Reactions of Aldehydes and Ketones

Reactions of Aldehydes and Ketones

Introduction

In this experiment, many tests were performed on an unknown to determine its identity. The tests that were used include: Tollens Test, Iodoform Test, and the preparation of solid derivatives with the use of both 2,4 – dinitrophenylhydrazone and semicarbazone¹. The reactivity of the carbonyl group is utilized in this experiment. The structure of a carbonyl group and the differences between an aldehyde and a ketone are as shown in Figure¹:

Figure 1: In organic chemistry, carbonyl groups (as shown in the aldehyde and ketone above) are the birthplace of many reactions¹.

 In addition, Figure 2 shows the mechanism of how a carbonyl group utilizes polarity to make the carbon into an electrophile¹. The polarity of the double bond between the carbon and oxygen results in a positive charge on the carbon and a partial negative charge on the oxygen¹.

Figure 2: Polarity is what makes the carbonyl group so easy to react with¹.

Figure 3 shows the next step in the S_N2 reaction where the nucleophile attacks the electrophile¹. The hydrogens on the carbon directly attached to the carbonyl group are acidic due to the resonance that stabilizes the negative charge on the oxygen¹.

Figure 3: The S_N2 mechanism for creating an alcohol from a ketone.

As shown in Figure 4, the basic hydrogens can be removed in the presence of a base to form an enolate anion that can act as a nucleophile and attack an electrophile¹. This experiment will use all the chemical properties of carbonyl groups to perform the tests and form the solid derivatives to determine the melting point of the solid derivatives and ultimately the identity of the unknown.

Figure 4: The acidic hydrogens can be removed in the presence of a base, forming an enolate anion that can act as a nucleophile¹.

1. Tollens Test:

Bernhard Tollens helped to discover/modify the Tollens reagent as he did some work with carbohydrates and their structures¹. The Tollens reagent tests for the presence of aldehydes in chemical compounds². The reagent is formed using the chemical equations in Figures 5 and 6¹. Once the reagent is formed, it can be used to test for an aldehyde group². If an aldehyde group is present, the reaction in Figure 7 will occur, and Ag¹⁺ will be reduced to a metal or form a silver mirror-like film on the inner wall of the test tube².

Figure 5: Step 1 of preparation of the Tollens reagent¹.

Figure 6: Step 2 of preparation of the Tollens reagent¹.

Figure 7: Reaction that occurs when Tollens reagent is added to a compound with an aldehyde group.

2. Iodoform Test:

The iodoform test shows if there are any methyl ketones in the unknown compound or not. The methyl ketones produce HCI₃ (Iodoform), and the compounds that lack methyl ketones do not produce HCI₃³. Iodoform is characterized by its yellow color and solidity³. Figure 8 shows the mechanism of the acidic hydrogens being removed from the ketone by a base to form an enolate anion¹. Then, in Figure 9, the enolate (nucleophile) is attacking the iodine (electrophile)¹. Figure 10 shows that the acidic hydrogens can be removed by the base until there are not any of the remaining¹. In addition, the enolate anion can also attack the iodine until there are not any more enolates able to be formed by the removal of acidic hydrogens as shown in Figure 10¹. Lastly, the carbonyl carbon is attacked by base, as in Figure 11, and is turned into an alcohol with CI₃ as the leaving group¹. CI₃, then attacks the hydrogen on the hydroxyl group, forming the iodoform¹.

Figure 8: Iodoform test begins due to acidic hydrogen being removed by the base.

Figure 9: The enolate anion that is formed when the acidic hydrogens are removed by the base acts as a nucleophile.

Figure 10: The  carbon becomes a nucleophile and attacks iodine.

Figure 11: The carbonyl carbon is attacked by the base and the CI₃ leaves as the leaving group.

3. Preparation of Solid Derivatives:

The solid derivatives of the unknown are formed using the reactivity of carbonyl groups. In Figure 12, the two derivatives formed in this lab are shown. Both derivatives are formed from a ketone.

Figure 12: The two solid derivatives formed in the experiment.

The above tests and preparations were completed in this lab using the compounds listed in Table 1: Table of Reagents. Lastly, the melting point of the solid derivatives of the unknown compound, listed in Table 2: Melting points of Aldehyde & Ketone Derivatives, were used to determine the identity of the unknown compound.

**SOURCES**

Experimental

The Tollens test was performed first. First, the test tubes were rinsed with 10% NaOH solution. 1 mL of 0.3 M AgNO₃ was obtained in a test tube, then 0.5 of 3M NaOH was added to the test tube of AgNO_3. A brown precipitate formed after this mixture.Next, 2M aqueous NH₄OH was added drop by drop while constantly shaking. NH₄OH kept being added until the brown precipitate that was formed dissolved. One drop of the unknown #166 was added, then. The test tube was shaken and left to sit at room temperature for 10 minutes. After 10 minutes, no black precipitate was observed, therefore, the test tube was placed in a warm water bath (45˚C) for 5 minutes. After five minutes, there was still no black precipitate formed, therefore the results for the Tollens test were negative.

The second test that was done was the Iodoform test. The unknown was used again. A large test tube was obtained, and 0.5 mL of water was added to this test tube, and then 2 drops of the unknown were added to this test tube. In addition, 0.5 mL of 3M NaOH was also added to the same test tube. After this, basic iodine solution was made by mixing 0.5 g of I₂, 1.009 g of KI, and 4 mL of H₂O. This 0.75 mL mixture of basic iodine was added to the test tube slowly, drop by drop while shaking until the brown iodine color stayed. If a bright yellow precipitate forms, the test is positive, since there was no yellow precipitate formed, the Iodoform test was negative.

To find the identity of the unknown, two other steps were taken: both including the formation of solid derivatives. First, a test tube was obtained. Then, 10 drops of the unknown #166 was added in 2 mL 95% ethanol. This solution was then added via pipette to 2 mL of the 2,4-dinitrophyenylhdrazine solution. A solid derivative was formed. This solid in the test tube was then placed in an ice bath to cool so that crystallization was maximized. Next, vacuum filtration was used to isolate and filter the solid. After the product was filtered, the product was placed in the oven for 10 minutes to dry even more due to it being more of a clay-like consistency. After it was dried, the melting point was obtained and determined.

The last part was the second preparation of derivatives. Semicarbazide was prepared by combining: 0.251 g of Semicarbazide hydrochloride, 0.351 g sodium acetate, and 2 mL of water in a test tube. The test tube mixture was shaken gently to allow the mixture to dissolve. The unknown was obtained again, and 0.5 mL of the unknown was added to the mixture in the test tube. The test tube was shaken again for a minute and then placed in a hot water bath for 5 minutes. The test tube was then allowed to cool at room temperature before placing it in an ice bath for 15 minutes to crystallize further. Vacuum filtration was then used to filter the product. Once dried, the melting point was obtained.

Results

Discussion

The identity of the unknown #133 was found based on the negative results of the Tollens test the positive results of the Iodoform test in addition to the melting points of the solid derivatives – semicarbazone and 2,4 – DNP. The results of the Tollens test were interpreted to be negative based on the formation of the silver mirror or black precipitate did not occur. The iodoform test was said to be positive, because a yellow precipitate formed. The melting point range of the solid derivative, 2,4 – DNP, was from 235-237 C. By comparing the temperature range with the values in Table 2, the unknown could have a few possible identities due to possible sources of error. One example of a possible error being that the sample used to take the melting point may have contained traces of water, which can ultimately affect the melting point.

The other melting point, with a range of 218-221 C was determined from the solid derivative, semicarbazone. This melting point helped us to determine the identity of the unknown more effectively than that derived from the 2,4 – DNP derivative, because it was closer to the value in the chart than the other melting point was. Using both the melting point from 2,4 – dinitrophenylhydrazone and the semicarbazone, the identity of the unknown was determined to be Benzaldehyde which has a 2,4 – DNP melting point of 237 C  and a semicarbazone melting point of 222 C.

**QUESTIONS?**

Conclusion:

In this experiment, unknown bottle #166 was determined to be 3-pentanone. The melting point of the solid derivative composed of 2,4- dinitrophenylhydrazone was about 20 degrees lower than the melting point due to the solid not being dried enough. The melting point taken of the solid derivative composed of semicarbazide was used to determine the identity of bottle number 166. A way to improve this experiment is to dry the product completely through vacuum filtration or possibly even in an oven.

References

1. University of Alabama at Birmingham. Reactions of Aldehydes and Ketones Handout. Accessed 20 June 2018.
2. North Carolina State University – Department of Chemistry. Tollen’s Test (Silver Mirror). Accessed 23 June 2018. https://projects.ncsu.edu/project/chemistrydemos/Organic/TollensTest.pdf
3. Jasperse. Minnesota State University. Carbonyl Unknowns. Accessed 23 June 2018. http://web.mnstate.edu/jasperse/Chem365/Carbonyl%20Unknown.pdf