Chapter 20: Q4P. (page 1049)

Question: Suppose you have just synthesized heptanoic acid from heptan-1-ol. The product is contaminated by sodium dichromate, sulfuric acid, heptan-1-ol, and possibly heptanal. Explain how you would use acid-base extractions to purify the heptanoic acid. Use a chart, like that in Figure 20-3, to show where the impurities are at each stage.

Short Answer

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Answer

The chart is as follows:

Step by step solution

Acid-base extractions

An acid-base extraction comes under liquid-liquid extraction. Acid and base are purified from a mixture depending on chemical properties.The solvents or standard solutions used are ether, sodium hydroxide, and hydrochloric acid.

Acid-base extractions for purifying heptanoic acid

When heptanoic acid, heptanal, heptan-1-ol, $H_{2} S O_{4}$ , and $N a_{2} C r_{2} O_{7}$ are shaken with water and ether, three compounds—heptanoic acid, heptanal, and heptan-1-ol—are formed with ether because they are soluble in ether. With water, $H_{2} S O_{4}$ and $N a_{2} C r_{2} O_{7}$ are formed because they are soluble in water.

Now, heptanoic acid, heptanal, and heptan-1-ol are shaken with aq. NaOH, heptanal and heptan-1-ol remain unchanged with NaOH, whereas heptanoic acid is converted to $C H_{3} C H_{2} C H_{2} C H_{2} C H_{2} C H_{2} C O O^{-} N a^{+}$ .

Here, in $C H_{3} C H_{2} C H_{2} C H_{2} C H_{2} C H_{2} C O O^{-} N a^{+}$ ,the ionized form of a carboxylic acid is water-soluble. Further, it is acidified with aq. HCl and ether. With HCl, it forms NaCl, and with ether, it forms heptanoic acid, and on evaporating ether, a pure heptanoic acid is formed.