Lab Report 4 Final Copy - Grade: A PDF

Preview

Summary

Lab Report 4 Final Copy...

Description

Aldehydes and Ketones: Identification of Unknown #350

Lead Author: Bradly Wurth Reviewer: Elijah Marsh Editor: Hannah Strickland

Chemistry 238 Section G5

Experiment 4

Introduction: An aldehyde contains a carbonyl group bonded to a hydrogen and a carbon (R group). A ketone contains a carbonyl group bonded to two carbons (two R groups).1 There are several ways to identify aldehydes and ketones. Two chemical processes for identification are the Tollen’s reagent test and and the iodoform test. Another way to identify aldehydes and ketones are through derivatives. Two derivates of ketones and aldehydes are 2,4-dintrophenylhydrozone and semicarbazone. The Tollen’s reagent test identifies aldehydes. Tollen’s reagent works by oxidizing an aldehyde into a carboxylic acid. This test will only work with aldehydes, because ketones cannot be easily oxidized. When the aldehyde is oxidized to a carboxylic acid it will form silver as a product. This is because diamminesilver is reduced to silver when the aldehyde is oxidized.2 The silver indicates a positive test result for an aldehyde. This mechanism is shown in figure 1. The iodoform test identifies methyl ketones, acetaldehyde, and secondary alcohols. The iodoform test is a haloform reaction. It works by the halogenation of a methyl ketone in the presence of a base. This will produce iodoform and a carboxylic acid. The yellow iodoform precipitate indicates a positive test result for a methyl ketone, acetylaldehyde, or a secondary alcohol.3 This mechanism is shown in figure 2. 2,4-dintrophenylhydrozone can be used to determine the functionality of an aldehyde or ketone. It works by interacting with the carbonyl group and forms 2,4dinitrophenylhydrazone. If the test is positive, the product will be a red precipitate. If the carbonyl group contains a ring then the product will be a darker red color. If the carbonyl group does not contain a ring then the product will be a lighter red color.4 This derivative can help identify ketones and aldehydes by comparing melting points. This mechanism is shown in figure 3. Semicarbazone, a derivative of imines, is a condensation reaction with semicarbazide and a ketone or aldehyde. This semicarbazone derivative can help identify ketones and aldehydes by comparing melting points.5 This mechanism is shown in figure 4. The chemicals used throughout the experiment are shown in table 1. The list of unknowns, along with their derivative melting points, are shown in table 2.

Figure 1: Figure 1 shows the mechanism for Tollen’s reagent with an aldehyde.

Figure 2: Figure 2 shows the mechanism for the iodoform test with a ketone.

Figure 3: Figure 3 shows the mechanism for 2,4-dinitrophenylhydrazine with a ketone or aldehyde to form 2,4-dinitrophenylhydrazone.

Figure 4: Figure 4 shows the mechanism for the preparation of semicarbazone with an aldehyde or ketone.

Table 1: Table of Reagents6 Compound 2,4-dintrophenylhydrozone

Molecular Weight (g/mol)

Boiling Point (°C)

Melting Point (°C)

Density (g/cm3)

198.138

378.6

198.0-202.0

1.7

ammonium hydroxide

35.046

38.0

-58.0

0.88

ethanol

46.069

78.9

-114.1

0.789

253.809

184.4

113.7

4.93

32.042

64.7

-97.8

0.79

potassium iodide

166.003

1323.0

681.0

3.12

semicarbazide hydrochloride

111.529

175.0 - 185.0

175.0 - 185.1

1.29

semicarbazone (acetone)

115.136

235.0

190.0

1.17

silver nitrate

169.872

440.0

212.0

5.35

sodium acetate

82.034

881.4

324.0

1.53

sodium hydroxide

39.997

1388.0

323.0

2.1

water

18.015

100.0

0.0

1.0

iodine methanol

Table 2: Melting Points of Aldehyde and Ketone Derivatives7 Compound

2,4-DNP (°C)

Semicarbazone (°C)

acetone

126

190

acetophenone

240

203

benzaldehyde

237

222

butanal

122

95

2-butanone

116

146

2-butenal

190

199

cycloheptanone

148

163

cyclohexanone

160

166

cyclopentanone

142

205

3,3-dimethyl-2-butanone

125

157

2,4-dimethyl-3-pentanone

88

160

heptanal

106

109

hexanal

104

106

2-hexanone

106

122

3-hexanone

130

112

2-methylcyclohexanone

136

191

3-methyl-2-butanone

117

113

4-methyl-2-pentanone

95

132

2-methylpropanal

182

125

pentanal

107

oil

2-pentanone

144

110

3-pentanone

156

138

1-phenyl-2-propanone

156

198

propanal

148

89

trichloroacetaldehyde

131

90

Experimental:

For the Tollen’s test, 1 mL of 3 M silver nitrate was added to 0.5 mL of sodium hydroxide in a test tube. Then, 2 M ammonium hydroxide was added dropwise to the solution until the silver oxide had completely dissolved. Next, a drop of unknown #350 was added to the solution. A stopper was placed on the test tube, and the tube was shaken. The test tube was allowed to sit for ten minutes at room temperature. Then, since there was no precipitate, the tube was heated in a hot bath for five minutes. The test tube was removed, the solution was a black liquid with no precipitate. For the iodoform test, one drop of unknown #350 was added to 0.5 mL of water in a test tube. Next, 0.53 mL of sodium hydroxide and 0.75 mL of iodine solution was

added dropwise to the test tube. Then, the test tube was shaken for one minute. A yellow precipitate was noted. For the preparation of the 2,4-dinitrophenylhydrazone derivative, 5 drops of unknown #350 were added to 2 mL of 95% ethanol in a test tube. Next, 2 mL of 2,4dinitrophenylhydrazine was added. A light orange precipitate immediately formed. After the formation of the precipitate, vacuum filtration was done. While in the filtration apparatus, the precipitate was washed with 2 mL of ethanol, and allowed to dry. After five minutes of filtration, the light orange precipitate was removed and placed on a watch glass. The watch glass was placed in an oven for 5 minutes to allow for drying. Then, the melting point of the dry precipitate was taken. For the preparation of the semicarbazone precipitate, 0.2 g of semicarbazone hydrochloride, 0.3 g of sodium acetate, and 3 mL of water were placed in a test tube. Then, 0.5 mL of unknown #350 was added. Next, 2 mL of 95% ethanol was added dropwise. A stopper was placed on the test tube, and was shared for one minute. The stopper was removed, and the test tube was heating in boiling water for five minutes. Next, the test tube was allowed to cool in an ice bath for ten minutes. White crystals were noted in the test tube. The solution was placed in a vacuum filtration apparatus, rinsed with 5 mL of ice water, and allowed to dry. Then, the precipitate was placed on a watch glass and put in the oven for five minutes. Finally, a melting point was taken. Results: For unknown #350, two tests were done to determine the chemical structure. These two tests were Tollen’s test and the iodoform test. For the Tollen’s test, the reaction formed a black liquid with no silver precipitate. This indicates that the test was negative. For the iodoform test, the reaction formed a yellow white precipitate. This indicates that the test was positive. These results are shown in table 3. For unknown #350, two derivatives were made to determine the melting point of the unknown. These two derivatives were 2,4-dinitrophenylhydrazone and semicarbazone. When the unknown was reacted with 2,4-dinitrophenylhydrazine to form 2,4-dinitrophenylhydrazone, it formed a light orange precipitate. The melting point of this derivative was a range of 152.0°C to 154.0°C. When the unknown was reacted to form the semicarbazone derivative, it formed a white precipitate. This derivative had a melting point range of 138.0°C to 142.0 °C. These results are shown in table 4. Table 3: Functional Group Test Results Test

Result

Reaction

Tollen’s test

negative

black liquid, no precipitate

iodoform test

positive

yellow-white precipitate

Table 4: Derivative Test Results Derivative

Reaction

Melting Point (°C)

2,4-dinitrophenylhydrazone

light orange precipitate

152.0 — 154.0

semicarbazone

white precipitate

138.0 — 142.0

Discussion: The Tollen’s test and iodoform test were performed to determine if unknown #350 was an aldehyde or ketone. When the Tollen’s test was performed, no silver precipitate formed. This indicates that there was no hydrogen bonded to the carbonyl to oxidize into a carboxylic acid. Because the unknown wasn’t oxidized, diamminesilver was not reduced to silver, thus no silver was formed.2 This indicates that there is not an aldehyde present in the unknown. The iodoform test will react with methyl ketones, acetaldehyde, and secondary alcohols. Because the unknowns, shown in table 2, are not acetaldehyde or secondary alcohols, the unknown most likely contained a methyl ketone. When the iodoform test was performed, a yellow-white precipitate was produced. This indicates that there was a methyl ketone present in the unknown. The methyl ketone was halogenated. This produces iodoform, which is a yellow-white precipitate, and a carboxylic acid. Because the yellow iodoform precipitate was produced, this indicates a positive result. Two derivatives were composed to determine the identity of unknown #350. The two derivatives were 2,4-dinitrophenylhydrazone and semicarbazone. When the unknown formed the 2,4-dinitrophenylhydrazone derivative, it was a light orange precipitate. The light orange color indicates that the carbonyl carbon in the ketone was composed of a carbon chain and not a carbon ring.4 After the derivative was obtained, the melting point was determined. The melting point was a range of 152.0°C to 154.0°C. This melting point range matches to the melting point of the chemical 3-pentanone. The melting point range for the pure 2,4-dinitrophenylhydrazone derivative of 3-pentanone is 156°C. Because the melting point range was lowered and broadened, this indicates that the derivative contained impurities.7 This impurities could have been sodium acetate, water, ethanol, or other impurities. When the unknown #350 formed the semicarbazone derivative, it was a white solid. The melting point for the derivative was a range of 138.0°C to 142.0 °C. This melting point range matches to the melting point of the chemical 3-pentanone. The melting point of the pure semicarbazone derivative of 3-pentanone is 138°C. The broadened melting point range indicates that impurities were present in the derivative.7 These impurities could have been ethanol, water, or other impurities. Unknown #350 was determined to be a methyl ketone. The 2,4dinitrophenylhydrazone derivative had a melting point range of 152.0°C to 154.0°C. The semicarbazone derivative had a melting point range of 138.0°C to 142.0°C. The

derivative melting points match to the compound 3-pentanone. The iodoform test does not match up with 3-pentanone, because it does not contain a methyl ketone. It will not give a positive result for the iodoform test. Another possibility for the unknown is 4-methyl-2-petanone. This chemical has a semicarbazone derivative melting point of 132°C, which matches to the data collected. The 2,4-dinitrophenylhydrazone derivative melting point (95°C) does not match with the data collected. This could be due to a number of possibilities such as impurities, or misuse of the melting point machine. Although the melting point doesn’t match, the structure does match. 4-methyl-2-pentanone is a methyl ketone that is connected to a carbon chain. This confirms with the iodoform test and the 2,4-dinitrophenylhydrazone derivative. Overall, the two possibilities for the unknown #350 are 3-pentanone and 4methyl-2-petanone. These chemicals are shown in figure 5.

Figure 5: Figure 5 shows the two possible chemical structures for unknown #350. It also shows the 2,4-dinitrophenylhydrazone derivative melting point and the semicarbazone melting point. Conclusion: The two chemical tests for aldehydes and ketones performed during this experiment were the Tollen’s reagent test and the iodoform test. Because the iodoform test was positive, this indicated that unknown #350 was a methyl ketone. This eliminated all of the aldehydes on the list of unknowns. Then, the 2,4dinitrophenylhydrazone derivative was made from the unknown. The precipitate formed was a light orange color, indicating that the methyl ketone contained a carbon chain and not a carbon ring. This eliminated all of the ketones containing rings from the list of unknowns. The melting point of this derivative and the semicarbazone derivative were determined, which narrowed down the list of unknowns to 3-pentanone and 4-methyl-2pentanone. If the unknown was 3-pentanone, an error could have occurred during the iodoform test, giving a false-positive. If the unknown was 4-methyl-2-pentanone, an error could have occurred in taking the 2,4-dinitrophenylhydrazone derivative melting point, thus giving the wrong melting point.

More tests could be done on the unknown, such as IR spectroscopy, NMR spectroscopy, and mass spectroscopy. These methods could give a better idea of what functional groups the unknown chemical contained, where the hydrogens and carbons were located, and which ways the chemical broke apart. Another way to improve this experiment, would be to dry the derivatives for a longer period of time to ensure that all impurities were removed. This could give a more accurate melting point, which would help identify the proper compound for the unknown.

References: 1Brown,

W. H.; Iverson, B. L.; Anslyn, E. V.; Foote, C. S. Organic Chemistry; Wadsworth Cengage Learning: Australia, 2014. (accessed Mar 1, 2017). 2Silver Method http://library.med.utah.edu/WebPath/HISTHTML/MANUALS/ FONTANA.PDF (accessed Mar 1, 2017). 3The Haloform Reaction http://pubs.acs.org/doi/abs/10.1021/cr60052a001 (accessed Mar 1, 2017). 4Fallow, M. W. 2,4-dintrophenylhydrozone http://www.orgsyn.org/demo.aspx? prep=cv2p0228 (accessed Mar 1, 2017). 5Ketones and Aldehydes http://crab.rutgers.edu/~alroche/Ch18.pdf (accessed Mar 1, 2017). 6The PubChem Project https://pubchem.ncbi.nlm.nih.gov/ (accessed Mar 1, 2017). 7Hill, R.; Barbaro, J. Experiments in Organic Chemistry, 3rd ed.; Contemporary Publishing Company of Raleigh: Raleigh, NC, 2005 8Impure Solids http://kirsoplabs.co.uk/lab-aids/impure-solids-melt-lower-temperatures/ (accessed Mar 1, 2017)....