Sorry to post again but I'm really stuck on this one!
I'm doing some post lab questions and I'm stuck on one. In lab we performed Tollens test, Fehlings test, and Jones Oxidation to differentiate our unknown between an aldehyde or a ketone. The question I'm stuck on is " older bottles of benzaldehyde are often contaminated with benzoic acid. Would this affect the tests carried out in the lab if you had a sample from a contaminated bottle? If so, how would it change the results?" Any help or hints?
No problem! I'm here to help.
In order to answer the question, we need to understand how the Tollens test, Fehling's test, and Jones Oxidation work to differentiate between aldehydes and ketones.
The Tollens test is a chemical test used to detect aldehydes. It involves the reaction of an aldehyde with Tollens' reagent (a solution of silver nitrate and ammonia). If an aldehyde is present, it will reduce the silver ions in the Tollens' reagent to form a silver mirror on the inner surface of the reaction flask.
Fehling's test is another chemical test used to detect aldehydes. It involves the reaction of an aldehyde with Fehling's solution, which is a mixture of copper sulfate and sodium hydroxide. If an aldehyde is present, it will be oxidized to a carboxylic acid, and the copper ions in the Fehling's solution will be reduced to form a red precipitate of copper(I) oxide.
Jones Oxidation, on the other hand, is a test specifically used to differentiate between primary and secondary alcohols. It involves the oxidation of primary alcohols to carboxylic acids using a mixture of chromic acid (CrO3) and sulfuric acid. Secondary alcohols are not oxidized under these conditions.
Now, back to the question. If the benzaldehyde from the old bottles is contaminated with benzoic acid, it can affect the results of the tests carried out in the lab. Here's how:
1. Tollens Test: The presence of benzoic acid can interfere with the Tollens test. Benzoic acid does not react with Tollens' reagent, so if your sample contains both benzaldehyde and benzoic acid, you may not observe a silver mirror formation, leading to a false negative result for the presence of an aldehyde.
2. Fehling's Test: Benzoic acid can also interfere with the Fehling's test. It will not be oxidized to a carboxylic acid like an aldehyde would be, but it can react with the copper ions in Fehling's solution, leading to a false positive result. This means that even if your sample only contains benzoic acid (no aldehyde), you may still observe the formation of a red precipitate, which would be misleading.
3. Jones Oxidation: In the Jones Oxidation, the presence of benzoic acid would not significantly affect the differentiation between an aldehyde and a ketone. This is because benzoic acid is a carboxylic acid and not an aldehyde or a ketone. The Jones Oxidation does not oxidize carboxylic acids, so it would not produce false results in this case.
In summary, the presence of benzoic acid in the benzaldehyde sample from the contaminated bottles can affect the Tollens test and Fehling's test by producing false negative and false positive results, respectively. However, it would not significantly affect the Jones Oxidation, as it is not targeting carboxylic acids.
Remember, it is important to be aware of potential contaminants and their possible effects on the tests being performed to obtain accurate results.