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Q.65.

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Organic Chemistry
Found in: Page 91

Short Answer

Question: Recall from Section 1.10B that there is restricted rotation around carbon-carbon double bonds. Maleic acid and fumaric acid are two isomers with vastly different physical properties and values for loss of both protons. Explain why each of these differences occurs.

Answer

The melting point of fumaric acid is higher than maleic acid due to its symmetrical structure.

The solubility of maleic acid is greater than fumaric acid due to greater polarity.

Maleic acid is more acidic ( is low) due to the stabilization of the conjugate base by hydrogen bonding.

Maleic acid is more basic ( is high) due to the destabilization of the conjugate base caused by removing the second proton.

See the step by step solution

Step by Step Solution

Step 1: Factors affecting the melting point

The melting point depends upon the intermolecular forces and symmetrical structure.

The symmetrical structures are arranged into a packed form, and therefore crystallization occurs. Hence, the melting point is higher in a symmetrical structure.

Step 2: Factors affecting solubility

Solubility depends upon the polarity of the molecules. The more the acidic proton, the greater will be the extent of hydrogen bonding in water. Hence, the more will be the solubility.

Step 3: Acidic constant

The high value of the acidic constant indicates the high basicity and low acidity and vice versa. The high acidity of the molecule depends upon the stabilization of the carbanion. The higher the stabilization of the conjugate base or carbanion, the more the acidity and lower value.

Step 4: Explanation

The melting point of maleic acid is lower than fumaric acid due to the structure of the molecules.

Maleic acid has two COOH groups on the same side. Therefore, the packed arrangement of the molecules in a crystal is not possible. Hence, the melting point of maleic acid is low.

The COOH groups of fumaric acid are trans to each other. Therefore, a packed arrangement of the molecules is possible. Hence, the melting point is high for fumaric acid.

The solubility of maleic acid is higher than fumaric acid. Maleic acid has two COOH groups. Therefore, the polarity of the molecule is more.

The conjugate base of maleic acid is formed by removing a proton, which is stabilized by the other COOH group due to the cis configuration. Therefore, the acidic constant value is high, and value is low.

Loss of proton by maleic acid

The conjugate base stabilization is due to the hydrogen bonding. The conjugate base of fumaric acid is not stabilized due to the cis configuration. Therefore, the acidic constant value is low, and value is high.

No extra stabilization of conjugate base of fumaric acid

The removal of the second proton gives the destabilized group in the case of maleic acid. The two groups in maleic acid are on the same side.

Therefore, both negative charges on the groups repel each other and destabilize.

Therefore, maleic acid is less acidic and has a high value.

In fumaric acid, due to the trans COOH groups, the second proton of the group is acidic and hence the value is low.

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