Separating benzoic acid, benzaldehyde and benzyl alcohol without chromatography
A 12.0 g laboratory mixture contains benzoic acid, benzaldehyde and benzyl alcohol. The mixture must be separated into three recovered compounds, each at ≥80% recovery, using only a separatory funnel, a rotary evaporator, a steam bath and the following reagents: saturated NaHCO3, 1 M NaOH, saturated NaHSO3, diethyl ether, anhydrous Na2SO4 and dilute HCl. No chromatography is allowed. The mixture is known to contain 30.0% benzoic acid, 40.0% benzaldehyde and 30.0% benzyl alcohol by mass. A student proposes extracting the ether solution with 1 M NaOH first, then with NaHCO3, then treating the remaining organic layer with NaHSO3. The supervisor rejects that order. In the first extraction with 100 mL of saturated NaHCO3, acidification of the aqueous layer gives 3.10 g of benzoic acid. The remaining organic phase weighs 8.40 g.
1. Explain why the supervisor rejects the NaOH-first sequence and give the correct order of operations. Include the fate of each component at each step.
Read the worked answer
NaOH first is rejected for three reasons. First, NaOH is a much stronger base than NaHCO3 and can promote base-catalysed reactions of benzaldehyde, especially the Cannizzaro reaction and aldol condensation, which would destroy part of the aldehyde and lower recovery. Second, NaOH would extract benzoic acid as sodium benzoate, making the subsequent NaHCO3 extraction redundant and complicating the pH control during acidification. Third, a strongly alkaline ether layer can emulsify and make clean phase separation difficult.
The correct order is:
Dissolve the mixture in diethyl ether and extract with saturated NaHCO3. Only benzoic acid is acidic enough to be deprotonated; it enters the aqueous layer as sodium benzoate. Benzaldehyde and benzyl alcohol remain in the ether.
Acidify the aqueous layer with dilute HCl to precipitate benzoic acid; filter, wash and dry.
Dry the ether layer over anhydrous Na2SO4 and remove most of the ether carefully.
Add saturated NaHSO3. Benzaldehyde forms a bisulfite adduct, which precipitates or separates; benzyl alcohol does not react and remains in the organic phase.
Filter the adduct, wash it, then treat it with dilute HCl or Na2CO3 to release benzaldehyde. Distil or dry the liberated aldehyde.
Recover benzyl alcohol from the remaining organic phase by removing the solvent.
This order uses the weakest reagent first, avoids destroying the aldehyde, and gives a clean chemical handle for each component.
2. Calculate the percentage recovery of benzoic acid and reconcile the 8.40 g organic phase with the known composition. Decide whether the benzoic acid step meets the ≥80% requirement.
Read the worked answer
Initial benzoic acid = 30.0% of 12.0 g = 3.60 g.
Recovered benzoic acid = 3.10 g, so:
% recovery = 3.10/3.60 × 100 = 86.1%
This meets the ≥80% requirement.
The non-acidic components are benzaldehyde (40.0% of 12.0 g = 4.80 g) and benzyl alcohol (30.0% of 12.0 g = 3.60 g), totalling 8.40 g. That matches the observed organic phase exactly. Therefore the 0.50 g of benzoic acid that was not recovered did not stay in the organic phase; if it had, the organic phase would weigh 8.90 g. The missing 0.50 g is accounted for as mechanical loss during acidification, filtration or transfer, or as emulsion hold-up in the aqueous layer.
So the mass balance is:
12.00 g feed = 3.10 g recovered acid + 8.40 g organic phase + 0.50 g lost acid
The benzoic acid step is acceptable on recovery, but the 0.50 g loss should be reduced by rinsing the aqueous layer with fresh ether and by careful acidification to avoid a fine, loss-prone precipitate.
3. Explain why NaHSO3 separates benzaldehyde from benzyl alcohol, how the aldehyde is released, and confirm the identity of the two fractions with simple tests. Then calculate the recoveries if 4.08 g of benzaldehyde and 3.24 g of benzyl alcohol are isolated.
Read the worked answer
Sodium bisulfite adds to the carbonyl group of aldehydes to give a water-soluble or crystalline α-hydroxy sulfonate adduct:
RCHO + NaHSO3 → RCH(OH)SO3Na
Benzaldehyde forms this adduct readily. Benzyl alcohol has no carbonyl group, so it cannot form a bisulfite adduct and stays in the ether layer. This is a chemical separation based on the presence of the aldehyde carbonyl, not on boiling point or solubility alone.
The adduct is filtered and washed, then treated with dilute HCl or aqueous Na2CO3. Acid converts the sulfonate adduct back to the free aldehyde, which can be extracted into ether, dried and distilled:
RCH(OH)SO3Na + HCl → RCHO + NaHSO3 + HCl (net release of RCHO)
Recoveries:
Benzaldehyde: 4.08/4.80 × 100 = 85.0%
Benzyl alcohol: 3.24/3.60 × 100 = 90.0%
Both exceed 80%.
Confirmation tests: benzaldehyde gives a silver mirror with Tollens reagent and a yellow-orange precipitate with 2,4-dinitrophenylhydrazine. Benzyl alcohol gives no silver mirror and no 2,4-DNP precipitate; it may give a positive test for a primary alcohol only after oxidation. The absence of a carbonyl test in the alcohol fraction confirms that the separation was clean.