Ishwar S. answered • 04/07/19
University Professor - General and Organic Chemistry
The reaction you are describing is the Cannizzaro reaction where benzaldehyde in the presence of a strong concentrated basic solution undergoes self-reduction to form benzyl alcohol and self-oxidation to form sodium benzoate.
1) The aqueous layer contains soluble sodium benzoate. When acidified, sodium benzoate is converted to insoluble benzoic acid, which is isolated via filtration. I am unable to draw organic structures here, but will write the reaction using the chemical formula.
C6H5COO-Na+ (aq) + HCl (aq) → C6H5COOH (s) + NaCl (aq)
2) Benzyl alcohol can be isolated from unreacted benzaldehyde by performing a distillation since they are both liquids. Benzaldehyde will distill in the receiving flask first since it has a lower boiling point of 178 °C. Benzyl alcohol with a higher boiling point of 205 °C will be left over in the distilling flask.
3) Without knowing the exact experimental procedure you followed, I will not be able to give an accurate answer to this question.
4) In a Cannizzaro reaction, the aldehyde cannot have any alpha hydrogens - hydrogen on a carbon adjacent to the carbonyl carbon (C=O). Without any alpha hydrogens, the hydroxide ion from NaOH can attack the carbonyl carbon to initiate the reduction and oxidation of benzaldehyde.
In an aldol condensation reaction, the alpha carbon adjacent to the carbonyl MUST have a hydrogen atom. This alpha hydrogen is acidic, which can be deprotonated by OH- to form a stable (due to resonance), but reactive enolate anion. The enolate is nucleophilic, which can attack the electrophilic carbonyl of a 2nd molecule of the aldehyde to form the product, an aldol.
The example below is the reaction of 2 moles of acetaldehyde in the presence of NaOH to form the aldol product.
CHO-CH3 + NaOH → CHO-CH2- + CHO-CH3 → CHO-CH2-CH(O-)-CH3 + H2O → CHO-CH2-CH(OH)-CH3
5) For this question, do you mean the mechanism between "sodium hydroxide" and benzaldehyde?
Esra Y.
Firstly thank you so much for answering^^ Here's the procedure: In a 50 mL beaker introduce benzaldehyde (7.5g) and a saturated solution of sodium hydroxyde (4.5 g of pellets in the minimum amount of water). Heat the mixture by means of bain-marie for 30 min. while stirring vigorously. Next, cool the beaker donwn and the minimum amount of cold water to dissolve the solid. Then transfer the mixture into a separating flask, extract with dichloromethane (2x20 mL) and collect the organic layers in a 100 mL beaker. The content of the beaker is mixed vigorously with a solution of sodium bisulfite (5 mL) in order to remove the unreacted benzaldehyde. If a precipitate is formed, filter through a Büchner funnel and wash it with dichloromethane (10 mL); finally dry it and weigh it out. The organic layer is washed with consecutively with a dilute solution of hydroxyde (5 mL) and water until neutral pH is reached. The resulting organic layer is dried over anhydrous sodium sulfate (~1g) and evaporated by means of a rotary evaporator. Weigh the mass of product (benzyl alcohol) obtained. The remeaning aqueous phase is cooled down in an ice bath and treated with concentrated HCl until pH=1. The resulting solid is fitered through a Büchner funnel, washed twice with coldwater and finally dried over filter paper. Recrystallization: Dissolve the solid completely in hot water (30 mL) and let the solution cool down slowly until benzoic acid crystallizes in the form of white needles. The solid is isolated by filtration and dried over filter paper. Weigh mass of the product (benzoic acid) obtained. 5) No, i meant sodium bisulfate and benzaldehyde.04/07/19