2.2 Alkanes

Structural formulae of alkanes
■ Alkanes

	►	General formula: CnH2n+2 with n = 1, 2, 3, ....
	►	A group of saturated hydrocarbons, which every carbon atom in the molecule bound to a maximum of four other atoms by single covalent bonds C-C and C-H only.

■ Molecular formula of the first ten alkane members.

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Number of carbon, n Molecular formula CnH2n+2 1 CH4 2 C2H6 3 C3H8 4 C4H10 5 C5H12 6 C6H14 7 C7H16 8 C8H18 9 C9H20 10 C10H22

Nomenclature of alkanes
■ Naming alkanes according to the IUPAC system

	►	Each member of the alkane series has a name suffix-ane.
	►	Prefixes is connected with the suffix-ane give their name alkanes. Number of carbon, n Prefix Name of alkane: Prefix + suffix-ane 1 Meth Methane 2 Eth Ethane 3 Prop Propane 4 But Butane 5 Pent Pentane 6 Hex Hexane 7 Hept Heptane 8 Oct Octane 9 Non Nonane 10 Dec Decane
	►	The following mnemonic method can be used to memorize the first ten prefixes. ○ Meth Eth Prop But Pent Hex Hept Oct Non Dec ○ Mama Eats Potato Ball, Papa Hate Hungry Ox, Nasty Dog

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

■ Structural formula of straight-chain alkanes

	►	The following table shows the example of molecular formula and structural formula of alkanes. Alkanes Molecular formula Structure formula Methane CH4 Ethane C2H6 Butane C4H10
	►	For the same molecular formula, the structural formula may be different. ○ For example, n-butane and 2-methylpropane have different structural formula is different hydrocarbons though both have the same molecular formula C4H10.

Physical properties of alkanes
■ Physical properties of alkanes series:

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Number of atom Name Formula Boiling point Melting point Density Physical state at room temperature 1 Methane CH4 -162℃ -183℃ 0.42gcm3 Gas 2 Ethane C2H6 -89℃ -182℃ 0.55gcm3 Gas 4 Butane C4H10 -0.5℃ -135℃ 0.59gcm3 Gas 5 Pentane C5H12 36℃ -130℃ 0.63gcm3 Liquid 6 Hexane C6H14 69℃ -95℃ 0.66gcm3 Liquid 17 Heptadexane C17H36 302℃ NA 0.77gcm3 Solid

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

	►	Melting and boiling point will gradually be increased with the number of C atoms increases.
	►	Physical state: The first four alkanes (C1-C4) are gases, C5 alkanes is liquid, and larger alkanes (C17 and above) are expected to be solid at room temperature and pressure
	►	Density shows a gradual upward trend of change. This is because of the addition of carbon atoms in the methylene group CH2 which equivalent to an increase of 14 units of relative molecular mass.

Chemical properties of alkanes
■ Chemical properties of alkanes

	►	Alkanes are saturated hydrocarbons. Therefore, alkanes are not very reactive chemical.
	►	At room conditions, alkanes do not react with most chemicals such as dilute acid, dilute alkali, oxidizing agents or reducing agents.
	►	Alkanes are neutral. Therefore, alkanes do not change the color of blue(or red) litmus paper.

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

■ Combustion of alkanes

	►	In the event of excess oxygen, all alkanes burn completely to produce carbon dioxide, water and energy.
	►	The equation for the complete combustion of a number of alkanes. ○ CH4(g) + 2O2(g) → CO2(g) + 2H2O(l) + energy
	►	The complete combustion of alkanes to produce a lot of heat energy.
	►	Incomplete combustion of alkanes (in air, 21% oxygen) will produce carbon dioxide, carbon monoxide, water and soot.
	►	Combustion of alkanes in the excess air will produce more soot when going down the homologous series of alkanes. This is because the number of carbon atoms per molecule increases as down the homologous series of alkanes.

✍ Worked-example 2.2(a) Compare the burning of pentane (C5H12) and octane (C8H18) in terms of the soot produced. Solution: Combustion of pentane (C5H12) = $\frac{\text{mass of carbon in 1mole pentane}}{\text{mass of 1mole of pentane}}\mathrm{\times }100$ = $\frac{\mathrm{5(12)}}{\mathrm{5(12)\; +\; 12(1)}}\mathrm{\times }100$ = $\frac{60}{72}\mathrm{\times }100$ = 83.33% Combustion of octane (C8H18) = $\frac{\text{mass of carbon in 1mole octane}}{\text{mass of 1mole of octane}}\mathrm{\times }100$ = $\frac{\mathrm{8(12)}}{\mathrm{8(12)\; +\; 18(1)}}\mathrm{\times }100$ = $\frac{96}{114}\mathrm{\times }100$ = 84.21% Percentage of carbon in the octane is higher than the percentage of carbon in pentane. Therefore, liquid octane burns with more flame soot compare to pentane.

■ Substitution Reaction

	►	When a mixture of alkanes and halogen are exposed to the sunlight or ultraviolet light , a substitution reaction will occur slowly.
	►	In this substitution reaction, hydrogen atom in the alkane molecule is replaced by halogen atom.
	►	For example, the substitution reaction between methane and halogen are as follows: ○ ${\mathit{CH}}_4(g)+{\mathit{Cl}}_2(g)\stackrel{\mathit{sunlight}}{\to }{\mathit{CH}}_3\mathit{Cl}(g)+\mathit{HCl}(g)$ ○ ${\mathit{CH}}_4(g)+{\mathit{Br}}_2(g)\stackrel{\mathit{sunlight}}{\to }{\mathit{CH}}_3\mathit{Br}(g)+\mathit{HBr}(g)$ Observation: Brown coloured bromine is decolourized.
	►	Any member of the alkane will also undergo substitution reaction when mixed with chlorine gas or bromine vapour in the presence of sunlight or ultraviolet light.

■ Cracking Reaction

	►	The long chain of alkane can be broken into shorter chain (more useful) molecules through catalytic cracking.
	►	As it is catalytic cracking it requires a catalyst, and this catalyst is either alumina, Al2O3 or silica, SiO2 . These reactions also need high temperatures (450ºC is usually used).
	►	Example: ○ C6H14(g) $\underset{\mathrm{∆}}{\overset{{\mathit{Al}}_2{O}_3}{\to }}$C4H10(g) + C2H4(g)
	►	The following animation shows the cracking reaction of hexane.

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