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Qualitative Test for Alkenes and Identifying an Unknown Compound ATR

Qualitative Test for Alkenes and Identifying an Unknown Compound ATR

Chemistry 262 

Aim

The experiment aimed to determine whether the compounds were either alkanes or alkenes using the potassium permanganate and pyridinium bromide perbromide.

Introduction

Compounds composed of hydrogen and carbon atom, such as alkanes, alkenes, and aromatics, are said to be hydrocarbons. Alkanes are saturated compounds that only have single bonds (C-C) and cannot further react with hydrogen. Alkenes are unsaturated compounds that contain at least one double bond (C=C) and can react with hydrogen under the right conditions. Alkynes are also an unsaturated compound that has one triple bond (C≡C). Lastly, there are aromatic compounds that are unique unsaturated carbon double bonds and have a ring structure. To be able to distinguish between alkenes and alkynes from aromatic compounds and saturated alkanes, simple reactions tests are conducted. Two standard qualitative tests for unsaturation in hydrocarbons are discoloration of aqueous bromine (Br2) solution and aqueous potassium permanganate (KMnO4).

The discoloration of an aqueous bromine test was used to find any presence of alkenes or alkynes in each compound. However, an ethanolic solution of pyridinium bromide perbromide (PBP) was used instead of aqueous bromine since this solution is hazardous. The PBP test contains bromines that react only with alkenes and alkynes but cannot react with an aromatic compound. The solution test displays an orange-brown color when the compound is unsaturated. This color, however, disappears, turning the solution colorless due to the addition reaction and addition of bromine to each carbon making double and triple saturated bonds.

The Potassium permanganate, or KMnO4, test it is performed to find the presence of alkenes in each compound. During this test, the unsaturated compound is reduced by the potassium permanganate ion forming a precipitate of brown manganese oxide. This reaction is an oxidation-reduction reaction because the MnO4 oxidizes the alkene. An -OH group is added to each carbon of the double alkene bond, where it becomes saturated. If the compound turns purple, then the test is negative, and if the compound turns into a brown color, then the test is positive for the presence of alkenes (Organic Chemistry Laboratory).

Physical Properties and Hazards

Table 1 Physical Properties and Hazards of Compounds

Compound

Name

Molecular

Structure

Molecular

Formula

Molecular Weight g/mol

B.P/

M.P

(°C)

Density

(g/cm3)

Physical state/

appearance

Hazard and

toxicity

Potassium permanganate

KMnO4

158.034

B.P:

N/A

M.P:

240.0

2.70

Odorless dark purple crystalline solid

May cause severe skin burns and eye damage, may

intensify fires, and harmful if

swallowed.

Pyridinium bromide

perbromide

C5H6Br3N

319.822

B.P: 115.3 M.P:

133.0

1.26

Red to brown needle crystals

May cause severe skin burns and eye damage and can cause respiratory

irritation.

Cyclohexane

C6H12

84.16

B.P:

80.74

M.P:  

6.47

0.799

Odorless, colorless liquid

Highly flammable, can cause skin irritation, drowsiness or dizziness, and be fatal if

swallowed.

Cyclohexene

C6H10

82.143

B.P:

82.0

M.P:  

-103.5

0.811

Colorless liquid with sharp odor

Highly flammable, harmful if swallowed and

toxic if in contact to skin.

Ethyl acetate

C4H8O2

88.106

B.P:   

-83.6

M.P:  

77.1

0.897

Colorless liquid with fruity odor

Highly flammable, can cause eye irritation, and may cause drowsiness or dizziness.

Acetone

C3H6O

58.08

B.P:  

56.0

M.P:

-95.0

0.791

Colorless liquid

Highly flammable, can cause eye irritation, and may cause drowsiness or dizziness.

1-octene

C8H16

112.24

B.P: 121    M.P:  

-101.7

0.715

Colorless liquid

Highly flammable and can cause skin irritation.

Table 1: A display of the compounds handled during the experiment with the corresponding molecular formula, molecular weight, density, melting and boiling point, physical appearance and hazards.

Procedure and Observations

The Pyridinium bromide perbromide test was done by labeling nine tubes: cyclohexane, cyclohexene, ethyl acetate, mineral oil, kerosene, 1-octene, unknown one, unknown two, and unknown three.  First two drops of each sample were placed into each correctly labeled test tube. When each sample was introduced to the tube two drops of PBP were placed into each test tube and then gently shaken. After putting the two drops of PBP, each tube was observed to see if there was a color change or no color change. If there was no color change no additional drops were added and the test was negative. If there was a color change more additional drops were added up to drops. If the sample remained colorless then the test was positive. 

The Potassium permanganate test was done by labeling 10 test tubes: control, cyclohexane, cyclohexene, ethyl acetate, mineral oil, kerosene, 1-octene, unknown one, unknown two, and unknown three. The control was prepared by pouring 1mL of acetone into the test tube and three drops of 2% KMnO4 solution. The other nine tubes were prepared by the addition 1mL to each test tube along with two drops of their respective sample. Then, starting with two drops, KMnO4 solution was added and gently tapped. If the sample remained a purple color even after the addition of all 20 drops, then the test was negative. If the purple color looked faded compared to the control, additional drops of KMnO4 were added and the tube was gently tapped. If the purple color remained faded and turns into a brownish color with precipitate, then the test was positive. 

Results  

Table 2: Reaction with 2% Ethanolic PBP

Name of compound  

# of PBP drops added  

Observations

Conclusion (+ or – test)

Cyclohexane

2

Yellow

-

Cyclohexene

12

Colorless

+

Kerosene

2

Yellow

-

1-octene

12

Colorless

+

Ethyl acetate

2

Yellow

-

Mineral oil

2

Yellow

-

Unknown 1

2

yellow

-

Unknown 2

12

Colorless

+

Unknown 3

12

Colorless

+

Table 2 shows the PBP test result after the addition of the drops. The (+) sign represent the positive test for alkenes and a (-) sign represent the negative test for alkenes.

Table 3: Reaction with 2% Aqueous KMnO4 Solution

Name of compound

# of 2% KMnO4 drops

added

Observations

Conclusion

(+ or – test)

cyclohexane

20

Purple

-

cyclohexene

2

Brownish clear, precipitate

+

kerosene

11

Purple

-

1-octene

16

Brownish clear, precipitate

+

Ethyl acetate

16

Purple

-

Mineral oil

2

Purple

-

Unknown 1

2

Purple

-

Unknown 2

20

Brownish clear, precipitate

+

Unknown 3

20

Brownish clear, precipitate

+

Table 3 shows the KMnO4 test result after the addition of the drops. The (+) sign represent the positive test for alkenes and a (-) sign represent the negative test for alkenes.

Discussion and Conclusion

Based on the results in Table 2 as well as Table 3, four of the samples in the PBP test were positive, meaning the samples contained double or triple bonds. The samples that were colorless after the test was completed result in a positive test, which indicates that bromine atoms attached to the unsaturated carbons bonds. For the potassium permanganate test, once KMnO4 changes color to brownish with precipitate, it shows a positive test with the presence of alkenes and attachment of the -OH group in each unsaturated carbon bond. The cyclohexane was negative due to only containing saturated carbon bonds. Cyclohexene resulted in positive due to the double bond in the ring. Since Kerosene is a mixture of saturated hydrocarbons, the result was negative, as well as mineral oil. For 1-octene, the result was positive due to the double bond it contained. Although ethyl acetate has double bonds, it has it between carbon and oxygen; therefore, the result came out as negative. The first unknow was toluene, which came out negative, the second unknown was 1-octene, which resulted in positive, and the third unknown was 3-hexyne, which resulted in positive.

IR experiment 

Instrument Details, Preparation of the sample, and procedure for recording the IR

The spectrometer used was called an attenuated total reflection or ATR; it works by measuring the IR beam that is internally reflected when the beam comes into contact with a sample. To get a good result, there must be good contact between the sample and the crystal surface. I

Two different Infrared Spectroscopy machine was used to experiment, one was used only for the solid example, and the other one could be used for both. An unknown liquid, IR-3, and an unknown solid, IR-4, were chosen. Kim-wipes, along with acetone, was used to clean the IR plate, and one drop of the unknown liquid was placed on the IR plate. The IR machine began reading the compound. The IR plate was cleaned with acetone and Kim-wipes, and with a small spatula, the unknown solid was placed on the IR plate. A Kim-wipe was ripped in half and folded for times. Then the folded wipe was placed above it while holding it in place with two fingers, and the pressure arm was placed above the sample area. The pressure arm locked into a precise position above the crystal area where force is applied to the sample pushed onto the diamond surface. Once the unknown solid sample was placed, the IR machine began reading the compound.

Before inserting the sample holder in the reservoir, “Col Bkg” was clicked on the top lefthand corner. A window popped up saying “please prepare to collect the background spectrum,” and “OK” was pressed. The computer makes some noise, and a specific spectrum was created, which resembled the outline of a mountain. Once the spectrum was completed, a new window popped, asking to add to window1 where “No” was clicked. Then the sample was loaded in the reservoir, and “Col Smp” on the upper left-hand corner was clicked. A window “confirmation” popped, saying that the sample spectrum can now be prepared, “OK” was clicked to generate the IR spectrum. Then “process” was click to find the percent transmittance and then “find peaks” were pressed and dragged until it was appropriate. After that the spectrum were printed. This was carried out twice, for the liquid and solid sample.

Evaluation of the unknown spectrum

Table 4 Functional Groups and Absorption Values for Liquid IR-3

Functional Group

Wavenumbers (cm-1)

Alkene (C-H, sp2)

1604.07 cm-1

Aromatic (C=C)

1589.49 cm-1

Hydroxyl Group (-OH)

3339.60 cm-1

Table 4 A display of the wavenumber values for characteristic absorptions and the functional group or groups that could produce them.

Table 5 Functional Groups and Absorption Values for Solid IR-4

Functional Group

Wavenumbers (cm-1)

Alkene (=C-H, sp2)

3058.8 cm-1 - 3025.7 cm-1

Carbonyl group (-C=O)

1701.1 cm-1

Alkane (-C-H)

2865.4 cm-1

Alkene aromatic (-C=C-) 

1664.1 cm-1- 1625.3 cm-1 - 1597.8 cm-1- 1577.2 cm-1

Table 5 A display of the wavenumber values for characteristic absorptions and the functional group or groups that could produce them.

Identification of the unknown

Liquid sample (IR-4): C9H10O Degrees of unsaturation:

Identification: It contains 1 ring and 4 double bonds.

Solid sample (IR-3): C6H5BrO Degrees of unsaturation: 

Identification: It contains 1 ring and 3 double bonds  References

“Acetone.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine.

“Cyclohexane.” National Center for Biotechnology Information. PubChem Compound

Database, U.S. National Library of Medicine.  

“Cyclohexene.” National Center for Biotechnology Information. PubChem Compound

Database, U.S. National Library of Medicine.

“Ethyl Acetate.” National Center for Biotechnology Information. PubChem

Compound Database, U.S. National Library of Medicine.

“Potassium Permanganate.” National Center for Biotechnology Information. PubChem

Compound Database, U.S. National Library of Medicine.

“Pyridinium bromide perbromide.” National Center for Biotechnology Information.

PubChem Compound Database, U.S. National Library of Medicine.

“Signature Labs Series.” Organic Chemistry Laboratory. CHEM 26200 Experiments. Cengage

Learning.

“1-octene.” National Center for Biotechnology Information. PubChem Compound Database, U.S. National Library of Medicine.

“39416-48-3.” 39416-48-3| C5H6Br3N-2 Pyridinium Bromide Perbromide Boiling, Melting, Flash Point| Density| Formula| Synthesis Route, https://www.guidechem.com/dictionary/en/39416-48-3.html.  

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