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?
Yes, the contamination of benzaldehyde with benzoic acid can have an effect on the Tollens test, Fehling's test, and Jones Oxidation that were carried out in the lab.
1. Tollens Test: The Tollens test is used to distinguish between aldehydes and ketones. In this test, an aldehyde is oxidized to a carboxylic acid while the Tollens reagent is reduced to metallic silver. However, if the benzaldehyde is contaminated with benzoic acid, it will already contain a carboxylic acid group. Therefore, even without the addition of Tollens reagent, the observed result will be a positive test due to the presence of the carboxylic acid. This could lead to a false positive result for the presence of an aldehyde.
2. Fehling's Test: Fehling's solution is used to determine the presence of reducing sugars, such as aldehydes. In the presence of an aldehyde, Fehling's solution undergoes a redox reaction, resulting in the formation of a brick-red precipitate of copper(I) oxide. However, if the benzaldehyde is contaminated with benzoic acid, which is not a reducing sugar, the observed result will be negative. This could lead to a false negative result for the presence of an aldehyde.
3. Jones Oxidation: Jones Oxidation is a strong oxidation reaction commonly used to convert primary alcohols to carboxylic acids. If the benzaldehyde is contaminated with benzoic acid, which is already a carboxylic acid, the primary aldehyde group in the benzaldehyde will not oxidize further. Therefore, the Jones Oxidation reaction will not proceed as expected, and the observed result will be negative for the formation of a carboxylic acid.
In summary, the contamination of benzaldehyde with benzoic acid can lead to erroneous results in the Tollens test, Fehling's test, and Jones Oxidation. It can produce a false positive result for the presence of an aldehyde in the Tollens test, a false negative result for the presence of an aldehyde in the Fehling's test, and a negative result for the formation of a carboxylic acid in the Jones Oxidation.
To determine whether the presence of benzoic acid in an older bottle of benzaldehyde would affect the tests performed in the lab, let's consider the principles behind each test and how they work.
1. Tollens Test: This test is used to differentiate aldehydes from ketones. In this test, Tollens' reagent (ammoniacal silver nitrate) is reduced by aldehydes to form a silver mirror. Ketones do not react with Tollens' reagent.
2. Fehling's Test: Fehling's reagent (a mixture of copper (II) sulfate and sodium hydroxide) reacts with aldehydes to produce a red precipitate of copper (I) oxide. Ketones do not react with Fehling's reagent.
3. Jones Oxidation: This test is specifically used to oxidize primary and secondary alcohols to their corresponding aldehydes and ketones, respectively. The Jones reagent (chromium trioxide in diluted sulfuric acid) is used in this test.
Now, let's consider how the presence of benzoic acid might affect these tests:
1. Tollens Test: Benzoic acid is not an aldehyde; therefore, it would not react with Tollens' reagent. If your sample from the contaminated bottle contains benzoic acid, it should not affect the result of this test. You would still be able to differentiate between an aldehyde and a ketone based on the reaction with Tollens' reagent.
2. Fehling's Test: Like Tollens' reagent, Fehling's reagent does not react with carboxylic acids, including benzoic acid. Therefore, if your sample contains benzoic acid, it should not interfere with the reaction between Fehling's reagent and the aldehyde, and you should still observe a positive result for the presence of an aldehyde.
3. Jones Oxidation: The presence of benzoic acid in the sample might interfere with this test. Jones Oxidation is specific to the oxidation of alcohols to aldehydes/ketones. If your contaminated sample contains benzoic acid, there could be a false positive result because the presence of benzoic acid might be mistakenly identified as an aldehyde.
In summary, the presence of benzoic acid in an older bottle of benzaldehyde should not affect the Tollens and Fehling's tests, but it could potentially interfere with the Jones Oxidation test. Always make sure to consider the possibility of contamination and accurately interpret your results in the lab.