Chemistry

UNIT 3: ALKENES AND ALKYNES

Key unit competency

Relate the physical and chemical properties of alkenes and alkynes to their

reactivity and uses

Learning objectives

• Explain the reactivity of alkenes in comparison to alkanes

• Explain the existence of geometrical isomerism in alkenes

• Describe the industrial process of preparing alkenes and alkynes

• Apply IUPAC rules to name alkenes and alkynes

• Carry out an experiment to prepare and test ethene gas

• Outline the mechanisms for electrophilic addition reactions for alkenes and

alkynes

• Write the structural formulae of straight chain alkenes and alkynes

• Apply Markovnikov’s rule to predict the product of hydrohalogenation of

alkenes

• Classify alkynes as terminal and non-terminal alkynes using their different

structures

• Appreciate the combustion reaction as source of fuels.

• Appreciate the uses and dangers of addition polymers (polythene used for

polythene bags, polypropene for plastic bottles etc.)

Introductory activity

Observe the following picture and answer the questions that follow.

1. What is the collective name of the substances used to manufacture the

items showed in the above picture?

2. a. What are the raw materials used in the manufacture of the substances

identified in 1)?

b. These raw materials may be obtained from different sources. Discuss

this statement.

c. Do you expect these raw materials be soluble or not in water? Justify

your answer.

3. Even though the items which appear in the picture above are interesting,

they also present some disadvantages. Discuss this statement.

3.1. Definition, structure and nomenclature of alkenes

Alkenes are a homologous series of hydrocarbons which contain a carbon-carbon

double bond. Since their skeleton can add more hydrogen atoms, they are referred

as unsaturated hydrocarbons.

The general formula of alkenes is C_nH_2n.

Alkenes are abundant in the nature and play important roles in biology. Ethene,

for example, is a plant hormone, a compound that controls the plant’s growth and

other changes in its tissues.

Ethene affects seed germination, flower maturation, and fruit ripening.

They are described as unsaturated hydrocarbons because they can undergo

addition reactions.

The double bond in alkenes is made of one sigma bond and one pi bond. This gives rise to the impossibility of rotation around the double bond. The hybridization state in alkenes is sp2 and the structure around each carbon doubly bonded is trigonal planar with a bond angle value of 120^o

.IUPAC names of alkenes are based on the longest continuous chain of carbon

atoms that contains the double bond.

The name given to the chain is obtained from the name of the corresponding

alkane by changing the suffix from –ane to –ene.

If the double bond is equidistant from each end, number the first substituent that

has the lowest number. If there is more than one double bond in an alkene, all of

the bonds should be numbered in the name of the molecule, even terminal double

bonds. The numbers should go from lowest to highest, and be separated from one

another by a comma.

The chain is always numbered from the end that gives the smallest number for the

location of the double bond.

In naming cycloalkenes, the carbon atoms of the double bond are numbered

1 and 2 in the direction that gives the smallest numbers for the location of the

substituents.

If a compound contains two or more double bonds, its location is identified by a

prefix number. The ending is modified to show the number of double bonds:

• a diene for two double bonds,

• a triene for two three bonds

• a tetraene for four double bonds

3.2. Isomerism in alkenes

Alkenes exhibit two types of isomerism: structural isomerisms and

stereoisomerism.

1. Structural isomerism

Alkenes show as well position isomerism, chain isomerism and functional isomerism.

In position isomerism, the position of the double bond changes but the length

of the chain remains the same.

Alkenes and cycloalkanes have two fewer hydrogen atoms than alkanes. That is

why, they have the same molecular formula. However, they belong to different

homologous series. Therefore, they are functional group isomers. This isomerism

that relates open chain compounds to ring chain compounds is referred to as ring

isomerism.

2. Stereoisomerism

Due to the impossibility of rotation around the double bond, alkenes give rise to

cis-trans or geometrical isomerism.

3.3. Preparation of alkenes

Different methods are used for the preparation of alkenes. Most of them are

elimination reactions.

1. Dehydration of alcohols

An alkene may be obtained by dehydration of an alcohol. The reaction involves

the loss of H and OH (water) from adjacent carbons of an alcohol to form an  alkene. The dehydration is carried out by heating an alcohol with concentrated sulphuric acid or 85% phosphoric acid.

If two or more alkenes may be obtained, the one having more substituents on

the double bond generally predominates. This is the Zaitsev’s rule.

This is due to the stability of the intermediate carbocation. The carbocation

produced in step 2 may undergo a transposition (rearrangement) of a hydride ion

or a methyl group giving a more stable carbocation and therefore a more stable

alkene.

Mechanism

2. Dehydrohalogenation of halogenoalkanes

alkenes. The reaction follows the Zaitsev’s rule.

3. Dehalogenation of dihalogenoalkanes

When a compound containing two halogen atoms on the adjacent carbon

atoms is treated with magnesium or zinc it transforms to an alkene.

3.4. Laboratory preparation and chemical test for ethene

Activity 3.4

Preparation of ethene Set up the apparatus as shown in the Figure below (Figure 3.1) and follow the instructions to perform the experiment on the preparation ofethane

Requirements:

Chemicals:

• Ethanol, aluminium oxide, lime water, mineral wool, bromine water,

acidified potassium permanganate solution (very dilute), water.

Additional apparatus:

• Boiling tube

• Rubber stopper with hole

• Delivery tube

• Trough

• Test- tube rack

• 5 test tubes

• 5 rubber stoppers for test tubes

• Spatula Procedure and setting

• Bunsen burner

• Glass rod

• Splint

• Matches 

1. Preparation of ethene:

- Pour some ethanol into the boiling tube to a 3 cm depth

- Add some glass wool to soak up the ethanol, using a glass rod to push the wool

down the tube.

 - Clamp the boiling tube in a horizontal position using a retort stand.

 - Put a small amount of aluminium oxide about half way along the boiling tube.

- Complete the set up of the apparatus as shown in the diagram above.

- Light the Bunsen burner, adjust it to a blue flame and heat the aluminium

oxide. (Make sure the test tube is filled with water when you start to collect the

gas produced.)

 - As the aluminium oxide gets hot the heat reaches the ethanol at the end of the

tube. The ethanol then changes to vapour, passes over the hot aluminium oxide

and is dehydrated to produce ethene gas.

- Collect 5 test tubes of the gas and put a stopper on each tube when it is filled.

- When the test tubes have all been filled, loosen the retort stand and raise the

apparatus so that the delivery tube no longer dips into the water. This avoids

suck back of water as the tube begins to cool which could cause the boiling

tube to crack. Turn off the Bunsen burner. 

2. Testing the properties of ethene

Addition of bromine:

- Taking great care, add about 1ml of the test tube of bromine water to one of

the test tubes of ethene.

 - Replace the stopper and shake the tube a few times.

 - Record your observations.

- Write down your conclusions 

- Addition of acidified potassium permanganate:

- Add about 1ml of very dilute potassium permanganate solution to one of the

test tubes of ethene and shake the tube a few times.

 - Record your observations.

- Write down your conclusions

 Combustion:

- Remove the stopper of one of the tubes filled with ethene and apply a light to

the mouth of the test tube using a lighted splint.

- Allow the gas to burn and when it has stopped burning add a small amount of

lime water to the test tube, stopper it and shake the tube a few times.

- Write down your observations.

Interpretation

When ethanol is heated in the presence of aluminium oxide, a gas is produced. This

gas does not react with lime water. This means that the produced gas is not carbon

dioxide. The equation of the reaction is: 

The gas decolourises bromine water. Bromine water is a test used to identify the

presence of a carbon-carbon double bond or triple bond. The bromine adds across

the double bond and a dibromoalkane is formed. The reaction between alkene and

bromine water is shown below:

If you shake an alkene with bromine water (or bubble a gaseous alkene through

bromine water), the solution becomes colourless. Alkenes decolourise bromine

water.

The Figure 3.2 shows Bromine water added to ethene: before the reaction (left)

the color of bromine appears, and after the reaction (right) the colour of bromine

disappears.

When ethene reacts with acidified potassium manganate (VII), the purple colour of

the permanganate solution turned to colourless or light pink indicating the presence

of the carbon – carbon double bond.The reaction is the following:

The gas burns with a smoky flame producing carbon dioxide and heat energy. The

carbon dioxide produced turns into milky lime water.

3.5. Physical properties of alkenes

• Alkenes which have less than 5 carbon atoms are gaseous at ordinary

temperature, the other are liquid up to 18 while others are solids as the

number of carbon atoms increases.

• Boiling points and melting points of alkenes are less than those of alkanes

but also increase as the molecular weight increase.

• Alkenes are insoluble in water but soluble in most organic solvents.

• Cis-alkenes have a slightly higher boiling point than the trans-isomers

because the dipole moments in trans structures cancel each others----.

3.6. Chemical properties

Alkenes are far more reactive than alkanes due to the carbon-carbon double bond.

These compounds are unsaturated and they can easily undergo addition reactions

to yield saturated products.

The double bond in alkenes is a region of high density of electrons. Therefore, this

region is readily attacked by electrophiles. An electrophile is an atom, a molecule

or an ion which is electron-deficient; i.e. it is a Lewis acid or an electron pair acceptor.

1. Addition of hydrogen halides

Hydrogen halides (HCl, HBr, HI) react with alkenes to yield halogenoalkanes. The

reaction is carried out either with reagents in the gaseous state or in inert solvent

such as tetrachloromathane. 

When hydrogen halides add to unsymmetrical alkenes, the reaction leads to the

formation of two products in two steps. The first step leads to the formation of

two different carbocations with the major product formed from the more stable carbocation. This is the Markownikov’s rule. That is “The electrophilic addition of an unsymmetric reagent to an unsymmetric double bond proceeds by involving the most stable carbocation.

2. Addition of water

The hydration of alkenes catalysed by an acid is an electrophilic addition. Ethene

can be transformed into ethanol. The first step consists of adding concentrated

sulphuric acid. The second step consists of the hydrolysis of the product of the

first step.

In industry the reaction is carried out at approximately 300 °C in the prence of

phosphoric acid as a catalyst.

3. Addition of cold concentrated sulphuric acid

When cold concentrated sulphuric acid reacts with alkene, an alkyl hydrogen

sulphate is obtained. If the starting alkene is unsymmetrical, two different alkyl

hydrogen sulphates are obtained. If the alkyl hydrogen sulphate is warmed in

the presence of water, an alcohol is obtained.

4. Addition of halogens

The addition of halogens (halogenation) on alkenes yields vicinal

dihalogenoalkanes. The reaction takes place with pure reagents or by mixing

reagents in an inert organic solvent.

When a chlorine or bromine molecule approaches an alkene, the pi electrons

cloud interact with the halogen molecule causing its polarisation.

3.6.1.2. Hydrogenation

In the presence of a catalyst (Pt, Ni, Pd), alkenes react with hydrogen to give

alkanes.

Alkenes are readily oxidised due to the presence of the double bond.

1. Reaction with oxygen

Ethene react with oxygen in the presence of silver as a catalyst to yield epoxyethane.

3. Reaction with potassium permanganate

Alkenes react with dilute potassium permanganate solution to give diols. The

reaction takes place in the cold.

The colour change depends on the medium of the reaction.

4. Hydroformylation

The hydroformylation is a process by which alkenes react with carbon monoxide

and hydrogen in the presence of rhodium catalyst to give aldehydes.

3.6.3. Addition polymerisation 

Alkenes undergo addition polymerisation reaction to form long chain polymers.i.e

a polymer is a large molecule containing a repeating unit derived from small unit

called monomers. A polymerisation reaction involves joining together a large number of small molecules to form a large molecule.

Many different addition polymers can be made from substituted ethene

compounds.

Each polymer has its physical properties and therefore many polymers have wide

range of uses.

Mechanism for the polymerisation of ethene.

1. Initiation

It is a free radical initiation.

2. Propagation

3. Termination 

where the part between brackets indicates a unit of the formula of the polymer

that repeats itself in the formula; n indicates the number of the units in a formula of

a polymer and is a very large number.

Summary of most alkene polymers obtained from alkenes as monomers and their

uses (Table 3.1)

3.7. Structure, classification and nomenclature of alkynes

A triple bond consists of one sigma bond and two pi bonds. Each carbon of the triple

bond uses two sp orbital to form sigma bonds with other atoms. The unhybridised 2p

orbitals which are perpendicular to the axes of the two sp orbitals overlap sideways

to form pi bonds.

According to the VSEPR model, the molecular geometry in alkynes include bond

angle of 180^o around each carbon triply bonded.Thus, the shape around the triple

bond is linear.

There are two types of alkynes: terminal alkynes and non-terminal (internal)

alkynes

A terminal alkyne has a triple bond at the end of the chain e.g.: : R-C ≡ C − H

A non-terminal alkyne has a triple bond in the middle of the chain: R − C ≡ C − R'

Examples

Alkynes are named by identifying the longest continuous chain containing the

triple bond and changing the ending –ane from the corresponding alkane to –yne

3.8. Laboratory and industrial preparation of alkynes

1. Preparation of ethyne

Activity: 3.8

Set up the apparatus as shown in the diagram below. 

Procedure:

• Place 2g of calcium carbide in a conical flask

• Using the dropping funnel, add water drop by drop.

• Collect the gas produced in the test tube.

• Remove the first tube and connect a second test tube.

• To the first test tube add two drops of bromine water. Record your observations

• To the second tube add two drops of potassium manganate (VII). Record your observations.

Ethyne (acetylene) can be prepared from calcium carbide which is obtained by

reduction of calcium oxide by coke at high temperature.

When bromine water is added to acetylene, the red colour of bromine is discharged.

The solution becomes colourless.

The decolourisation of bromine water is a test for unsaturation in a compound.

When potassium manganate (VII) is added to acetylene, its purple colour is

discharged.

2. Alkylation of acetylene

The hydrogen atom of ethyne as that of other terminal alkynes is slightly acidic and therefore it can be removed by a strong base like NaNH₂ or KNH₂.The products of the reaction are acetylides. Acetylides react with halogenoalkanes to yield higher alkynes.

3. Dehydrohalogenation

The dehydrohalogenation of vicinal or geminal dihalogenoalkanes yields alkynes

4. Dehalogenation

The dehalogenation of a tetrahalogenoalkane yield an alkyne.

3.9. Physical properties of alkynes

Alkynes are non-polar compounds with physical properties similar to those of

alkenes with the same number of carbon atoms. Their linear structure gives them

greater intermolecular forces than alkenes

3.10. Chemical reactions of alkynes

Addition reactions

As unsaturated hydrocarbons, alkynes are very reactive. Because they are unsaturated

hydrocarbons, alkynes undergo addition reactions. Alkynes can add two moles of reagents.

Even though they have a higher electron density than alkenes, they are in general less reactive because the triple bond is shorter and therefore the electron cloud is less accessible.

1. Addition of hydrogen halides

Alkynes react with hydrogen halides to yield vicinal dihalogenoalkanes, the reaction follows the Markownikov’s rule. The reaction takes place in four steps.

2. Addition of water

Alkynes react with water in the presence of sulphuric acid and mercury sulphate at 60^o C to give carbonyl compounds.

3. Hydrogenation

The hydrogenation of alkynes in the presence of palladium catalyst gives alkanes

The reaction requires two moles of hydrogen for a complete saturation.

In the presence of Lindlar catalyst, the alkynes are partially hydrogenated giving alkenes4

A Lindlar catalyst is a heterogeneous catalyst that consists of palladium deposited on

calcium carbonate and poisoned with different lead derivatives such as lead oxide

or lead acetate. A heterogeneous catalyst is the one which is in the phase different

from that of the reactants.

4. Reaction with metals

Terminal alkynes react with active metals to yield alkynides and hydrogen gas.

Internal alkynes do not react as they do not have an acidic hydrogen atom.

5. Reaction with metal salts

When a terminal alkyne is passed through a solution of ammoniacal silver nitrate, a white precipitate of silver carbide is formed.

When a terminal alkyne is passed through a solution of ammoniacal copper(I)

chloride, a red precipitate of copper(I)carbide is formed. 

The reactions above are used to:

• Differentiate between terminal and non-terminal alkynes.

• Differentiate ethene and ethyne

The reaction shows that hydrogen atoms of ethyne are slightly acidic, unlike those

of ethene.

3.11. Uses of alkenes and alkynes

Activity 3.11

Look at the picture below and appreciate the importance of alkenes and alkynes.

• Alkenes are extremely important in the manufacture of plastics which have

many applications such as: packaging, wrapping, clothing, making clothes,

artificial flowers, pipes, cups, windows, ...

• Ethene is a plant hormone involved in the ripening of fruits, seed germination,

bud opening;

• Ethene derivatives are also used in the making of polymers such as polyvinylchloride (PVC), Teflon,...

• Alkenes are used as raw materials in industry for the manufacture of alcohols, aldehydes, ...

• Alkynes are used in the preparation of many other compounds. For exampleethyne is used in the making of ethanal, ethanoic acid, vinyl chloride, trichloroethane, ...

• Ethyne (acetylene) is used as a fuel in welding and cutting metals.

• Propyne is used as substitute for acetylene as fuel for welding.