Postlab Questions 1. A student claims that a given unknown is a carboxylic acid

Question

Postlab Questions

1.     A student claims that a given unknown is a carboxylic acid because the sample dissolves in water, 10% NaOH, and 10% NaNCO3. Critique this conclusion.

2.     Explain how solubility tests could be used to differentiate between:

a. a phenol and a carboxylic acid.

b. a phenol that contains electron-withdrawing groups and a phenol that contains electron-donating groups.

c. a carboxylic acid and an amine.

d. 1-bromooctane and dibutyl ether.

e. 2-pentyne and pentane.

f.   2-nitropentane and 2-aminopentane.

3.     What chemical tests could be used to differentiate between:

a. cyclohexanone, cyclohexanol, cyclohexylamine, and cyclohexanecarboxylic acid.

b. 4-methyl-1-pentanol, 4-methyl-2-pentanol, and 2-methyl-2-pentanol.

4.     In the Tollens test, what substance is being oxidized and what substance is being reduced?

Explanation / Answer

1. The conclusion is partially wrong.

Because alcohols can also dissolve in water, 10% NaOH and 10% Na2CO3. Because they can form hydrogen bond with water and can react with bases to form salts which can dissolve in water.

Carboxylic acids can also dissolved in water,10% NaOH and 10% Na2CO3. Because they dissociates in aqueous medium to give conjugate base and H+. But in that case if alkyl tail is long enough then they can not soluble in aqueous solution, due to hydrophobic interaction of alkyl chain.

4. Tollens' reagent is a chemical reagent used to determine the presence of an aldehyde, aromatic aldehyde and alpha-hydroxy ketonefunctional groups. Tollens' reagent can be prepared by following reactions.

2 AgNO3 + 2 NaOH Ag2O (s) + 2 NaNO3 + H2O

Ag2O (s) + 4 NH3 + 2 NaNO3 + H2O 2 [Ag(NH3)2]NO3 + 2 NaOH

(Tollens' reagent)

The ionic equations for the overall reaction between Tollens' reagent and aldehyde are shown below; R refers to an alkyl group.

2 [Ag(NH3)2]+ + RCHO + H2O 2 Ag(s) + 4 NH3 + RCO2H + 2 H+

Here aldehyde (RCHO) is oxidized to carboxylic acid (RCO2H), and  [Ag(NH3)2]+ is reduced to Ag(s).

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