Abstract
The purpose of this experiment was to characterize aldehydes and ketones using Tollen’s test and Iodoform test. It also aims to identify an unknown substance as well as determine whether glucose is an aldehyde or a ketone using the same tests. In order to achive these purposes, four test tubes where prepared for each type of test. For Tollen’s test, each of the tubes contained 1 drop of the analyte to which 1mL of the Tollen’s reagent was added. For the Idoform test, 4 drops of each substance was used, to which 2 mL of sodium hydroxide (aq) and iodine-KI solution were added. Results showed that aldehydes were reactive to Tollens reagent but not to the Iodorform reagent, methyl Ketones on the produced reverse results. The identity of the unknown was determined to be 3-pentanone, while glucose sample was an aldehyde. The results of the experiment were consistent with the chemical structures of the compounds, hence, it was concluded that the experiment was a success.
Introduction
Aldehydes and ketones both have the carbonyl group as their functional group. Nevertheless, they have different chemical reactions. Aldehydes have the following molecular structure:
where R is an alkyl group and the middle carbon atom is called the carbonyl carbon. Aldehydes can be easily oxidized by mildly oxidizing agents such as the silver ion in Tollen’s reagent (Ag(NH3)2+ OH–). The Tollen’s reagent can be used to determine whether a compound is an aldehyde. When reacted with aldehydes the silver atom is oxidized from +1 to 0, while the carbonyl carbon is oxidized from +1 to +3. This means that the carbonyl group has to lose 2 electrons which will be taken up by two silver monocations (Chemistry Beta). The Balanced chemical reaction for aldehydes and the Tollen’s reagent is:
2Ag(NH3)+2 + RCHO + 3OH- 2Ag0 + RCOO- + 4NH3 + 2H2O
A positive result for Tollen’s test is the production of silver like mirror, which is the Ag0 species in the chemical reaction above; while a negative result shows no formation of the silver mirror compound.
Ketones, on the other hand have the following chemical structure:
where R is an alkyl group, and the methyl group attached and the carbonyl carbon could also be another alkyl group, but in this example, the compound is a methyl ketone. Ketones cannot be easily oxidized by mildly oxidizing agents as they need stronger oxidizing chemical species. The oxygen atom in ketones withdraws some of the electrons from the methyl group making the alpha hydrogen (hydrogen attached to the -CH3). This allows the easier abstraction of the alpha hydrogen to be replaced by other atoms such as Iodine which could easily compensate for the electron withdrawing effect of the carbonyl oxygen. The presence of ketones can be confirmed through the Iodoform Test. The Idoform test takes advantage of the presence of alpha hydrogen at the methyl groups. The three iodine atoms from test reagent replace all three hydrogen atoms in the methyl ketone, making the alpha carbon a good living group. An hydroxide ion eventually makes a nucleophilic attack to the carbonyl carbon and the iodoform molecule (CHI3) leaves to form the carboxylate ion. The iodoform molecule is yellowish in color which serves as positive sign that a methyl ketone is present (Davis et al 2402).
In this experiment aldehydes and methyl ketone samples were characterized using the two aforementioned tests. An unknown sample was also checked for the presence of either aldehyde or methyl ketone.
Results
Discussion
In a molecule, different atoms have different electron withdrawing capacities. This is the case with aldehydes and ketones. For chemical species, the carbonyl carbon and oxygen withdraws more electrons compared to their neighboring atoms. The result is the formation of partials charges in the molecule, especially near the carbonyl group. This makes the carbonyl group and its neighboring atoms highly reactive to different reagents. The carbonyl carbon in aldehydes, for example, is a higher partial positive charge than in a methyl ketone; hence aldehydes can easily react with mildly basic or oxidizing reagents such as the Tollen’s reagrent. In ketone, however, the methyl carbon could allow its electrons to be pulled by the carbonyl group but the result is that its hydrogens could be easily abstracted be replaced by other electron reach atomic species such as Iodine. This makes methyl ketones produce positive results in the Iodoform test through the following reaction mechanism:
Note that to arrive at the iodoform (CHI3) and carboxylate ion (RCOO-), there are three elimination addition reaction pairs to replace all three hydrogens in –CH3 with three iodine atoms arriving at the compound in black square, which is the attached by hydroxide ion (nucleophilic attack) at the carbonyl carbon. A substitution elimination product then forms (red square). Based on this theory, it can be expected that citral and straight chain glucose will produce positive results for Tollen’s test and negative for the Iodoform test (McMurry 866). On the other hand, 2-heptanone will produce positive test in the Iodoform test and negative on the Tollen’s test. The chemical structures of these chemical species are shown in table 3.
Based from the results of the experiment, since glucose reacted with Tollen’s reagent, then it can be safely concluded that its straight chain conformation is an aldehyde. The structure of glucose support’s this result; as shown in table 3, thecarbonyl carbon has a hydrogen attached to it, which is the characteristic for aldehydes. With regards to the unknown, considering the chemical structures for its possible identity as well as the results of the experiment, it can be concluded that it is neither an aldehyde nor a methyl ketone, because it produced negative results for both Tollen’s test and Iodoform test. The unknown is therefore 3-pentanone, because pentanal is an aldehyde and 2-pentanone is a methyl ketone.
It should be noted that some test results were not highly conclusive such as the presence of traces of greyish precipitates by very few only for the Tollen’s test of 2-heptanone and the presence of traces of greyish and yellowish precipitates by very few only for the Idoform test of citral. Nevertheless, it can be safely concluded that these inconsistencies are due to contaminants in either the prepared samples or the reagents used. Since the test reagents used were not freshly prepared by the class, they must have been contaminated.
Experimental Procedure
For Tollen’s test, four test tubes containing one drop of each compound found in table 3 were prepared. To each of these test tubes, 1 mL of Tollen’s reagent was added. They were then let to stand in room temperature for 1 minutes and all observations were recorded.
For the Iodoform test 4 drops of each substance in table 4 were put into separated test tubes. Each of the tubes where then given 2 mL of 10% sodium hydroxide solution and 5 mL of iodine-potassium iodide solution with a concentration of 0.5 M. Observations where then recorded. Note that all test reagents were prepared for the class prior to the performance of the experiment.
Works Cited
Chemistry Beta. Mechanism for Tollen's classification test for Aldehydes?. 2014. Web. 29 October 2014. <http://chemistry.stackexchange.com/questions/13918/mechanism-for-tollens-classification-test-for-aldehydes>.
Davis, F. A.; Haque, M. S.; Ulatowski, T. G.; Towson, J. C. Asymmetric Oxidation of Ester and Amide Enolates Using New (Camphorylsulfony1) oxaziridines. Journal of Organic Chemistry, 51.1 (1986): 2402.
McMurry, John E. Organic Chemistry (2nd ed.), Brooks/Cole, (1998). p. 866.
