Alkenes and Alkynes – Form 5 Chemistry Notes (O Level)

Alkenes (The Olefins)

Alkenes are unsaturated hydrocarbons, containing at least one carbon to carbon double bond in their molecular structures. They form a homologous series with the general formula C_nH_2n, where n is a whole number that can take values such as 2, 3, 4…

Nomenclature of Alkenes

Alkenes are named according to IUPAC rules. The root is derived from the longest carbon chain that must contain the carbon to carbon double bond. This carbon chain is numbered such that the carbon atom bearing the double bond takes the smallest possible number. The name begins with prefixes, if any, and ends with the suffix –ene.

The first five members of this homologous series are given in the following table.

Alkene	Molecular Formula	Structural Formula
Ethene	C₂H₄	CH₂=CH₂
Propene	C₃H₆	CH₃CH=CH₂
But-1-ene	C₄H₈	CH₃CH₂CH=CH₂
Pent-1-ene	C₅H₁₀	CH₃CH₂CH₂CH=CH₂
Hex-1-ene	C₆H₁₂	CH₃CH₂CH₂CH₂CH=CH₂

Ethene

Ethene is the simplest and the most important member of the alkene series.

Industrial Manufacture of Ethene

Industrially, ethene is manufactured by cracking long chain alkanes obtained by the fractional distillation of petroleum.

Laboratory Preparation of Ethene

Ethene is prepared in the laboratory by the dehydration of ethanol with excess concentrated sulphuric acid at a temperature of 180℃.

Procedure: The reaction occurs in two stages:

• Ethanol and conc. sulphuric acid are mixed in a round-bottomed flask, where they react in the cold to produce ethylhydrogen sulphate.

C₂H₅OH(l) + H₂SO₄(aq) → C₂H₅HSO₄(aq)(ethylhydrogen sulphate) + H₂O(l)

• The mixture is then heated gently to about 180℃. On heating, the ethylhydrogen sulphate decomposes to ethene and sulphuric acid is regenerated.

C₂H₅HSO₄(aq)(ethylhydrogen sulphate) →^{180℃, Δ} C₂H₄(g)(ethene) + H₂SO₄(aq)

• The ethene obtained is then passed through sodium hydroxide solution to remove any gaseous impurities, which are mainly carbon dioxide and sulphur dioxide (formed from the slight oxidation of the ethanol by the hot concentrated sulphuric acid).
• The pure ethene gas is collected over water since it is only slightly soluble in water.

Precaution: To prevent the sodium hydroxide from being sucked back into the reaction mixture, an empty flask is inserted between the round-bottomed flask and the flask containing the sodium hydroxide.

Remark: The overall reaction can be regarded as a dehydration of ethanol with the hot concentrated sulphuric acid, because the overall change is the removal of a molecule of water from ethanol, i.e.

C₂H₅OH(l)(ethanol) →^{conc. H₂SO₄} C₂H₄(g)(ethene) + H₂O(l)

Another method of preparing ethene from ethanol is by the catalytic dehydration of ethanol vapour, using a heated catalyst of aluminium oxide or broken porcelain. Asbestos wool soaked in ethanol is heated so that ethanol vapour passes over the hot catalyst; the ethene formed is collected over water.

C₂H₅OH(l)(ethanol) →^{broken porcelain or Al₂O₃, heat} C₂H₄(g)(ethene) + H₂O(l)

Physical Properties of Ethene

• Ethene is a colourless gas with a faint sweetish smell.
• It is only slightly soluble in water.
• It is slightly less dense than air.
• It is neutral to litmus.

Uses of Ethene

1. Ethene is used as a fuel.
2. It is a raw material for the manufacture of plastics, e.g. polyethene.
3. In agriculture, ethene is used to hasten the ripening of fruits.
4. It is used in the manufacture of synthetic fibres and detergents.
5. It is used in the manufacture of important organic compounds like ethane, ethanol, etc.

Chemical Properties of Alkenes

Alkenes are generally more reactive than alkanes because of their unsaturated nature, i.e. due to the presence of the carbon to carbon double bond in their molecular structures.

1. Combustion: Alkenes burn exothermically in oxygen with a yellow luminous smoky flame (due to their high carbon to hydrogen ratio), producing carbon dioxide and water vapour, e.g.

   C₂H₄(g) + 3O₂(g) → 2H₂O(g) + 2CO₂(g)

2. Addition reactions: An addition reaction is a reaction in which an unsaturated molecule reacts with another molecule, forming a saturated product or a product with its degree of unsaturation reduced. Generally, ethene will undergo an addition reaction with the reagent X–Y, where both X and Y are monovalent species, as follows:

   H₂C=CH₂ + X–Y → XCH₂–CH₂Y

   1. Addition of hydrogen (hydrogenation): Ethene, like other alkenes, undergoes an addition reaction with hydrogen when heated to a temperature of about 150℃ to 200℃ in the presence of a nickel catalyst, to yield ethane.

      C₂H₄(g) + H₂(g) →^{nickel, 150℃–200℃} C₂H₆(g)

      Hydrogenation is used in the manufacture of margarine from vegetable oils. These oils have unsaturated molecules, and when reacted with hydrogen in the presence of a nickel catalyst, the hydrogen adds across the carbon to carbon double bonds in the molecules and the oils harden, forming margarine.

   2. Addition of halogens (halogenation): Chlorine and bromine in tetrachloromethane add readily at room temperature across the carbon to carbon double bonds of alkenes. The reaction with iodine is slower. Examples:

      CH₂=CH₂ + Cl₂(g) →^CCl₄ CH₂Cl–CH₂Cl(1,2-dichloroethane)

      CH₂=CH₂ + Br₂(g) →^CCl₄ CH₂Br–CH₂Br(1,2-dibromoethane)

      Remark: When bromine reacts with any alkene, its reddish-brown colour disappears rapidly (even in the dark), i.e. bromine is rapidly decolourised by an alkene. This reaction is often used as a test for unsaturation.

   3. Addition of hydrogen halides: Ethene combines rapidly with hydrogen iodide (vapour) at room temperature to produce iodoethane.

      CH₂=CH₂ + HI(g) → CH₃CH₂I(iodoethane)

      Hydrogen chloride and hydrogen bromide undergo similar reactions with ethene but more slowly.

   4. Addition of concentrated sulphuric acid: Ethene readily combines with conc. H₂SO₄ at room temperature to form ethylhydrogen sulphate.

      CH₂=CH₂(g) + H₂SO₄(aq) → C₂H₅HSO₄(aq)(ethylhydrogen sulphate)

3. Reaction of alkenes with acidified potassium permanganate: Alkenes readily decolourise the purple colour of acidified potassium permanganate in the presence of water, forming diols, e.g.

   CH₂=CH₂(g) + H₂O(l) + [O](from acidified KMnO₄) → HOCH₂–CH₂OH(ethane-1,2-diol)

Tests for Alkenes

Two common tests for alkenes are:

1. Alkenes rapidly decolourise a solution of bromine in water (bromine water) or bromine in tetrachloromethane, even in the dark.
2. Alkenes decolourise the purple colour of acidified potassium permanganate solution.

The Distinctive Test Between Alkenes and Alkanes

Alkenes rapidly decolourise bromine water, while alkanes do not.

Alkynes

Alkynes are unsaturated hydrocarbons because they contain a carbon to carbon triple bond in their molecular structures. They form a homologous series with the general molecular formula C_nH_2n−2, where n is a whole number that can take values such as 2, 3, 4…

Nomenclature of Alkynes

Alkynes are named according to IUPAC rules. The root is derived from the longest carbon chain that must contain the carbon to carbon triple bond. This carbon chain is numbered such that the carbon atom bearing the triple bond takes the smallest possible number. The name begins with prefixes, if any, and ends with the suffix –yne.

The first five members of this homologous series are:

Alkyne	Molecular Formula	Structural Formula
Ethyne	C₂H₂	CH≡CH
Propyne	C₃H₄	CH₃C≡CH
But-1-yne	C₄H₆	CH₃CH₂C≡CH
Pent-1-yne	C₅H₈	CH₃CH₂CH₂C≡CH
Hex-1-yne	C₆H₁₀	CH₃CH₂CH₂CH₂C≡CH

Ethyne

It is the simplest and the most important alkyne. It is commonly called acetylene.

Laboratory Preparation

Ethyne is usually prepared in the laboratory by the action of cold water on calcium carbide.

CaC₂(s) + 2H₂O(l) → Ca(OH)₂(aq) + C₂H₂(g)

The gas is passed through acidified copper(II) sulphate to remove phosphine, the main impurity usually present. The pure gas is then collected over water as it is only slightly soluble in water.

Physical Properties of Ethyne

• Ethyne is colourless and has a characteristic sweet smell when pure.
• It is slightly soluble in water.
• It is slightly less dense than air.

Uses of Ethyne

• A mixture of ethyne and oxygen, commonly known as oxy-acetylene gas, burns with a very hot flame that is used for cutting and welding metals.
• It is used as fuel in hunters’ and miners’ lamps.

Chemical Properties of Alkynes

1. Combustion: Alkynes burn with a much more smoky and luminous flame compared to alkenes (due to their very high carbon to hydrogen ratio), producing carbon dioxide and water vapour, e.g.

   2C₂H₂(g) + 5O₂(g) → 2H₂O(g) + 4CO₂(g)

2. Addition reactions: Alkynes undergo addition reactions with two moles of another compound to produce a saturated product.
   1. Addition of hydrogen: Ethyne combines with twice its own volume of hydrogen over a nickel catalyst at a temperature of about 200℃ to form ethane.

      C₂H₂(g)(ethyne) + 2H₂(g) →^{nickel, 200℃} C₂H₆(g)(ethane)

   2. Addition of halogens: At ordinary temperature, ethyne combines readily with bromine in the presence of a metallic halide (e.g. silver bromide) as a catalyst, forming 1,1,2,2-tetrabromoethane.

      HC≡CH + 2Br₂(l) → HCBr₂–CHBr₂(1,1,2,2-tetrabromoethane)

      Remark: In this reaction, the reddish-brown colour of bromine is very rapidly decolourised. Hence this reaction is used as a test for unsaturation, but cannot be used to distinguish alkenes and alkynes.

   3. Addition of hydrogen halides: Ethyne combines readily with hydrogen iodide at room temperature to form 1,1-diiodoethane.

      HC≡CH + 2HI → CH₂I–CH₂I(1,1-diiodoethane)