S5: Chemistry

     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

        Activity 3.1.1

1. Describe the formation of a carbon-carbon double bond. What is the 
    hybridisation state of a carbon doubly bonded? 

2. What is the shape of the molecule around the double bond? Explain.

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 

Example: Ethene

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 resistance of rotation around the double bond.
The hybridization state in alkenes is 
and the structure around each carbon doubly bonded is trigonal 
planar with a bond angle value of 

Activity 3.1.2

Refer to the IUPAC system used in the nomenclature of alkanes, name the 

following compounds.

                             

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

                        

Checking up 3.1

1. Write the structural formula of:
    a. 4-ethylhept-3-ene
    b. 5-isopropyl-2,6-dimethylundec-3-ene
    c. 3-ethyl-2,4,5-trimethyl oct-2-ene
    d. 3-ethyl-2-methylcyclohexene
    e. Buta-1,2-adiene

2. Name each of the following compounds according to the IUPAC system.

              

               

3.2. Isomerism in alkenes

         Activity 3.2

1. What is meant by isomers and what are the types of isomers?

2. Which types of isomerism can be exhibited by alkenes? Give your reasons 

Alkenes exhibit two types of isomerism: structural isomerisms and stereoisomerism.

1. Structural isomerism

Alkenes exhibit 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.

Example:

Alkenes and cycloalkanes have the same molecular formula because they both 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 resistance of rotation around the double bond, alkenes give rise to cistrans 

or geometrical isomerism. Sometimes known as E- and Z- Isomers

          

Checking up 3.2

1. State the necessary condition for the existence of cis-trans isomerism in 
    alkenes?

2. Which of the following alkenes can exhibit a cis-trans isomerism?

                

3.3. Preparation of alkenes

      Activity 3.3

Different methods can be used to prepare alkenes. Discuss the possible 
reactions which may be involved in the preparation of alkenes and propose the 

mechanisms, where it is possible.

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

elimination reactions.

An alkene may be obtained by dehydration of an alcohol. The reaction involves 
the loss of H and OH 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.
From the secondary carbocation, two products can be obtained and the reaction 

follows the Zaitsev’s rule.

                 

                 

From the tertiary carbocation, two products can be obtained and the reaction 
follows the Zaitsev’s rule.

The dehydration of alcohols leading to alkenes may also be effected by heating 

alcohols in the presence of alumina.

                   

2. Dehydrohalogenation of halogenoalkanes
Halogenoalkanes react with hydroxide ions in ethanolic solution to yield alkenes. 

The reaction follows the Zaitsev’s rule.

      Examples

When a compound containing two halogen atoms on the adjacent carbon 

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

    Examples:

         

 When the two halogen atoms are attached to non-adjacent carbon atoms, a cyclic 

alkane is formed. 

           

Checking Up 3.3

1. Refer to the IUPAC system, name the alkenes formed when the following 
    alcohols are dehydrated in the presence of sulphuric acid.
a. Pentan-2-ol
b. 2-methylpropan-1-ol
c. 2,3-dimethylbutan-2-ol
d. 2-methylcyclohexanol
e. 2-methylbutan-2-ol
2. What are the products of the dehydrohalogenation of the following 
    compounds? Show the major product.
f. 1-bromo-2-methylpropane
a. 2-bromo-3-methylpentane
b. 2-bromo-2,3-dimethylbutane

c. 3-chloro-3-ethylpentane

3. Write the formula of the compounds formed when each of the following 

dihalogenoalkanes react with magnesium.

           

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 of 

        ethene.

                           

                         Figure 3.1: Laboratory preparation of ethene 

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.

                        

                      Picture 3.1: Test for unsaturation

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.

                  

Checking Up 3.4

1. Ethene is prepared by dehydration of ethanol in the presence of alumina, 
    explain other reactions that produce ethene. 
2. Describe the chemical test used to identify the presence of a carbon-carbon 
     double bond in an organic compound. 
3. Explain how ethene can be differentiated from carbon dioxide using a 

     chemical test?  

 3.5. Physical properties of alkenes

Activity 3.5

1. How does the physical state of alkenes change with molecular mass. 
2. Put in a test tube 5ml of cyclohexene. Add 5ml of water and mix. Record 
    your observations. 
3. Put in a test tube 5ml of cyclohexene. Add 5ml of tetrachloromethane 
    and mix. Record your observations.
4. Cis-but-1-ene and trans-but-2-ene exhibit geometric isomerism. State 

    which one of them is less volatile and why? 

• 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----.

Checking Up 3.5

Which one of the following compounds has higher boiling point? Explain.
a. Cis-butene and trans-butene 
b. Ethene and propene

        3.6. Chemical properties
        3.6.1. Addition reactions 

        3.6.1.1. Electrophilic additions

 Activity 3.6.1

1. Explain the following terms and give two examples for each.
     a). Addition reaction
     b). Lewis acid
2. Distinguish other name given to a Lewis acid.
3. Predict if Lewis acids can react with alkanes. 
4. Justify if Lewis acids react with alkenes. 

5. Differentiate the reactivity of alkenes and alkanes.

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.

Electrophilic addition reactions take place in two steps:

     i. Formation of a carbocation

               

  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.

The order of stability of the carbocations is:

                 

                  

In the presence of peroxide, the reaction follows a free radical mechanism and it 

does not follow the Markonikov’s rule.

                  

                  

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.

    Example:

Reaction of ethene with bromine in an inert organic solvent gives:

                      

The reaction follows the mechanism below:

                   

he reaction with bromine is a useful test for alkenes.
The brown red colour of bromine is discharged in alkenes.

With bromine water, the reaction gives a mixture of organic products.

  Example:

Bromine water containing sodium chloride gives a mixture of three organic products.

Example:

3.6.1.2. Hydrogenation

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

           

This reaction is very useful when transforming vegetable oils into fats such as 

margarine by hydrogenation. The process is referred as hardening.

 Checking up 3.6.1

1. Predict the products formed when alkenes (But-1-ene and 
     3-methylpent-2-ene react with each of the following reactants:
      i. HCl
     ii. Water in acidic medium
    iii. Cold sulphuric acid
    iv. hydrogen
2. Outline the mechanism of the reaction between 2-methylpent-2-ene 

     with hydrogen bromide. 

   3.6.2. Oxidation reactions

     Activity 3.6.2

1. Explain the terms oxidation, oxidising agent based on examples
2. Explain the terms reduction, reducing agent and give examples
3. The combustion of alkenes yields products, illustrate it by a reaction and 
    indicate the types of products generated.
4. Explain what happens when alkenes react with oxidising agents.      
    Alkenes are readily oxidised due to the presence of the double bond.

1. Reaction with oxygen

i. Transformation to epoxides

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

        

Epoxyethane is a very reactive substance. It reacts with water to give 1,2-ethanediol 

which is used in the making of polyesters, detergents, and so on.

       

 ii. Combustion

Alkenes burn in oxygen to give carbon dioxide, water and energy

    Example:

    2. Reaction with ozone

An alkene reacts with ozone to give an ozonide.
The reaction is carried out at low temperature (below ) in non-aquous medium.
                     

On hydrolysis, the ozonide splits into two carbonyl compounds. The reaction which 
is an oxidative cleavage is referred to as ozonolysis.

Since the by-product is hydrogen peroxide, the hydrolysis is carried out in the 

presence of a reducing agent.

         

The interest of the ozonolysis reaction is that it can help to identify the location of 
the double bond in an alkene.

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.

            

This reaction also is used to test for the presence a double bond.

An alkane does not react with KMnO4 (left), but an alkene reacts with KMnO4 

producing a dark brown precipitate of MnO2 (right) (Figure 3.3).

                            

                      Picture 3.2: Reaction of alkenes and KMnO

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.

                    

Checking up 3.6.2

1. Write the equations of the reaction between 3-methylpent-2-ene with:
     a. Oxygen in the presence of silver catalyst.
     b. Cold dilute potassium permanganate solution
    c. Ozone
2. Describe the purpose of the reaction between alkenes and ozone.
3. Describe the observations when butane and but-2-ene react separately 

    with potassium manganate (VII) solution.

3.6.3. Addition polymerisation

Activity 3.6.3

1. The students of a given class are asked to form separate couples of students. 
    In each couple, the students hold each other by their two hands. Now 
    each couple is asked to free one hand per student so that each student of 
    each couple can hold a hand of another student from a different couple. 
    What will be the result of such an arrangement compared to the first one?
2. From this example, predict what will happen in an addition reaction of 

    many molecules of one or different alkenes?

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)

                                      Table 3.1: Polymers of alkenes and their uses

          

Checking Up 3.6.3

1. Explain the terms 
     a. addition polymerisation
     b. monomer 
     c. polymer
2. The use of some plastic bags is banned in our country. Analyse the 
scientific and environmental reasons of this prohibition and suggest 

alternative solutions.

Project Work
Although they have many uses, plastics have side effects and therefore some of 
them are being replaced by more eco-friendly plastics.
 
Design a project for the making of plastics using starch from plants. In your 
project you will:
1. Perform the extraction of starch
2. Make plastics using starch you will have extracted
3. Test the properties of your plastics

4. Differentiate between bioplastics and biodegradable plastics.

3.7. Structure, classification and nomenclature of alkynes

       Activity 3.7

1. Explain the formation of a carbon-carbon triple bond.
2. What is the hybridisation state of a carbon atom triply bonded and what 
is the shape of the structure around it.

3. Differentiate between the following compounds 

4.

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 180o
 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.: :

A non-terminal alkyne has a triple bond in the middle of the chain:

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.

                             

Checking Up 3.7

1. Name according to the IUPAC system, each of the following compounds.

                             

5. Write structural formula for:
a. 2,5- dimethyl-3-hexyne
b. 6-isopropyl-5-propyldec-3-yne

c. 5-ethyl-4-methloct-1-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

               

               Figure 3.3 Laboratory preparation of ethyne

   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.

              

 A more quick industrial production consists of heating methane alone at high 

temperature for 0.01-0.05second.    

        

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.  

             

Checking Up 3.8

1. Using chemical equations, describe the preparation of ethyne(acetylene)
2. By which reactions higher members of the alkynes family are prepared?
3. Suggest a synthesis for each of the following compounds using 
      acetylene as the starting organic material.
a. Propyne
b. 2-butyne

c. 3-hexyne

3.9. Physical properties of alkynes

Activity 3.9

Alkynes have the general formula 
 They have two fewer hydrogen atoms 
than alkenes, and four fewer H than alkanes. Do you expect alkynes to be more 
or less volatile than alkenes? Explain by referring to the nature of the chemical 

bonding and the structure of alkenes and 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

         

Alkynes are water insoluble but they dissolve in each other and in non-polar solvents.

Checking up 3.9

1. Which of 3,4,4-trimethylpent-1-yne and oct-3-yne has a high volatility? 
    Explain 

2. Table salt (NaCl) is water soluble but hex-2-yne is not. Explain why.

3.10. Chemical reactions of alkynes

  Activity 3.10

Alkynes have a carbon-carbon triple bond. That is why they have a higher electron 
density than alkenes. Do you expect alkynes to be more reactive than alkenes?

Which types of reactions can be exhibited by 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.

Example:

                    

2. Addition of water

Alkynes react with water in the presence of sulphuric acid and mercury sulphate 
at to    give carbonyl compounds.
             
Example:

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.

Example:

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

alkenes

    

 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. 

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.

   

    4. 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.

Checking up 3.10 

1. Write the formula(s) and the name (s) of the products of the reaction of 
    pent-1-yne with:
     a. water 
    b. hydrogen chloride 
    c. sodium metal 
2. Outline the mechanism of the reaction between but-2-yne with 
     hydrogen bromide.

   3.11. Uses of alkenes and alkynes
           Activity 3.11
         Look at the picture below and mention the importance of alkenes and alkynes.  
                  
  
     Figure 3.5: Some plastic materials (A& B), tomatoes which are ripening (C)
     and a person who is welding (D)

 • 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;

                                

      Picture 3.3: Ethene is a plant hormone which causes bananas to ripen. 

      image source m.yukie .mobi.

 • 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 example 
   ethyne 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.

Checking up 3.11

Alkenes, alkynes and their derivatives have many applications in our daily life. 

Discuss this statement.

3.12. End unit assessment

I. Multiple choice questions. Choose the best answer in the following by 
    noting the corresponding letter.

1. Which of the following is given off during ripening of fruits and 
   vegetables?
b. Ethane
c. Ethene
d. Ethyne
e. Methane

2. Loss of hydrogen halide is called:
a. Halogenation
b. Dehydration
c. Dehydrohalogenation
d. Hydrogenation

3. Alkenes can be oxidized using powerful oxidizing agent in acidified 
    medium.
a. Potassium manganate (VII)
b. Sodium manganate (VI)
c. Calcium manganate (VI)
d. All of them

4. The molecular formula of-------- fit the general formula
 a. Alkanes
b. Alkynes
c. Alcohols
d. Alkenes

5. Example of addition reactions include all but one of the following. 
     Which is the odd one out?
a. Combustion of propene.
b. Reaction of with propene.
c. Reaction of HBr with but-2-ene.
d. Polymerization of ethene 

6. Which statement is incorrect about reactions of propene?
a. Reaction with  and gives 1-bromo propan-2-ol as the main 
    product.
b. Polymerization of propene gives polypropene, of which the isotactic and 
   syndiotactic forms are commercially valuable.
c. Reaction with in the presence of a radical initiator yields 
2-bromopropane as the major product.
d. No correct answer

7. Which one of the following statements is incorrect?
a. The electrophilic addition of HBr to but-2-ene involves a secondary 
    carbonium ion intermediate.
b. In the presence of a radical initiator, HBr reacts with but-1-ene to give 
1-bromobutane as the major product.
c. In the presence of a radical initiator, HBr reacts with but-1-ene to give a 
   Markovnikov addition product.
d. The major product of the electrophilic addition of HBr to hex-1-ene is 
     2-bromohexane.

8. What type of reaction do alkynes undergo across triple bond?
a. Elimination reaction
b. Substitution reaction
c. Addition reaction
d. Halogenation
    96 Chemistry Senior Five Student Book

9. Acetylene is also called: 
a. Ethyne
b. Ethene
c. Ethane
d. Methane

10. What product(s) will be obtained from the acid-catalysed hydration of 
pent-2-yne?
a. pentanal
b. pent-2-one and pentan-3-one
c. pentan-2-one
d. pentan-3-one

II. Open questions

11. Give all possible isomers of

12. Explain the following observations
a. When bromine in presence of dichloromethane is added to propene, 
   only one product is formed i.e. 1,2-dibromopropane.
b. When bromine water  is added to propene, a mixture of 
    products namely 1,2-dibromopropane and bromopropan-2-ol are 
    obtained.
c. When bromine in presence of carbon tetrachloride and sodium chloride 
is added to propene, a mixture of products namely, 1,2-dibromopropane 

and bromo-2-chloropropane are formed.

13. Show how the following conversions may be accomplished

                 

14. a. In an experiment it was found that 35g of pure alkene reacted 
          with 100g of bromine .

i. Calculate the molecular mass of the alkene
ii. Write the molecular formula of the alkene
iii. Write the structural formulae and the systematic names of one of 
     any two alkenes in (ii)
b. Using equations only show the mechanism for the reaction of one 

     of alkenes in (iii) with bromine.

15. Three hydrocarbons D, E and F, all have the molecular formula C6H12. 

D decolourises an aqueous solution of bromine and shows geometric 
isomerism. E also decolourises an aqueous solution of bromine but does 
not show geometric isomerism. F does not decolourise an aqueous solution 
of bromine. Draw one possible structure each for D, E and F.

16. Alkenes such as ethene and propene have been described as the building 
blocks of the organic chemical industry. Discuss this statement, giving 
examples. What particular features of the chemistry of alkenes make them 

suitable for this role and why are alkanes less suitable.