Properties of hydrocarbon

Title: Experiment 12, Properties of hydrocarbons. Objective: To determine the properties of hydrocarbons. Introduction:

Hydrocarbons are organic compounds that consist of only C and H atoms.

They include the alkanes, alkenes, alkynes and the aromatic hydrocarbons. Almost

all useable supplies of hydrocarbons are obtained from fossil fuels. These compounds are classified as saturated or unsaturated hydrocarbons. This classification only applies to the aliphatic compounds (alkanes, alkenes and alkenes), that is the straight or branch chain hydrocarbons depending on the number of bonds in between the carbon atom which is

the basis of hydrocarbons structural differences. Since alkanes have only single bonds, thus it is said to be saturated, alkenes and alkynes which have carbon-carbon double or triple bonds are said to be unsaturated. For aromatic hydrocarbons, they are cyclic compounds whose structure is related to that of benzene, with 6 electrons in a six-membered ring. In this experiment, hexane will be used as an example of saturated hydrocarbons (alkanes),cyclohexene will be used as an unsaturated hydrocarbon (alkenes) and toluene as an aromatic hydrocarbon. i.

Combustion

Hydrocarbons are currently the main source of the world’s electric energy and heat sources (such as home heating) because of the energy produced when burnt. Often this energy is used directly as heat such as in home heaters, which use either petroleum or natural gas.

Common properties of hydrocarbons are the facts that they produce steam, carbon dioxide and heat during combustion and that oxygen is required for combustion to take place. Typical combustion reaction: 2 C4H10 + 13 O2 --> 8 CO2 + 10 H2O + heat energy ii.

Reaction with bromine

Bromine reacts readily with alkenes at room temperature in the absence of light to form the corresponding dibromide compound. This reaction can be followed by the disappearance of the brown bromine colour from the reaction medium, as long as the alkene is in excess.

Alkanes and toluene on the other hand can show reaction in the presence of sunlight only, which will produce hydrogen bromide (HBr) as a side product when react with bromine.

The presence of HBr can be tested for by placing a piece of moist blue litmus paper above the reaction vessel. Evolution of HBr will turn the litmus paper red, which indicate the HBr is acidic. This can be used as an additional test for alkanes. Aromatic hydrocarbons will not react with bromine either in the dark, or in the sunlight. iii.

Reaction with potassium permanganate

Potassium permanganate reacts readily with alkenes to form glycols or other oxidation products. This is referred to as Baeyer’s test for alkenes. Neither alkanes nor aromatic hydrocarbons show reaction with potassium permanganate. The reaction can be followed by

the disappearance of the purple colour of the KMnO4. The presence of clear or brown colour indicate the presence of manganese oxide, MnO2.

Apparatus and Materials: Bunsen burner, Evaporating dish, test tubes, test tube rack, hexane (C6H10), cyclohexene (C6H10), toluene (C7H8), 5% bromine (Br2 dissolved in either trichloroethane or in methylene chloride; either solvent is fine), 0.5% potassium permanganate (KMNO4) dissolved in water 98.5% H2SO4.

Results: i.

Combustion:

a. Hexane: Burn in orange flame with little amount of smoke and soot. b. Cyclohexene: Burn vigorously in orange flames with large amount of smoke and soot.

c. Toluene: Burn very vigorously in orange flames with large amount of smoke and soot.

d. Unknown compound A: Burn in orange flames with little amount of smoke and soot. e. Unknown compound B: Burn vigorously in orange flames with large amount of smoke and soot. ii.

Reaction with bromine: Compounds

Observations In the dark

Exposed to sunlight The mixture turns to pale yellow

Hexane

The mixture turns to pale yellow in colour after adding bromine.

in colour after adding bromine

and turns back to colourless after exposed to sunlight.

Cyclohexene

Pale yellow colour of bromine

water becomes colourless once added into cyclohexene solution

Toluene

Unknown A

Two layers of mixtures are

Two layers of mixtures are

yellow, lower layer is colourless.

white, lower layer is colourless.

formed. Upper layer is cloudy

The mixture turns to pale yellow in colour after adding bromine.

formed. Upper layer is cloudy The mixture turns to pale yellow in colour after adding bromine

and turns back to colourless after exposed to sunlight.

Unknown B

Pale yellow colour of bromine

water becomes colourless once added into unknown B solution

iii.

Reaction with potassium permanganate: a. Hexane: Two layers of mixtures are formed with upper layer is colourless while lower layer is dark purple colour.

b. Cyclohexene: Two layers of mixture are formed with upper layer is colourless while lower layer is brown colour with brown precipitate.

c. Toluene: Two layers of mixtures are formed with upper layer is colourless while lower layer is dark purple colour.

d. Unknown compound A: Two layers of mixtures are formed with upper layer is colourless while lower layer is dark purple colour. e. Unknown compound B: Two layers of mixture are formed with upper layer is colourless while lower layer is brown colour with brown precipitate.

Discussion:

For the part of combustion, we know that all hydrocarbons undergo combustion with

producing carbon dioxide, water and heat energy in complete combustion. Equation:

a. Hexane: 2C6H14 + 19O2

12CO2 + 14H2O

b. Cyclohexene: 2C6H10 + 17O2 c. Toluene: C7H8 + 9O2

12CO2 + 10H2O

7CO2 + 4H2O

From the observation, alkenes and aromatic hydrocarbon produced more soot and smoke than alkanes when undergo combustion. Soot and smoke that produced was carbon monoxide and even carbon, which produced when there is insufficient oxygen for those hydrocarbon undergo complete combustion. Incomplete combustion is more likely to occur in aromatic than alkenes than alkanes. The hexane, C6H14 contain relatively less carbon atoms per all atoms present in a molecule, so there is a more complete combustion and little soot is observed. The

cyclohexene, C6H10 contain more carbon content per all atoms present in one molecule than C6H14 does. So soot and smoke formed which indicate incomplete combustion. The toluene

C7H8 contains more carbon content per all atoms present in one molecule than C6H10 does, so more soot is observed, indicating that there is much incomplete combustion. The following

pictures show the sequence in increasing order of the time taken required for the completion of combustion reaction of hydrocarbon. Accurate sequence is hexane, unknown A, cyclohexene, unknown B and toluene.

For the part of reaction with bromine, alkanes react with bromine only when it is exposed to sunlight. It undergoes free radical substitution by replacing one of the hydrogen in hydrocarbon with one bromine atom. Sunlight will act as catalyst to break the bond, which called as photochemical reaction. This is because alkanes contain sigma-bonds, which is stronger bond and thus need sunlight to act as catalyst to break the bond. The mechanism is shown below

For alkenes, from the observations we noted, alkenes react readily with bromine although did not exposed to sunlight. This is because it undergo halogenations by addition reaction due to it contain pi-bonds, which can be broken easily result from their imperfect

overlap. So, no sunlight is needed to act as catalyst to break the pi-bonds. Cyclohexene reacts with bromine to form 1,2-dibromocyclohexane as a product. The mechanism is shown below.

For toluene, it is also undergo free radical substitution, which means react with bromine with the presence of sunlight but, the reaction happens to the methyl group instead of the

benzene ring. The organic product formed is (bromomethyl)benzene. The mechanism is shown below. Step 1 (Initiation)

Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.

Step 2 (Propagation)

(a) A bromine radical abstracts a hydrogen to form HBr and a benzyl radical, then (b) The benzyl radical abstracts a bromine atom from another molecule of Br2 to form the benzyl bromide product and another bromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat.

Step 3 (Termination) Various reactions between the possible pairs of radicals allow for the formation of Br2 or the product, benzyl bromide. These reactions remove radicals and do not perpetuate the cycle.

Unknown compound A showed same observation with hexane while Unknown compound B showed same observation with cyclohexene.

Hexane (Before After)

Cyclohexene (Before After) Toluene (Before After)

Unknown A (Before After) Unknown B (Before After) For reaction with potassium permanganate, alkenes is the only one that showed positive result, thus it is called Baeyer permanganate test. The appearance of brown colour indicate the presence of manganese dioxide, MnO2. This is because acidified potassium permanganate that act as oxidizing agent is reduced into brown precipitate which is manganese dioxide and cyclohexene is oxidized to diol.

Toluene can produce carboxylic acid from the reaction of potassium permanganate but it is necessary to heat the mixture first. If not, no reaction will occur. Unknown A liquid got the same observation as hexane and unknown B liquid got the same as cyclohexene.

Hydrocarbons compounds before adding with acidified KMNO4

Hydrocarbon compounds after adding with acidified KMNO4 Conclusion:

For combustion reaction, all compounds produced smoke, soot and orange flames. However, hexane and compound A produce less soot compare to the other three compounds. Cyclohexene and compound B produced more soot than the others three compound due to

more carbon content per atoms present. For the reaction with bromine, reaction is only occur in the presence of UV light for alkanes and aromatic compounds by free radical substitution reaction but cyclohexene react readily with bromine by addition reaction. For the reaction with

acidified potassium permanganate, hexane and toluene has no reaction but cyclohexene react with potassium permanganate with leaving brown colour solution that indicate the presence of

manganese dioxide, MnO2. From the experiment, we can conclude that unknown compound A is saturated hydrocarbon while unknown compound B is unsaturated hydrocarbon.