CHY2024 - Experiment 3 - Reactions of Aliphatic and Aromatic Hydrocarbons PDF

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PRACTICAL # 3 DATE OF EXPT. ___02/02/2021________________ LAB GROUP Tuesday 11am-2pm NAME Jermaine Chambers I.D. 1904634

Reactions of Aliphatic and Aromatic Hydrocarbons OBJECTIVE To observe the properties of saturated, unsaturated and aromatic hydrocarbons. DISCUSSION Hydrocarbons are organic compounds made up entirely of carbon and hydrogen atoms. Their principal natural sources are coal, petroleum, and natural gas. Hydrocarbons are grouped into several series by similarity of molecular structure. Some of these are the alkanes, alkenes, alkynes, and aromatic hydrocarbons. Alkanes Also known as the paraffins or saturated hydrocarbons, the alkanes are straight or branched-chain hydrocarbons having only single bonds between carbon atoms. They are called saturated hydrocarbons because all their carbon-carbon bonds are single bonds. The first 10 members of the alkane series and their molecular formulae are listed below: Methane CH4 Ethane C2H6 Propane C3H8 Butane C4H10 Pentane C5H12

Hexane C6H14 Heptane C7H16 Octane C8H18 Nonane C9H20 Decane C10H22

Like most organic substances, the alkanes are combustible. The products of their complete combustion are carbon dioxide and water. The reactions of the alkanes are of the substitution type; that is, some atom or group of atoms is substituted for one or more of the hydrogen atoms in the hydrocarbon molecule. For example, in the bromination of methane, a bromine atom is substituted for a hydrogen atom. This reaction does not occur appreciably in the dark at room temperature but is catalyzed by ultraviolet light. The equation is

Ultraviolet CH4 + Br2

CH3Br + HBr Light

Methane

Methyl bromide (Bromoethane)

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Alkenes Also known as the olefins, the alkenes are a series of straight or branched-chain hydrocarbons containing a carbon-carbon double bond in their structures. They are unsaturated hydrocarbons. The first two members of the series are ethene (C2H4) and propene (C3H6). Their structural and condensed structural formulae are: H

H

C

C

H

H

H C H

H

C H

H2C

CH2

H

Ethene (ethylene)

C H

CH3CH=CH2 Propene (propylene)

The functional group of this series is the carbon-carbon double bond (C = C); it is a point of high reactivity. Alkenes undergo addition-type reactions; that is, other groups are added to the double bond, causing the molecule to become saturated. For example, when hydrogen is added, one H atom from H2 is added to each carbon atom of the double bond to saturate the molecule, forming an alkane: Ni Catalyst

CH3CH = CH2 + H2

CH3CH2CH4 Heat and Pressure

Propene

Propane

When a halogen such as bromine is added, one Br atom from Br2 is added to each carbon atom of the double bond to saturate the molecule: CH2 = CH2 + Br2  CH2CH2   Br Br 1,2-Dibromoethane (Ethylene dibromide) Evidence that bromine has reacted is the disappearance of the red-brown color of free bromine. Other reactions of olefins also show the increased reactivity of the alkenes over the alkanes. Unsaturated hydrocarbons can be oxidized by potassium permanganate. The reaction is known as the Baeyer test for Unsaturation. Evidence that reaction has occurred is the rapid disappearance (within a few seconds) of the purple color of the permanganate ion. The resulting reaction products will not be colorless. Potassium permanganate is a very strong oxidizing agent and gives similar results when it reacts with other oxidizable substances, such as alcohols. Alkynes Also called the acetylenes, the alkynes are another class of unsaturated hydrocarbons, but they contain a carbon-carbon triple bond in their structures. The first two members of this series are acetylene (ethyne) and propyne: H CC H Ethyne (Acetylene)

CH3C  CH Propyne 2

Acetylene is the most important member of this series and can be prepared from calcium carbide and water. The equation for this reaction is CaC2(s) + 2H2O(l)  CH  CH(g) + Ca(OH)2(aq) Mixtures of acetylene and air are explosive. The alkynes undergo addition type reactions similar to those of the alkenes. Aromatic Hydrocarbons The parent substance of this class of hydrocarbons is benzene (C6H6). From its formula benzene appears to be a highly unsaturated molecule; the corresponding six-carbon alkane contains 14 hydrogen atoms per molecule (C6H14). However, the chemical reactions of benzene show that its behavior is like that of the saturated hydrocarbons in many respects. Their reactions are primarily of the substitution type. In the past, benzene was used extensively in student laboratories to illustrate the properties of aromatic hydrocarbons. Within the last decade, studies have shown benzene to be a cancer-causing substance and it is being eliminated from many experiments. We will use toluene in this experiment as an alternative to benzene. The carbon atoms in a molecule are arranged in a six-membered ring structure, with one hydrogen atom bonded to each carbon atom. The following diagrams represent the benzene molecule; in the second and third structures it is understood that a carbon and a hydrogen atom are present at each corner of the hexagon or benzene ring. H C

H

H

C

C

C

C

CH3

H

C

H

H

Benzene (C6H6)

Toluene (C6H5CH3)

MATERIALS AND EQUIPMENT Droppers, test tubes, rubber bands, test tube racks, measuring cylinders, stoppers, labelling paper, black sugar papaer. Liquids: cyclohexene (C6H10), cyclohexane (C6H12), and toluene (C6H5CH3). Solutions: 4% bromine (Br2) in dichloromethane and 0.01 M potassium permanganate (KMnO4).

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PROCEDURE In the following reactions, cyclohexane will be used to represent the saturated hydrocarbons; cyclohexene, the unsaturated hydrocarbons and toluene the aromatic hydrocarbon. A.

Combustion

Obtain about 1 mL (no more) of cyclohexane in an evaporating dish and start it burning by carefully bringing a lighted match or splint to it. Repeat with an equally small volume of cyclohexene. Note the characteristics of the flames. B.

Reaction with Bromine in dichloromethane

Take six clean dry test tubes and label A-F. Place about 1 mL of cyclohexane in test tubes A and B, 1 mL cyclohexene in test tubes C and D and 1 mL into test tubes E and F. Wrap test tubes A, C and E with black sugar paper. Add 4-5 drops bromine in dichloromethane to each test tube. Place test tubes A, C and E in the dark for 15 minutes. Allow test tubes B, D and F to be exposed to sunlight for 15 minutes. Note all observations after 15 minutes.

C.

Reaction with Potassium Permanganate (Baeyer Test)

The Baeyer test for Unsaturation in hydrocarbons involves the reaction of hydrocarbons with potassium permanganate solution. Evidence that reaction has occurred is the rapid disappearance (within a few seconds) of the purple color of the permanganate ion. Potassium permanganate is a very strong oxidizing agent and gives similar results when reacted with other oxidizable substances, such as alcohols. Label 3 test tubes G, H and I. Add 1 mL cyclohexane into test tube labelled G, 1 mL cyclohexene in the test tube labelled H and 1 mL toluene into test tube I. Add a few drops potassium permanganate solution into each test tube and shake. Note all observations.

Results A. 1.

COMBUSTION Describe the combustion characteristics of cyclohexane and cyclohexene. [2 Marks] Both hydrocarbons produce carbon dioxide and water during combustion test. When the oxygen is limited, the product will be carbon monoxide and water. Cyclohexene burns and produces more soot because of the higher percentage of carbon compared to cyclohexane. Cyclohexane also produces a larger flame that lasts for longer when lit when compared cyclohexene.

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2.

(a) Write a balance equation to represent the complete combustion of i.

Cyclohexane

[2

Marks] C6H12+ 9O2

ii.

6CO2 + 6H2O + energy

Cyclohexene

[2

Marks] C6H10(l) + 8.5O2(g)

B. and C.

6CO2 (g) + 5H2O(l) + energy

Reaction with Bromine and Potassium Permanganate

Date Table: Describe your observations in the table below. Test

[9 Marks]

Cyclohexane

Cyclohexene

Toluene

(Saturated

(Unsaturated

(Aromatic

Reaction with Br2 in the

Hydrocarbon) Reddish brown

Hydrocarbon) Reddish brown

hydrocarbon) Reddish brown colour

dark

colour remains

colour turns

remains unchanged

unchanged

colourless

Reaction with Br2 when

Reddish brown

Reddish brown

Reddish brown colour

exposed to sunlight

colour turns

colour turns

turns colourless

colourless

colourless

Purple colour of

Purple colour of

Purple colour of

KMnO4 remains

KMnO4 turns

KMnO4 remains

Reaction with KMnO4

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unchanged

colourless and

unchanged

brown precipitate formed.

3.

Which of the two hydrocarbons reacted with bromine in the dark?

[1 Mark]

Cyclohexene 4.

Write an equation (structural formula) to illustrate how cyclohexane reacts with bromine when the reaction mixture is exposed to sunlight.

[2 Marks]

5. Of the compounds used in this experiment (cyclohexane, cyclohexene, toluene) which was the most reactive with bromine solution? Explain your conclusion. Cyclohexene, because it is an alkene and alkenes contain a double bond it makes them very reactive. This explains why the cyclohexene readily reacted with the bromine rather than the cyclohexane. Even though aromatic hydrocarbons may appear like unsaturated hydrocarbons their structures and properties are generally different, so they are not considered alkenes. So, cyclohexane and toluene will not readily react with bromine but they are catalyzed by UV light or sunlight. [2 Marks]

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6. On the basis of the chemical tests used in this experiment, is it possible to clearly distinguish between an alkane and an alkene? Explain your conclusion. Yes, the presence of double bonds in alkenes makes them more reactive than alkanes. The higher reactivity of the alkenes over alkanes allows us to distinguish them. This explains why the cyclohexene rapidly reacted with the bromine to turn from brown to colorless and the cyclohexane underwent no reaction with bromine in the dark and therefore there were no color change. So, in conclusion an alkene will turn bromine colorless and an alkane will not react with bromine unless in sunlight.

[4 Marks]

7. Which of the three hydrocarbons tested gave a positive Baeyer test?

[1 Mark]

Cyclohexene

[25 Marks]

POST-LAB QUESTIONS 1. Write the structure of the major organic product for the following reactions: [3 x 1 Mark] a.

CH3-CH=CH2 + HBr 

b.

+ Br2

c.

→

CH3-CH=CH-CH3 + KMnO4 + H2O 

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2.

Write an equation (structural formula) to illustrate how 2-pentene reacts with bromine.

[2 Marks]

CH2=CH-CH2-CH2-CH3+Br

CH2Br-CHBr-CH2-CH3

3. Using curved arrows, outline the mechanism for the following giving the name of the reaction and the reaction mechanism.

i.

Cyclohexane and bromine

[6 + 2 Marks]

No reaction is shown until the mixture is not treated with ultraviolet radiation, the reaction would result into bromo-cyclohexane and HBr. The reaction proceeds through free radical mechanism.

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ii.

Cyclohexene and bromine

[4 + 2 Marks]

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The name of the reaction is Free radicle substitution

4. What do you think would be observed if hexene was reacted with bromine and potassium permanganate?

[2 Marks]

Because hexene is an alkene I think it would react with bromine and a color change would take place from red brown to colorless and it would change from purple to yellow brown

5. Write the reaction for toluene with bromine in the presence of sunlight. Give the name of the reaction and the reaction mechanisms.

[4 Marks]

The name of the radical substitution.

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[25 Marks]

Total = 50 Marks

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