PROPERTIES OF CIS-TRANS ISOMERS OBJECTIVES: To perform a cis/trans isomerization
ID: 541155 • Letter: P
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PROPERTIES OF CIS-TRANS ISOMERS OBJECTIVES: To perform a cis/trans isomerization. > To characterize differences between cis and trans isomers DISCUSSION: Maleic acid and fumaric acid have the same molecular formula·c,no. and each contains a double bond and two carboxyl groups, -COOH. The compounds are isomers because their structures differ in the geometry, or arrangement in space, of the atoms in the molecule. The formulas below show the structure of the two isomers HOOC COOH cis-isomer trans-isomer in the trans isomer the carboxyl groups are on opposite sides of the molecule and in the cis form they are on the same side. The fact that two isomers can be isolated indicates that rotation of the molecule at the double bond is restricted. As a result of this geometrical difference, cis and trans isomers have different physical properties and somewhat different chemical properties. For example, the carboxyl group is a fairly polar group, and will tend to attract electrons. In the cis isomer both carboxyl groups are on one side. Therefore, the molecule will be polar. The trans isomer is symmetrical and thus nonpolar. This difference in polarity will be reflected in the solubility. Polar materials are more likely to dissolve in water than are nonpolar substances. Both isomers are acids because the carboxyl groups can give up hydrogen to form a carboxylate ion, -Coo. In the cis isomer the negative charge is stabilized by proximity to the other carboxyl group. Therefore, the proton on the carboxyl group will be more easily removed and solutions of the isomer will be more acidic. (However, the very stable arrangement that forms makes the second proton less likely to be lost.) This can be detected by lower pH and faster reactions with agents such as magnesium metal that react with acid. In this experiment you will convert maleic acid to fumaric acid by heating it in an aqueous solution containing some hydrochloric acid (HCl). The HCl is a catalyst because it provides hydrogen ion, H', to convert the double bond in the acid to an alcohol. This intermediate compound will have an -OH on one of the carbons and a second -H on the other bond; however, the single carbon-carbon bond can rotate such that water is lost from the alcohol, both cis and trans isomers can form. The more stable isomer will be formed in greater amount. You will compare some of the properties of the two acids and attempt to conclude which isomer is maleic and which is fumaric acid, as well as conclude which acid is the most stable.Explanation / Answer
1) Fumaric acid being the trans isomer will have the least stearic hindrance as the carboxylic acid groups will be on the different sides of the double bonds-hence causing the least electronic repulsion. Hence will be more stable as compared to Maleic acid.
D) Malic acid is cis isomer while Fumaric acid is trans isomer.
2) Fumaric acid can form intermolecular hydrogen bonds (strong) while maleic acid forms intramolecular hydrogen bonds (weak), hence fumaric acid has higher boiling point than maleic acid."
3) Maleic acid is more acidic than Fumaric acid , due formation of strong conjugate base.
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