Aldehydes and Ketones Lab Report PDF

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Aldehydes and Ketones Lab Report, Spring 2021...

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Aldehydes and Ketones Lab Report Writer - Matthew Estacio Reviewer - Maddie Looney Editor - Nora Cipkowski

Introduction: The purpose of the Aldehydes and Ketones lab involved identifying an unknown ketone or aldehyde by performing several tests. The Tollen’s Reagent test was used to identify the presence or absence of an aldehyde (Reference 1). The Iodoform test was used to identify the presence or absence of a methyl ketone, and the mechanism of the iodoform formation reaction was drawn . Two derivatives were created by reaction with 2,4-DNP and semicarbazide hydrochloride. Finally, the melting point of the derivatives was taken and compared to provided literature values to determine the correct identity of the unknown. The carbonyl group of aldehydes and ketones is the source of many reactions in organic chemistry. The polarity of the carbon oxygen double bond makes the carbonyl group very reactive, causing the carbon to be partially positive and the oxygen to be partially negative. A resonance structure places further negative charge on the oxygen, causing the carbonyl group to undergo several reactions. In these reactions, the carbonyl group acts as an electrophile that can be attacked by many different nucleophiles. The carbonyl group also makes the hydrogens attached to the carbonyl carbon (ɑ -carbon) mildly acidic. This effect is caused by the negative charge on the ɑ -carbon being stabilized by pushing electrons onto the adjacent oxygen. Because of this, the hydrogens can be removed under basic conditions, and the enolate anion can act as a nucleophile with many different electrophiles. Table 1: Table of Reagents Compound

M.W. (g/mol)

B.P. (℃)

M.P. (℃)

Density (g/mL)

Ammonia

17.031

-33.34

-77.73

0.674

Ammonium hydroxide

35.04

24.7

-91.5

0.9

Ethanol

46.07

78.37

-114.1

0.789

Iodine

126.904

184

113.7

4.93

Iodoform

393.73

218

121

4.01

Methanol

32.04

64.7

-97.6

0.791

Potassium iodide

166.0028

1330

681

3.12

Semicarbazide

75.08

186.3±23

96

1.7±0.1

Semicarbazide hydrochloride

111.53

n/a

175-177

1.2908

Silver

107.87

2162

962

10.5

Silver (I) oxide

231.735

n/a

280

7.14

Silver nitrate

169.87

440

212

4.35

Sodium acetate

82.0343

881.4

324

1.53

Sodium hydroxide

39.997

1388

318

2.13

Sodium nitrate

84.9947

380

308

2.26

Water

18.01528

100

0.0

1.00

2,4-Dinitrophenylhydrazine

198.14

378.6

198-202

1.7±0.1

2,4-Dinitrophenylhydrazone

278.264

434.6

169-161

1.47

Mechanism: I.

Tollen’s Test (Silver Mirror) 2 AgNO3 + 2 NaOH → Ag2O(s) + 2NaNO3 + H2O

Eq. 1

Equation 1 shows the first step in creating the Tollen’s reagent. Silver nitrate reacts with sodium hydroxide to form solid silver (I) oxide. Ag2O(s) + 4 NH3 + 2 NaNO3 + H2O → 2[Ag(NH3)2]NO3 + 2 NaOH

Eq. 2

Equation 2 shows the second and final step in creating the Tollen’s reagent. Aqueous ammonia is added until the solid dissolves, forming a clear solution. 2[Ag(NH3)2]+ + RCHO + H2O → 2 Ag(s) + 4 NH3 + RCOOH + 2H+

Eq. 3

Equation 3 shows the reaction of the Tollen’s reagent with an aldehyde group. II.

Iodoform Test

Figure 1 Figure 1 shows how the iodoform test occurs by a base removing the slightly acidic hydrogens from the carbonyl ɑ carbon. This forms the enolate ion.

Figure 2 Figure 2 shows the enolate anion acting as a nucleophile, attacking iodine.

Figure 3 Figure 3 shows how the enolate anion can act as a nucleophile twice more until all three hydrogens have been replaced by iodine atoms.

Figure 4 Figure 4 shows how the carbonyl carbon is attacked by excess base in the solution because of the strong electron-withdrawing groups. The -CI3 acts as the leaving group, forming HCI3, iodoform.

III.

Preparation of Solid Derivatives

Figure 5 Figure 5 shows the formation of semicarbazone and 2,4-dinitrophenylhydrazone (2,4 DNP) by taking advantage of the reactivity of the carbonyl carbon with nucleophiles.

Experimental: Four different tests were performed in this lab in order to determine the identity of the unknown. The Tollens test, the iodoform test, a reaction with 2,4 - Dinitrophenylhydrazone

(DNP), and a reaction with a semicarbazide reagent. Tollens Test: A clean test tube was obtained and rinsed with 10% NaOH solution. A tollens reagent was created by adding 1 mL of 0.3 M AgNO3 and 0.5 mL of 3 M NaOH to the test tube. 2 M of aqueous ammonium hydroxide were then added dropwise while the test tube was shaken constantly. This was done until the precipitate that formed initially had dissolved. One drop of unknown was added to the test tube. The test tube was then shaken and incubated at room temperature for 10 minutes. The color of the solution after incubation was observed and recorded. Iodoform Test: 1-2 drops of unknown were dissolved in 0.5 mL of water in a large test tube. Methanol was used in place of water if the unknown did not dissolve. In a separate test tube an iodine solution was created by adding 0.5 g of iodine, 1.0 gram of potassium iodide, and 4.0 mL of water. To the original test tube, 0.5 mL of 3 M NaOH were added. Then 0.75 mL of the basic iodine solution was added drop by drop while the test tube was being shaken. This was done until the brown color of the iodine persisted in the solution. The presence, or lack thereof, of a yellow precipitate was recorded. If methanol was used, 2.5 mL of water was added to help precipitate the iodoform. 2,4 - DNP: 10 drops of unknown and 2 mL of 95% were placed in a test tube. A pipette was used to add this mixture into a separate test tube containing 2 mL of 2,4 DNP solution. After the precipitate had formed, the test tube was cooled in an ice bath. Then the solid was isolated via vacuum filtration using a Hirsch funnel. The solid was then collected in a capillary tube and placed in the melting

point apparatus where the melting point was determined and then recorded. Semicarbazone: The semicarbazide reagent was prepared by dissolving 0.2 grams of semicarbazide hydrochloride and 0.3 g sodium acetate in 2 mL of water in a test tube. The test tube was shaken gently in order to encourage everything to dissolve. 0.5 mL of the unknown compound was added to the test tube. If an emulsion was not formed, 95% ethanol was added drop-by-drop until a clear solution was produced. The test tube was then stoppered and shaken vigorously for about a minute. The stopper was then removed and the test tube was placed in a boiling water bath for 5 minutes. The test tube was then allowed to cool to room temperature, then it was cooled in an ice bath for 10 minutes. If no crystals formed after cooling, the inside of the test tube was scratched with a glass stirring rod to induce crystallization. The solid was then collected via vacuum filtration through a Hirsch funnel. A capillary tube was used to collect some of the solid, and was then placed in the melting point apparatus where the melting point was determined and recorded. Using the results from the first two tests, and the melting point of the solid formed after each of the last two reactions, the unknown was identified as 2-butanone.

Results: Tollen’s Test: After testing Unknown 959, there was no elemental silver precipitate at the bottom of the test tube; therefore, it was a negative result.

Iodoform Test:

After testing the unknown, there was a yellow precipitate that was formed; therefore, it was a positive result. 2,4 Dinitrophenylhydrazone: When a solid was formed, its melting point was tested. It had a melting point of 109-116 °C, which was compared to Table 2. Semicarbazone: A solid product was formed, so its melting point was tested. The product had a melting point of 144.6 - 146°C. This was then compared to Table 2. Table 2. Ketone and Aldehyde Derivatives’ Melting Point °C

Based on the lack of an elemental silver precipitate formed from the Tollen’s test, a significant amount of yellow precipitate formed during the Iodoform test, the melting point of the solid derivative for the 2,4-dinitrophenylhydrazine reaction being 109-166 ℃, and the melting point of the solid derivative for the semicarbazone reaction being between 144.6-146 ℃, the product was determined to be 2-butanone (Figure 6).

Figure 6 Figure 6 shows the structure of 2-butanone. 2-butanone does not contain an aldehyde group, corresponding with the negative test result observed from the Tollen’s test. 2-butanone contains a methyl ketone, corresponding with the positive result of the Iodoform test. The melting point of the 2,4-dinitrophenylhydrazine reaction solid derivative was 109-166 ℃ , and the melting point of the semicarbazone reaction solid derivative was 144.6-146 ℃, both corresponding to the values listed for 2-butanone (Table 2).

Discussion: The Tollen’s Test oxidizes aldehydes, which results in a silver reflective substance on the edges of the test tube. This is an example of a positive result. If a ketone is used in a Tollens reaction, there would be no visible change; which means that this would be a negative result. This specific unknown had a negative result. This means that the unknown did not contain an aldehyde (Reference 2). In this lab, the Iodoform test was used to test for the presence of a methyl group in aldehydes and ketones. If the unknown had a methyl ketone or methyl aldehyde present, the product would have been a bright yellow color change. On the other hand, if the unknown did not have a methyl attached to a ketone or aldehyde, the product would have had no visible color change. Unknown 959 had a yellow color at the end of the test, and, therefore, contained a methyl group attached to the carbonyl group.

The reaction with 2, 4 Dinitrophenylhydrazone resulted in a product with the melting point of 109 - 116 °C. This melting point only matched one possible unknown in Table 2, which was 2-Butanone. Then the reaction with Semicarbazone was performed and produced a solid. The melting point of this solid was measured at 144.6 - 146 °C. This melting point also matched with one possible unknown, which was also 2-Butanone. All of these results come together to prove that the unknown was 2-Butanone. Based on the Tollens Test and the Iodoform Test, the unknown had a methyl group attached to a carbonyl group and was not an aldehyde, indicating the presence of a methyl ketone. 2-Butanone is a methyl ketone with melting points that correspond to the melting points observed in the reactions with 2,4-dinitrophenylhydrazine and semicarbazone (Table 2). In Equation 3, the Tollens Reagent attacks the aldehyde of the molecule if it is there, which is why there was no reaction with 2-Butanone. Unusual observations of this experiment was a small precipitate during the Tollens Test, but not the full silver-mirror precipitate to indicate a positive result.

Conclusion: The most important data collected were the results of each test performed in this lab as well as the melting points collected after each solid derivative was formed. The negative result from the Tollens test gave us the information that our unknown did not have an aldehyde group in it. The positive result gained from the iodoform test told us that the unknown must have had a methyl attached to a ketone. Finally, the melting points of the solid derivatives allowed us to compare those to the melting points on the list of possible unknowns, which allowed us to narrow down the unknown to 2-butanone. These results showed that these tests were effective in identifying the presence or absence of specific functional groups, specifically aldehydes and

ketones. One issue that was encountered during the lab was understanding how to operate the melting point apparatus, so in the future it would make the procedure run smoother if instruction on how to use the apparatus was accessible prior to the lab (Reference 3). The melting point apparatus also takes quite a while to heat up, so it might be beneficial to start the apparatus while performing the reaction so it’s already heated to the proper temperature, causing the lab to take less time to perform.

References: Reference 1. Libretexts. “Tollens' Test.” Chemistry LibreTexts, Libretexts, 13 Sept. 2020, chem.libretexts.org/Bookshelves/Organic_Chemistry/Supplemental_Modules_(Organic_C hemistry)/Aldehydes_and_Ketones/Reactivity_of_Aldehydes_and_Ketones/Tollens_Test. Reference 2. BYJU. “Iodoform Test - Description and Mechanism - Compounds That Test Positive.” BYJUS, BYJU'S, 28 Oct. 2020, byjus.com/chemistry/iodoform-test/. Reference 3. Libretexts. (2020, August 14). 6.1D: Step-by-step procedures for melting point determination. Retrieved March 01, 2021, from https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Book %3A_Organic_Chemistry_Lab_Techniques_(Nichols)/06%3A_Miscellaneous_Techniques /6.01%3A_Melting_Point/6.1D%3A__Step-byStep_Procedures_for_Melting_Point_Determination...