Reactions of Carbonyls
While most carbonyl functional groups share these two properties, within the carbonyl family of compounds, there are significant differences in reactivity. Aldehydes are generally more reactive than ketones and more easily oxidized. Carbonyls directly bonded to aromatic groups are generally less relative than Carbonyls that are directly bonded to alkyl substituents. In this experiment, we will perform each reaction on the known compounds that correspond to each chemical test first, to determine whether the reagents are working and to help you interpret a positive or a negative test.
It is important that after the tests, the unknown sample will not be able to be unequivocally determined. Results: During this experiment, there were three different tests that were completed to determine what compound the unknown sample was. I started off with unknown sample #5 which was clear in color. The first test completed was the iodoform test. Once all three of the test tubes contained 2 mL of 5% NaOH and 3 drops of the unknown, 3-pentanone, or acetophenone, iodine solution was added by the drop until the yellow color of the iodine was slightly visible.
The test tube containing the unknown and 3-pentanone only needed 3 drops each while acetophenone required 11 drops. The test tube containing acetophenone tested positive by forming a white/yellow precipitate at the bottom of the test tube. The unknown sample and 3-pentanone never changed color which means they tested negative. The Benedict’s test was completed next because it had to sit in the hot water bath for 10 minutes. After 3 drops of the compounds (unknown, glucose, and benzaldehyde) were added into the test tube, 2 mL of ethanol were then mixed in.
The next step was the addition of 1 mL of Benedict’s solution to each test tube. The three test tubes were all blue in color before entering the hot water bath. After 7 minutes in the hot water bath, the only test tube to test positive (the blue solution actually turned a dark red color) was the test tube containing glucose. The test tubes containing the unknown and benzaldehyde did not change color once in the hot water bath thus resulting in a negative test result. The final test that was completed was the chromic acid test.
Once 3 drops of the compound (unknown, benzaldehyde, and 3-pentanone) were added to the test tube, 1 mL of reagent grade acetone was mixed with the compound. Next, 1 drop of chromic acid (orange/yellow color) was added into the three test tubes. The test tubes containing the unknown and benzaldehyde compounds turned a greenish/blueish color which resulted in a positive test. The test tube containing 3-pentanone never changed color, so this resulted in a negative test. The following results stated above are located in a table which is included in the observations.
Conclusion: The Benedict’s test was used in order to determine if the unknown solution was either an “aliphatic aldehyde” or an “aromatic aldehyde. ” The oxidizing agent in Benedict’s reagent is a water soluble complex formed between cupric ions (Cu 2+) and citrate ions. An aliphatic aldehyde that is added to Benedict’s reagent is oxidized to the corresponding carboxylic acid and Cu2+ is reduced to the cuprous (Cu+) in Cu2O. In a positive test the blue color of the reagent turns greenish and a reddish precipitate of cuprous oxide deposits in the tube.
Benedict’s reagent oxidizes aliphatic aldehydes but cannot oxidize aromatic aldehydes. The Reaction is as followed: The Iodoform test may be used to distinguish between methyl ketones and no methyl ketones. When iodine and OH- are added to a methyl ketone, a yellow precipitate called “iodoform (CHI3) is synthesized. One must be careful as some secondary alcohols, ethanol, and acetaldehydes may also give a positive test. Non methyl ketones will not form a yellow precipitate. A methyl ketone will react in the following manner with the I2 and OH-.
The Chromic test is based on the reduction of chromium (IV), which is orange, to chromium (III), which is green, when an alcohol is oxidized by the reagent. A change in color of the reagent from orange to green represents a positive test. Primary alcohols are oxidized by the reagent to carboxylic acids: secondary alcohols are oxidized to ketones. Tertiary alcohols are not oxidized at all by the reagent. Hence, the reaction can be used to distinguish tertiary alcohols from primary and secondary alcohols.
Questions: 1. Using chemical tests only, how would you distinguish between the following pairs of compounds? a. 2-methyl-2-propanol and butanal (Chromic Test) b. Benzaldehyde and propanal (Iodoform Test) c. 2-pentanone and 3-pentanone (Iodoform Test) 2. Why does Benedict’s test give a positive result for aliphatic aldehydes, but not aromatic aldehydes? a. The reason is Benedict’s reagent oxidizes aliphatic aldehydes, but cannot oxidize aromatic aldehydes.