. (See figure. 3 for image) (4) | Fig. 3| | Due to the structural differences, both these acids experience different properties (ex. Melting point). As previously noted, the conversion of maleic acid to fumaric is not ‘possible’ due to the double bonds. However, this experiment actually converts maleic to fumaric by means of heat. Maleic anhydride is heated and, through vacuum filtration, maleic acid crystals are isolated. HCl is added to this, and the solution is attached to a reflux condenser.
This solution is then cooled, and instead of converting back to its unstable form, the acid anhydride gains a water molecule and reforms in to the more stable fumaric acid compound. (See equations below) Maleic Anhydride + Water Maleic Acid + H + Maleic Acid + HCl Fumaric Acid + H + Finally, the melting point range of both acids as well as their mixture will be used to identify the purity of such isomers. Pure isomers will have similar melting points when compared to literature values and a smaller gap in the melting point range.
Table #1 Starting Products and Materials IUPAC Name| Molecular Formula| Molecular Weight (g/mol)| Physical Properties| Amounts used| | | | M. P. /B. P| Density (g/cm? )| Mass/Volume| Moles| Maleic anhydride| C4H2O3| 98. 06 g/mol| 53 °C/ 202 °C| 1. 314| 3. 5 g| 0. 036 mol| (Z)-Butenedioic acid(Maleic)| C4H4O4| 116. 1 g/mol| 138 °C/135°C| 1. 59| –| –| (E)-Butenedioic acid(Fumaric)| C4H4O4| 116. 1 g/mol| 287 °C (M. P. )| 1. 635| –| –| Hydrochloric acid| HCl| 36. 46 g/mol| ? 26 °C(38% solution)/110 °C (20. 2% solution); 48 °C (38% solution)| –| 5. mL| 0. 06 mol| Water| H2O| 18. 0153 g/mol| O°C/100°C| 0. 998 (liquid at 20 °C) 0. 92 (solid)| 8. 0 mL| 0. 44 mol| Procedure: As per laboratory manual, pages: 37-40. Data and Results: Substance| Melting Point Range (°C)| Maleic Acid| 139-145| Fumaric Acid| 260-270| Combined sample | 130-150| Part A: Filtrate of Maleic crystals ————————————————- Mass of Maleic Anhydride: 3. 5 g ————————————————- Mass of crystals scraped= 1. 12 g ————————————————-
Percent Recovery= (1. 12g / 3. 5g) x 100% ————————————————- = ~ 32% Discussion: The conversion of maleic acid to fumaric acid was successful! Confirmation of this could be seen after analyzing the different melting points; fumaric acid has a higher melting point than maleic acid. In the experiment, maleic anhydride was first heated and-through the process of vacuum filtration- the crystals were isolated. The percent recovery of the isolated crystals was 32%. The melting point range was observed to be 139-145°C.
This range was close to the literature value of 135-138°C. Possible sources of error may have been due to some impurities reacting with the maleic anhydride prior to filtration. This would have caused the slightly higher melting point that was observed. Another source of error may be due to the timing assigned to heating and cooling. Heating and/or cooling for too long or too little may have caused for the incomplete crystallization of the crystals. This may have caused the low percent recovery. Also, the less pure a crystal, the wider and higher its melting point range.
HCl was then added to the maleic acid. The mixture was attached to a reflux condenser and heated. The solution was then cooled and, through vacuum filtration, fumaric acid crystals were isolated. The melting point range was observed to be 260-270°C. This range was fairly close to the literature value of 287°C. A possible source of error may have been due to maleic acid not completely converting to fumaric acid. This could have occurred during the cooling process possibly indicating that not enough time was assigned to this process.
This would have caused the melting point range to be lower than the literature value; which is actually true in this case. The sources of error associated with the conversion of maleic anhydride to maleic acid crystals (as mentioned above) may have also occurred in this part of the experiment. Next, maleic acid crystals were combined with fumaric acid crystals. The combined melting point range was observed to be 130-150°C. Theoretically, the melting point range should have been in between the melting points of both maleic and fumaric acid crystals. This was not the case.
Experimentally, the melting point range determined was closer to that of the maleic acid crystals; indicating that the mixture contained more maleic acid (i. e. the mixture may not have been 1:1). The last portion of the experiment focused on illustrating the properties of such molecules using the molecular modelling kits. Questions were answered, discussed, and handed in at the end of the lab. Conclusion: Maleic acid was successfully converted into fumaric acid. The melting point range of maleic, fumaric, and a mixture of both crystals was determined and found to be 139-145°C, 260-270°C, 130-150°C, respectively.
Molecular modelling kits were also used to build different compounds in order to better visualize and understand the different compounds in organic chemistry. References: 1) http://en. wikipedia. org/wiki/Newman_projection; October 20th, 2008 2) http://content. answers. com/main/content/wp/en-commons/thumb/b/b8/200px-Newman_projection_ethane. png; October 20th, 2008 3) McIntosh J. M. (2008) The ABC’s of Organic Chemistry. 2008 Edition. (page 9) 4) http://images. google. ca/imgres? imgurl=http://capital2. capital. edu/faculty/wbecktel/cistrans. gif&imgrefurl=http://capital2. apital. edu/faculty/wbeck tel/Isomers. htm&h=269&w=560&sz=4&hl=en&start=9&sig2=VSKmaSksRE2JrtgPGZJNAA&um=1&usg=__O4ZboIVyV1VStoIyAOa6di5dXUQ=&tbnid=4UCuJ0azgLyIaM:&tbnh=64&tbnw=133&ei=l1L9SNy2GpnAMfiage4C&prev=/images%3Fq%3Dmaleic%2Band%2Bfumaric%2Bacid%26um%3D1%26hl%3Den; October 20th, 2008 5) http://msds. chem. ox. ac. uk/; October 20th, 2008 6) http://en. wikipedia. org/wiki/Maleic_anhydride; October 20th, 2008 7) http://en. wikipedia. org/wiki/Fumaric_acid; October 20th, 2008 8) http://en. wikipedia. rg/wiki/Maleic_acid; October 20th, 2008 9) http://en. wikipedia. org/wiki/Hydrochloric_acid; October 20th, 2008 10) http://en. wikipedia. org/wiki/Water; October 20th, 2008 Post-Lab Questions & Answers Q1. What would be the result this experiment if fumaric acid were used as the starting material? Are there any practical difficulties involved in this alternate experiment? Fumaric Acid Maleic Acid Trans-isomers are more stable than cis-isomers. This is mainly due to the fact that trans-isomers experience less repulsion between atoms than do cis-isomers.
Repulsion between the OH-group and O atom occur within the maleic acid compound. This is why maleic acid is less stable than fumaric. If fumaric acid were used as the starting material, it would not convert to maleic acid, under the experimental conditions due to the stability mentioned above. Moreover, fumeric acid has a lower solubility than maleic acid and would take a lot of energy and time to precipitate it. Q2. Write the IUPAC name for fumaric acid and maleic acid. The IUPAC name for: * Fumaric acid is (Z)-Butenedioic acid * Maleic acid is (E)- Butenedioic acid