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PLANNING AND DESIGNING LAB

Title

Alkanes and Alkenes

Hypothesis

Alkenes are much more reactive than alkanes.

Aim

The purpose of carrying out this experiment is to identify Alkanes and Alkenes by their reactions

with different substances and determine their properties.

Introduction

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and

carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups,

called hydrocarbyls. Aromatic hydrocarbons (arenes), alkanes, alkenes, alcohol, esters and

alkyne-based compounds are different types of hydrocarbons. The majority of hydrocarbons are

found naturally occurring in crude oil, where decomposed organic matter provides an abundance

of carbon and hydrogen which, when bonded, can catenate to form seemingly limitless chains.

Hydrocarbons are divided into two classes known as aliphatic compounds and aromatic

compounds. Aliphatic compounds are namely alkanes and alkenes.

In this experiment, the hydrocarbons that are being used to identify and determine their

properties are cycloalkane and cycloalkene which are from group alkane and alkene

respectively. Alkanes are the simplest family of hydrocarbons compounds that contain carbon-

hydrogen bonds and carbon-carbon single bonds. The carbon-hydrogen bonds are only very

slightly polar and so there are no parts of the molecules which carry any significant amount of

positive or negative charge which other things might be attracted to. The net effect is that alkanes

have a fairly restricted set of reactions. Thus, alkanes can only undergo combustion,

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halogenations and cracking process. Alkanes are not very reactive and have little biological

activity. The formula of alkane is CnH2n+1 in which n started from 1,2,3,4 and so on. For example,

the first three are:

Methane, CH4 where n = 1; Ethane, C2H6 where n = 2; Propane, C3H8 where n = 3

Alkanes with more than three carbon atoms can be arranged in numerous ways, forming different

structural isomers. An isomer, in part, similar to a chemical anagram but unlike an anagram, may

contain varying number of atoms and components, for which in a chemical compound can be

structurally arranged in a multitude of different combinations and permutations. The simplest

isomer of an alkane is the carbon atoms are arranged in a single chain with no branches.

However the chain of carbon atoms may also be branched at one or more points. The number of

possible isomers increases rapidly with the number of carbon atoms.

Alkenes are unsaturated hydrocarbons containing a carbon-carbon double bond. Alkenes are

relatively stable compounds but are more reactive than alkanes due to the presence of a carbon-

carbon double bond. The majority of the reactions of alkenes involve the rupture of this carbon-

carbon double bond, forming new single bonds. Thus, the main reaction of alkene is addition

such as hydrogenation, halogenation and oxidation. The formula of alkene is CnH2n where n starts

from 2,3,4,5 and so on. Thus, the first member of alkene family is ethene with a formula of C2H4.

As predicted by the VSEPR model of electron pair repulsion, the molecular geometry of alkenes

includes bond angles about each carbon in a double bond of about 120°. The angle may vary

because of steric strain introduced by non-bonded interactions created by functional groups

attached to the carbons of the double bond.

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 Materials

10 Test tubes

Test tube rack 

100 ml beaker

Test tube holder (clamp)

Alkane

Alkene

Concentrated sulphuric acid solution, H2SO4

2% Bromine in tetrachloromethane

0.50% potassium permanganate solution, KMnO4

10% sodium hydroxide solution, NaOH

10% sodium carbonate solution, Na2CO3

Distilled water

Blue litmus paper

 Procedure

A. Bromine Test

In a clean test tube, 1 ml of the alkane is added to 3 ml of 2 % bromine in tetrachloromethane

(Br2 /CCl4). The test tube is shaken well and is observed after two to three minutes.

A second test tube is prepared similarly. The first test tube is placed in a laboratory locker and

the second is placed in bright sunlight.

Both of the test tubes are allowed to stand for 10-15 minutes and both were then compared.

The color of the solution in each test tube is observed.

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A blue litmus paper is placed into the test tube to test whether or not hydrogen bromide was

evolved. The results are recorded.

Steps 1-5 are repeated with alkene, with just one test tube of sample. (There is no need for

sunlight reaction.)

B. Aqueous Potassium Permanganate (Baeyer’s Test)

In a clean test tube, 1 ml of alkane is added to a mixture of 3 ml of dilute potassium

permanganate solution (0.5% KMnO4 solution) and 3 ml of dilute sodium carbonate

solution(10% Na2CO3 solution).

The tube is shaken for 1-2 minutes and the results are noted.

Steps 1-2 are repeated with alkene and the results are noted.

C. Sulphuric Acid Test

1. In a clean test tube, 1 ml of alkane is added and with gently shaking, 3 ml of concentrated

sulphuric acid is added very cautiously.

2. The tubes are shaken well and the results are noted.

3. It is observed whether heat evolved and whether the hydrocarbon dissolves.

4. The contents are discarded by pouring them into a beaker containing at least 50 ml of water.

5. Steps 1-4 are repeated with alkene and the results are noted.

D. Sodium Hydroxide Test

1. In a clean test tube, 1 ml alkane is added to 3 ml of dilute sodium hydroxide solution (10%

NaOH solution) and is shaken well.

2. The changes are observed and the results are noted.

3. Steps 1-2 are repeated with alkene and the results are noted.

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Results Table

Reagent Used Observation

Alkane Alkene

2% Br2 in CCl4 (in the dark)

2% Br2 in CCl4 (in sunlight)

0.50% KMnO4 + 10% Na2CO3

Concentrated H2SO4

10% NaOH

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Expected Results

Reagent Used Observation

Alkane Alkene

2% Br2 in CCl4 (in the dark) There is no reaction. The blue

litmus paper remains its color.

The yellowish brown solution

turns cloudy. The blue litmus

paper turns pink.

2% Br2 in CCl4 (in sunlight) The yellowish brown solution

turns colorless. The blue litmus

paper turns pink

0.50% KMnO4 + 10% Na2CO3 The solution contains double

layers and remains purple in

color.

The solution contains double

layers. The purple solution

turns brown color and there are

brown precipitates present.

Concentrated H2SO4 The solution is clear. It has

double layers. There is no heat

evolved.

The solution contains layers.

The solution turns cloudy and

there is heat released.

10% NaOH The solution remains colorless

with double layers.

The colorless solution turns

yellow with double layers.

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Discussions

In this experiment, hopefully all of the tests conducted will be successful. The hydrocarbons that

are used will be identified and determined by their properties. In this experiment, two different

hydrocarbons are used which are alkane and alkene. If however it is assumed that the alkane is

Cyclohexane which has chemical equation of C6H12 and is produced by reaction of benzene and

hydrogen. It has chemical structure as shown below:

Meanwhile, the alkene is cyclohexene and is produced by partial hydrogenation of benzene.

Cyclohexene has chemical formula C6H10 and its chemical structure is as shown below:

The following discussion could be made for the expected results of the reactions of these

hydrocarbons:

The first test conducted in this experiment is bromine test. The test is been done by adding 1 ml

cyclohexane to 3 ml of 2 % bromine in tetrachloromethane in a clean test tube. It is done again in

another new clean test tube. One of the test tubes is then placed under the sunlight while the

other test tube is placed in the dark in a laboratory locker. The process that will take place is

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called halogenation, a substitution reaction which needs the presence of ultraviolet light or

sunlight as it is a photochemical reaction that will produce bromocylohexane and hydrogen

bromide. Thus, there will be no reaction in the test tube placed in the dark. The blue litmus paper

does not change in color when put into the test tube placed in the dark. However, there are

reactions in test tube placed under the sunlight. The yellowish brown solution in the test tube

turns colorless as the hydrogen atom will be replaced with bromine atom and produced

bromocyclohexane and hydrogen bromide. The blue litmus paper turns pink as hydrogen

bromide is acidic.

The experiment is then repeated by replacing cyclohexane with cyclohexene. However, for

cyclohexene there is no reaction in the dark as it is much more reactive and undergoes addition

reaction. The double bond of the alkene breaks down becomes single bonds which bromine

atoms bonded to each of the carbon atoms that shared the double bond. Thus, the yellowish

brown solution turns cloudy as it forms dibromocyclohexane and the blue litmus paper turns pink

as it is acidic. After conducting this test, the mixture solutions then are discarded in a

proper waste container.

The second test conducted is Baeyer’s Test which is aqueous potassium permanganate test. This

test is been done by adding 1 ml of cyclohexane to a mixture of 3 ml of dilute potassium

permanganate solution and 3 ml of dilute sodium carbonate solution in a clean test tube. The test

tube is then shaken for 1 to 2 minutes. There is no reaction in the solution as it remains purple in

color except that a double layer is formed on the surface of the solution. The double layer formed

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is due to the oil present in the benzene ring of the cyclohexane. The test is then repeated by using

cyclohexene instead of cyclohexane. The reaction that takes places is oxidation which causes the

purple solution to turn brown color. There is also brown precipitate formed and the surface of the

solution has double layers. The purple solution changes color to brown because potassium

permanganate is reduced to manganese dioxide, which is the brown precipitate. The double

layers exist due to the same reason as in the cyclohexane.

The test that is conducted after that is the sulphuric acid test. 1 ml of cyclohexane is added into a

clean test tube and with gently shaking, 3 ml of concentrated sulphuric acid is added cautiously.

The tubes are then shaken for a while. Then, quickly the test tube is placed on the palm of my

hand to test whether heat evolved. Alkanes are not reactive as this reaction is an addition

reaction. Hence the solution remains colorless but there is a double layer that formed on the

surface of the solution. For cyclohexene, the solution turns cloudy and there is heat released.

Cyclohexene reacts with concentrated sulphuric acid to produce alkyl hydrogen sulphates. There

is heat released due to the breaking down of the carbon-carbon double bond in cyclohexene

which releases energy that produces a net evolution of heat energy. The mixture of solution is

then discarded into a beaker containing with at least 50 ml of distilled water to dilute the

concentrated sulphuric acid in the mixture as it is too acidic before pouring it into the sink.

The last test is sodium hydroxide test that is conducted by adding 1 ml of cyclohexane to 3 ml of

dilute sodium hydroxide solution and is then shaken well. There was no change or reaction

occurs in the solution except that a double layer is formed on the surface of the solution. The test

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is then repeated by using cyclohexene to replace cyclohexane. The colorless solution turns

yellow and has a double layer on the surface of the solution for cyclohexene.

Throughout all the tests, it can be noted that cyclohexane is compared with cyclohexene to

identify and determine their properties. It can be seen that cyclohexene is much more reactive

compared to cyclohexane as it undergoes addition reactions. However, cyclohexane does not

participate in any addition reaction due to its carbon-carbon single bond. Instead, cyclohexane

undergoes halogenations which is a substitution reaction in the first test but remain unreactive on

the other tests.

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Variables

The dependent variable is the factor that is measured in order to determine how it responds

to changes made to the independent variable (i.e. the value of the dependent variable depends on

the independent variable). In this experiment the dependent variable is the reactivity of the

molecules (which we measured by the rate at which they decolored bromine water). It is

important to control all variables other than the dependent variable and independent variable to

make the experiment valid – ie so that measured changes to the dependent variable are only due

to the changes made to the independent variable. Some examples from this experiment include

temperature, time of measurement, light intensity, and the fact that we used corresponding

molecules (same # of carbons with the same shape).

A control is a sample that is left unchanged so that all other results can be compared to it (ie to

determine if the independent variable really had any effect on the dependent variable). In this

case samples of bromine water were left, cyclohexene and cyclohexane unmixed over the same

time period as the experiment. The fact that they did not change colour indicates that the

observed color changes in the experiment were due to the reaction between cyclohexene and

bromine water.

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Limitations/Sources of Error

Human errors, such as measuring incorrectly, inadvertently contaminating a solution by dropping

another substance into it, or using dirty instruments, are examples of how making a simple

mistake affects the experiment. Equipment limitations also cause errors if instruments are not

calibrated properly or if an instrument is unable to take a measurement because of calibration

limitations. Taking measurements during an experiment is another source of observation errors.

Measurement values may not be well defined, which means that some items may have a range of

values rather than a single value. Finally, inconsistent sampling techniques also cause errors.

Every time an experiment is done, each step must be repeated the same way as it was previously.

If this does not happen, different results are likely.

Precautions

1. Students must always wear lab coats all the time when conducting the experiment.

2. Students must wear gloves when handling with dangerous chemicals such as

concentrated sulphuric acid as it is corrosive.

3. Ensure that chemicals such as cyclohexane, cyclohexene and potassium permanganate

are handled in the fume chamber.

4. Dispose all waste and chemicals such as bromine water in an appropriate waste container.

5. Ensure that contents containing concentrated sulphuric acid are discarded into a beaker

containing at least 50 ml of water before pouring it into the sink.

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REFERENCES

Fesseden R.J. and Fesseden, J.S. Organic Chemistry, 4th Edition, California, U.S.A, Pacific

Grove, 1990, Pages 103, 431, 477.

Hornback, J.M. 1998. Organic Chemistry. Brooks/ Cole Publishing Company, Pacificgrove. p.p

816-817

Maitland, J.J. 1997. Organic Chemistry.w.w Norton and Company, USA. pp. 616

Michelle, J. Sienko, Robert A. Plane, Chemistry, 5th Edition, London, McGraw-Hill

Book Company, 1981, Pages 455-465

Personal Book References, Essential Chemistry SPM, Longman Publication, 2009.

Personal Book References, Module, Teaching and Learning Chemistry, Form 4, Malindo

Publications, 2009

Shriner, Hermann, Morill, et. Al. 2004. The Systematic Identification of Organic

Compounds. John Wiley and Sons Inc.USA. p. 130