In: Chemistry
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?
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).