Alkanes
Senior 3 · Chemistry
Alkanes
Lesson 3 of 4
30 min
Open the air hole of a Bunsen burner and the gas burns with a steady, almost invisible blue flame. Close it and the flame turns tall, yellow and flickering, and a cold evaporating dish held in it comes out black with soot. The fuel has not changed: only the supply of oxygen has. Burning is the reaction that matters most in daily life, and it is one of only two reactions that alkanes take part in easily.
All alkanes are made of non-polar molecules held together by weak attractions between molecules. Almost every physical property follows from that.
Table 3.1: Physical properties of alkanes
| Property | What is observed | Reason |
|---|---|---|
| state at 25 °C | C₁ to C₄ gases; C₅ to about C₁₇ liquids; from about C₁₈ waxy solids | longer molecules attract each other more strongly |
| boiling point | rises with chain length; falls with branching | more contact between molecules, stronger attraction |
| solubility | insoluble in water; soluble in each other and in other organic solvents | alkane molecules are non-polar |
| density | liquids less dense than water (hexane 0.66 g/cm³), so they float on it | molecules are light and loosely packed |
| colour and smell | colourless; the gases have almost no smell | no groups that absorb visible light |
| electrical conductivity | do not conduct | covalent molecules, no ions or free electrons |
Because alkane liquids float on water and do not mix with it, a petrol spill spreads across a puddle as a thin film, and water must never be thrown on burning petrol or kerosene: the burning liquid floats and spreads.
Definition
Complete combustion: burning in plenty of oxygen, so that all the carbon becomes carbon dioxide and all the hydrogen becomes water. Example: a blue gas flame.
Incomplete combustion: burning in too little oxygen, so that some carbon becomes carbon monoxide or soot (carbon). Example: a yellow, smoky flame.
Alkanes burn in oxygen and give out a lot of heat: one mole of methane releases about 890 kJ. With plenty of air the products are carbon dioxide and water:
With too little air, the carbon is only partly oxidised, to carbon monoxide or to soot:
In the apparatus of Figure 3.1 the pump draws the gases from a burning candle through the cold U-tube and then through limewater. A colourless liquid collects in the U-tube: it turns white anhydrous copper(II) sulfate blue and boils at 100 °C, so it is water. The limewater turns milky, so carbon dioxide is present. The fuel must therefore contain hydrogen and carbon.
Warning
Safety: carbon monoxide kills. Carbon monoxide has no colour and no smell. It binds to the haemoglobin in red blood cells far more strongly than oxygen does, so the blood can no longer carry oxygen; headache, dizziness and sleepiness come before unconsciousness. Never burn charcoal, kerosene or gas, or run a generator, in a closed room or a closed vehicle. If someone shows these signs, move them into fresh air at once and get medical help.
Worked example
Problem. (a) Write a balanced equation, with state symbols, for the complete combustion of pentane, a liquid. (b) Calculate the mass of carbon dioxide formed when 11 g of propane burns completely. (C = 12, H = 1, O = 16)
Solution. (a) Balance C first, then H, then O. Five C give ; twelve H give ; oxygen atoms needed , which is :
(b) , so mol. From , 1 mol of propane gives 3 mol of : mol. , so mass g.
Definition
Substitution reaction: a reaction in which one atom or group in a molecule is replaced by another atom or group. Example: a hydrogen atom of methane is replaced by a chlorine atom.
Photochemical reaction: a reaction started by light, usually ultraviolet light. Example: methane and chlorine react in sunlight but not in the dark.
A mixture of methane and chlorine kept in the dark does not react. In sunlight or ultraviolet light the yellow-green colour of chlorine fades and steamy fumes of hydrogen chloride form, which turn damp blue litmus paper red. One hydrogen atom at a time is replaced by a chlorine atom:
With more chlorine, substitution goes on until every hydrogen is replaced:
The products are chloromethane, dichloromethane, trichloromethane (chloroform) and tetrachloromethane. Bromine reacts in the same way but more slowly: a few drops of bromine added to hexane keep their orange-brown colour in the dark, and lose it only slowly in bright sunlight, giving fumes of hydrogen bromide:
Alkanes do not react with acids, alkalis, or oxidising agents such as acidified potassium manganate(VII), which stays purple. Their C–C and C–H bonds are strong and they have no functional group.
Worked example
Problem. Ethane reacts with chlorine in ultraviolet light to give a product with one chlorine atom. Write the equation, name the organic product, and draw its displayed formula in words (which atoms are bonded to each carbon).
Solution. One H is replaced by Cl:
The product is chloroethane, : the first carbon carries three H atoms; the second carries two H atoms and the Cl atom; the two carbons are joined by a single bond.
Crude oil contains more long-chain alkanes than people need, and too few short ones for petrol. Heating long alkanes to about 500 °C over a catalyst such as aluminium oxide breaks them into shorter alkanes and alkenes, the family of the next unit:
Cracking is a thermal decomposition. It always gives at least one product with a C=C double bond, because there are not enough hydrogen atoms for all the products to be alkanes.
Warning
Common mistakes. (1) In combustion, balance oxygen last, and remember that each gives two oxygen atoms; a half, as in , is removed by doubling every coefficient. (2) Substitution of an alkane needs light; addition (next unit) does not. (3) Alkanes do not decolourise bromine water quickly; that fast test belongs to alkenes.
Tip
General combustion equation: . Check it with methane: n = 1 gives , and .
FuseSchool: Halogenation. Watch how ultraviolet light starts the substitution of an alkane by a halogen, and compare it with how alkenes react with halogens.
Practice
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