2.3 Alkenes

Molecular formula of alkenes
■ Alkenes

	►	General formula: CnH2n with n = 2, 3, 4 ....
	►	It is an unsaturated hydrocarbon molecules have a functional group of double covalent bond, C=C .
	►	Contain at least one double bond between two carbon atom.

■ This video contains information on the molecular and structural formula of alkenes.

■ Molecular formula of the first nine alkene members.

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Number of carbon, n Molecular formula (CnH2n) 2 C2H4 3 C3H6 4 C4H8 5 C5H10 6 C6H12 7 C7H14 8 C8H16 9 C9H18 10 C10H20

Nomenclature of alkenes
■ Naming alkenes according to the IUPAC system

	►	Each member of the alkene series has a name suffix-ene.
	►	Prefixes is connected with the suffix-ene give their name alkenes. Number of carbon, n Prefix Name of alkane: Prefix + suffix-ene 2 Eth Ethene 3 Prop Propene 4 But Butene 5 Pent Pentene 6 Hex Hexene 7 Hept Heptene 8 Oct Octene 9 Non Nonene 10 Dec Decene
	►	Three members of the first retain their original name, the ethene (C2H4) , propene (C3H6) and butene (C4H8)
	►	For members with atoms carbon 4 and above, a Greek prefix used to show on the number of carbon atoms in molecules such as those used for the alkane series.

Physical properties of alkenes
■ Physical properties of alkanes series:

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Alkene Ethene Propane Butane Pentene Hexene Formula C2H4 C3H6 C4H8 C5H10 C6H12 Boiling point -108℃ -47℃ -6℃ 30℃ 63℃ Density N/A > ethene > propene > butene > pentene

■ Changes in the physical properties when going down the homologous series of alkenes.

	►	Melting and boiling point will gradually increase as the number of C atoms increases.
	►	Density show gradually increase with the addition of one methylene group CH2 which equivalent to the addition of relative molecular mass of 14 units.

Chemical properties of alkenes
■ Chemical Properties of Alkenes

	►	Functional group of alkene is the double bond between two carbon atoms, C=C.
	►	The presence of double bond bonds in alkenes cause alkenes are more reactive than alkanes.
	►	Chemical reaction of alkenes: ○ Combustion reaction ○ Addition reaction ○ Polymerisation reaction

■ This video contains information on the chemical properties of alkenes.

■ Combustion of alkenes

	►	All alkenes burn completely in air to produce CO2 and H2O.
	►	The equation for the complete combustion of a number of alkenes: ○ C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l) + energy ○ C3H6(g) + (9/2)O2(g) → 3CO2(g) + 3H2O(l) + energy
	►	The complete combustion of alkenes produces a lot of heat energy. So alkenes are good fuel.
	►	Combustion alkenes produce more soot and sparkle than alkanes. This is because alkenes are unsaturated hydrocarbons and the percentage of the carbon per atoms is higher than alkanes.

■ Addition reaction

	►	The presence of double bond bonds in alkenes cause alkenes are more reactive than alkanes.
	►	Alkenes undergo addition reaction with many chemicals such as halogens, hydrogen, hydrogen halide and water.
	►	Addition of hydrogen (Hydrogenation) ○ Alkenes react with hydrogen to produce alkanes. $C_n{H}_{\mathrm{2n}}+H_2\begin{array}{c}\to \\ \mathit{nickel},180C\end{array}{C}_n{H}_{\mathrm{2n}+2}$ ○ Convert unsaturated vegetable oils to saturated fats, and used to make margarine and cooking fats. Example: ${\mathit{CH}}_3{\mathit{CH}}_2\mathit{CH}={\mathit{CH}}_2+H_2\begin{array}{c}\mathit{nickel}\\ \to \\ 180C\end{array}{\mathit{CH}}_3{\mathit{CH}}_2{\mathit{CH}}_2{\mathit{CH}}_3$
	►	Addition of halogen (Halogenation) ○ Alkenes react with halogen to produce dihaloalkane. CnH2n + X2 → CnH2nX2 ○ Can be used to produce PVC plastics
	►	Addition of water (Hydration ) ○ Alkenes react with water in the presence of strong acid(catalyst) to form alcohol. $\begin{array}{c}\\ C_n{H}_{\mathrm{2n}}\end{array}\begin{array}{c}\\ +H_{2}O\end{array}\begin{array}{c}{H}_3{\mathit{PO}}_4\\ \underset{\mathrm{60atm},300C}{\to }\end{array}\begin{array}{c}\\ C_n{H}_{\mathrm{2n}+1}\mathit{OH}\end{array}$
	►	Addition of hydrogen halide – alkenes react with hydrogen halide gas to produce haloalkane. ○ Alkenes react rapidly with hydrogen halide gas to produce haloalkane. CnH2n + HX → CnH2n+1X
	►	The following animation summarizes the addition reaction of alkenes.

■ Polymerisation

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A chemical process of consolidation of small molecules called monomers to produce a very large molecules.

■ Alkene polymerisation

	►	Addition polymerisation of ethene involves combining a large number of ethylene molecules to form a polymer called polietena (polythene).
	►	The nature of polythene produced depends on the temperature and pressure where the polymerization is conducted.
	►	Polymerization of ethylene at a temperature of about 200°C, 1200 atmosphere pressure in the presence of some oxygen will produce a low-density polyethylene. This type of polythene widely used to make plastic bottles and containers, plastic bags.

Comparing alkane with alkene
■ The following table shows the relative molecular mass of the two homologous series of alkanes and alkenes

	►	Alkane Molecule formula Relative molecular mass Alkene Molecule formula Relative molecular mass Ethane C2H6 30 Ethene C2H4 28 Propane C3H8 44 Propene C3H6 42 Butane C4H10 58 Butene C4H8 56
	►	Each member of the alkanes and alkenes differs from the next member by -CH2 group. So, relative molecular mass of each member differs from the next member by 14./td>

■ Similarities of alkanes and alkenes

	►	Both are hydrocarbon compounds.
	►	Both are insoluble in water but soluble in organic solvents.
	►	Both do not conduct electricity.
	►	Both are low densities.
	►	Both are low melting points and boiling point.
	►	Both burn in excess oxygen to form carbon dioxide and water.

■ This video contains information on the differences between alkane with alkene.

■ Alkanes differs from alkenes in some aspects:

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Differences Alkanes Alkenes General formula CnH2n+2 CnH2n Type of hydrocarbon Saturated hydrocarbon Unsaturated hydrocarbon Functional group N/A Double cavalent bond between two carbon atoms, C = C. Type of bond in the molecule Single covalent bond only Single and double covalent bonds Type of chemical reaction ○ Substitution reaction ○ Cracking reaction ○ Addition reaction ○ Polymerisation reaction Reaction with ○ bromine water ○ acidified potassium manganate(VII) solution No change ○ Decolourizes the brown coloured bromine water. ○ Decolourizes the purple coloured solution.

Laboratory Activity 2.3.1 : Comparing the Properties between Alkane and Alkene in terms of their conbustion flame

Laboratory Activity 2.3.2 : Comparing the Chemical Properties between Alkane and Alkene

Generalisation of the homologous characteristic of alkanes and alkenes
■ Physical Properties

	►	Physical properties of both homologous series of alkanes and alkenes changed gradually according to the same pattern. ○ Melting point, boiling point, and density of alkanes and alkenes increases gradually as the number of atoms per molecule increases. ○ Physical condition(at room conditions): Change gradually from gaseous, liquid and solid state.
	►	Both alkanes and alkenes are insoluble in water and does not conduct electricity.

■ Chemical Properties

	►	All members of the alkanes show similar chemical properties. Alkanes are saturated compounds. So all alkanes are chemically unreactive except undergo substitution reaction with halogen in the presence of sunlight or ultraviolet light.
	►	All members of the alkenes show similar chemical properties. This is because every member of alkenes has the same functional group, the double bond which carried out the addition reaction with various chemicals.
	►	Alkanes and alkenes show different chemical properties because alkanes are saturated compounds while alkenes are unsaturated compounds.
	►	Alkanes and alkenes can be distinguished by using bromine water or acidified potassium manganate (VII) solution.

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