Maleic Acid and Fumaric Acid Conversion

Experiment 3: Cis to Trans Isomerization of Maleic Acid (4)

Prelab 16 pts Lab Report 101 pts

Purpose: (2)

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

Background: (5)

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 a different arrangement of bonds. 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 (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 and the same bonding arrangement of atoms, but different spatial arrangements of the individual atoms. 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 are 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 (Figure 2). [In many organic structural drawings, Rand R' represent an atom or a group of atoms (e.g., CH3 or C6H6).]

Experiment 3J: Maleic Acid Isomer Page 1 10/23/2008

The isomer with the R and R' bonded to the same carbon does not exhibit cis-trans isomerism and is a structural isomer of the other two 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 of each other 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 (R group) 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 (remember cis is same) 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 metals, and simple protic acids (ie HCl). Most carbon-carbon double bond isomerization processes involve a carbon-carbon single bond intermediate that can undergo a bond rotation to give either cis or trans isomer (Figure 3).

Trans isomers are generally more stable than the corresponding cis isomer because the large "R" groups are farther apart and steric hindrance is minimized. Steric hindrance: arises from 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 isomer.

A simple example of a cis-to-trans isomerization is the conversion of maleic acid to fumaric acid. Maleic acid is cis-butenedioic acid and fumaric acid is trans-butenedioic acid. A proposed mechanism for the cis-to-trans isomerization reaction is an electrophilic addition of a hydrogen ion to form a carboncation intermediate 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 (Figure 3).

Rxn 11

Cis and trans isomers may have different physical and chemical properties. Trans isomers generally have more symmetry, a smaller dipole moment, a higher melting point, and lower solubility Cis