Synthesis of benzyl alcohol and benzoic acid (Cannizzaro reaction)
Random Experiments Int. - Experiments and syntheses・11 minutes read
Dissolve potassium hydroxide and mix it with benzaldehyde to initiate the Cannizzaro reaction, resulting in the formation of benzyl alcohol and benzoic acid. After extracting and purifying the products through distillation and crystallization, yields of 79% for benzyl alcohol and 88% for benzoic acid are achieved.
Insights
- The synthesis process described involves the Cannizzaro reaction, where 30 grams of benzaldehyde reacts with a potassium hydroxide solution, resulting in the formation of both benzyl alcohol and benzoic acid through a dual reduction and oxidation mechanism facilitated by hydroxide ions. This reaction highlights the unique ability of hydroxide ions to act as nucleophiles, effectively splitting the benzaldehyde into two distinct products.
- After the reaction, the separation and purification steps are crucial for obtaining the final products: benzyl alcohol and benzoic acid. The process includes washing the organic layer, drying it, and conducting a simple distillation to yield 12.1 grams of benzyl alcohol with a 79% yield, while the benzoic acid is crystallized from the aqueous phase, resulting in 15.2 grams with an 88% yield. These steps emphasize the importance of careful handling and purification to achieve high-quality chemical products.
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Recent questions
What is potassium hydroxide used for?
Potassium hydroxide, also known as caustic potash, is a strong alkaline compound commonly used in various applications, including the production of biodiesel, soap making, and as a pH regulator in chemical processes. It serves as a powerful reagent in organic synthesis, particularly in reactions that require a strong base. In laboratory settings, potassium hydroxide is often utilized to create alkaline solutions for various chemical reactions, such as the Cannizzaro reaction, where it facilitates the conversion of aldehydes into alcohols and acids. Its ability to dissolve fats and oils makes it valuable in cleaning products and industrial applications as well.
How do you make benzyl alcohol?
To synthesize benzyl alcohol, one typically employs the Cannizzaro reaction, which involves the reaction of benzaldehyde with a strong base, such as potassium hydroxide. The process begins by dissolving potassium hydroxide in distilled water and then mixing it with freshly distilled benzaldehyde in a controlled environment. The reaction leads to the reduction of half of the benzaldehyde to benzyl alcohol while the other half is oxidized to benzoic acid. After forming a stable emulsion and allowing the mixture to react overnight, the resulting product can be extracted and purified through distillation, yielding benzyl alcohol with a significant yield.
What is the Cannizzaro reaction?
The Cannizzaro reaction is a chemical reaction that involves the disproportionation of non-enolizable aldehydes in the presence of a strong base, typically resulting in the formation of an alcohol and a carboxylic acid. In this reaction, one molecule of the aldehyde is reduced to the corresponding alcohol, while another molecule is oxidized to the carboxylic acid. This reaction is particularly useful for aldehydes that lack alpha-hydrogens, making them unsuitable for other types of reactions, such as aldol condensation. The Cannizzaro reaction is significant in organic chemistry for synthesizing valuable compounds, including benzyl alcohol and benzoic acid from benzaldehyde.
How do you purify benzoic acid?
To purify benzoic acid, one typically begins by acidifying an aqueous solution containing the crude product with concentrated hydrochloric acid, adjusting the pH to around 1 or 2. This process helps to precipitate benzoic acid from the solution. After acidification, the mixture is refrigerated overnight, allowing the benzoic acid to crystallize. The resulting crystals can then be collected through filtration and dried to remove any remaining moisture. This purification method is effective in achieving a high yield and purity of benzoic acid, which is characterized by its melting point of approximately 117 degrees Celsius.
What is the yield in chemical reactions?
Yield in chemical reactions refers to the amount of product obtained from a reaction compared to the theoretical maximum amount that could be produced based on the starting materials. It is typically expressed as a percentage, known as the percent yield, which is calculated by dividing the actual yield (the amount of product actually obtained) by the theoretical yield (the maximum possible amount) and multiplying by 100. Yield is an important measure of the efficiency of a chemical reaction and can be influenced by various factors, including reaction conditions, purity of reactants, and the presence of side reactions. High yields indicate effective reactions, while low yields may suggest inefficiencies or losses during the process.
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Summary
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Synthesis of Benzyl Alcohol and Benzoic Acid
- Begin the synthesis by dissolving 27 grams of potassium hydroxide in 18 milliliters of distilled water, heating the solution until it reaches its maximum temperature, then allowing it to cool while stirring.
- In a 250 milliliter round bottom flask, add 30 grams of freshly distilled benzaldehyde to the cooled potassium hydroxide solution, then seal the flask with a stopper and clamp, shaking until a stable emulsion forms.
- The reaction involves the Cannizzaro reaction, where half of the benzaldehyde is reduced to benzyl alcohol while the other half is oxidized to benzoic acid, facilitated by hydroxide ions acting as nucleophiles.
- After the emulsion thickens, remove the clamp, loosen the stopper, and allow the mixture to heat up overnight; if the smell of benzaldehyde persists after two days, proceed to the next step.
- Add a small amount of distilled water to the mixture and shake vigorously to create a cloudy solution, then transfer it to a separating funnel, washing the flask with distilled water and extracting five times with 30 milliliters of diethyl ether.
- Collect the organic layer containing benzyl alcohol, wash it with 10 milliliters of diluted sodium bicarbonate solution and 20 milliliters of distilled water, then dry the solution with anhydrous sodium sulfate until it becomes clear.
- Set up for simple distillation using a water bath, aiming to remove as much diethyl ether as possible; the boiling point of diethyl ether is 35 degrees Celsius, and benzyl alcohol should be distilled under vacuum at a boiling point of 205 degrees Celsius, yielding 12.1 grams of benzyl alcohol with a 79% yield.
- For benzoic acid, wash the aqueous solution with distilled water, acidify with concentrated hydrochloric acid to achieve a pH of 1 or 2, refrigerate overnight for crystallization, and then filter and dry the resulting 15.2 grams of benzoic acid, achieving an 88% yield with a melting point of 117 degrees Celsius.
