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In: Chemistry

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

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?

Solutions

Expert Solution

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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