AS 1.7 L4 Reactions of Alkanes Outcomes – To be able to describe the reactions of alkanes in terms of combustion and substitution by chlorine showing the mechanism of free radical substitution. (1.7.2e) Independent Learning (HW) – pp 116-123 and ALL questions therein Connector: Explain as fully as possible the difference between a saturated hydrocarbon and an unsaturated hydrocarbon. Draw out the displayed formulae of a pair of products formed by cracking decane Crowe2008

Saturated or Unsaturated? Saturated means “full up”. Alkanes are saturated. Every carbon atom has already used all four of it’s bonds to join to four other atoms. No other atoms can be added. Alkenes are unsaturated. They have a double bond that could instead become two single bonds. This means that other atoms can be added. It is not “full up”.

Draw out the displayed formulae of a pair of products formed by cracking decane Heat pressure catalyst + ethene H C octane

Alkanes are saturated compounds and are generally quite unreactive, however they can be burnt, or cracked, and under the right conditions will react with certain halogens. Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water. Write balanced equations for the complete combustion of (a) propane, (b) butane or

Notes on combustion The hydrocarbons become harder to ignite as the molecules get bigger. This is because the bigger molecules don't vaporise so easily. Bigger molecules have greater Van der Waals (weak intermolecular) attractions which makes it more difficult for them to break away from their neighbours and turn to a gas. Provided the combustion is complete, all the hydrocarbons will burn with a blue flame. The bigger the hydrocarbon, the more likely you are to get a yellow, smoky flame, due to incomplete combustion.

2. Incomplete combustion Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide. The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke. Carbon monoxide is produced as a colourless poisonous gas. What’s the problem with CO? Carbon monoxide binds irreversibly with haemoglobin in red blood cells and so reduces the bloods ability to carry and supply oxygen to cells.

3. The chlorination of methane When chlorine and methane are mixed in the dark nothing happens. If they are exposed to uv light from a lamp or the sun, then a series of substitution reactions occur to produce a mixture of products: This is an example of a photochemical reaction. Why is this called a substitution reaction?

Explaining the chlorination of methane – reaction mechanism The mechanism involves a chain reaction. During a chain reaction, for every reactive species you start off with, a new one is generated at the end - and this keeps the process going. The over-all process is known as free radical substitution, or as a free radical chain reaction.

This process is known as homolytic fission. Chain initiation The chain is initiated (started) by UV light breaking a chlorine molecule into free radicals. unpaired electron Cl2                 2Cl   chlorine molecule chlorine radicals The Cl-Cl bond breaks. One electron goes to each atom The curly half-arrows are used to show the movement of one electron. Cl Cl Cl Cl This process is known as homolytic fission.

Chain propagation reactions These are the reactions which keep the chain going. CH4  +  Cl                  CH3    +  HCl chlorine radical methyl radical CH3    +  Cl2                CH3Cl  +  Cl   chlorine radical The new chlorine radical can then react with methane or chloromethane molecules. CH3Cl    +  Cl                CH2Cl  +  HCl   CH2Cl    +  Cl2                CH2Cl2  +  Cl   And so on until CCl4 is formed

Chain propagation reactions - using curly arrows H3C H + Cl CH3 + H-Cl Homolytic fission New HCl bond formed from electron from chlorine radical and one of the electrons in the C-H bond Draw the curly arrows for the next step which is the methyl radical reacting with a chlorine molecule CH3    +   Cl Cl                CH3 Cl  +  Cl  

Chain termination reactions These are reactions which remove free radicals from the system without replacing them by new ones. 2Cl                  Cl2 CH3    +  Cl                  CH3Cl CH3    +  CH3                  CH3CH3

Further information about the reaction between chlorine and methane The propagation steps occur extremely rapidly, and can produce an explosive chain reaction. If the amount of chlorine available is limited, chloromethane and hydrogen chloride are the two main products. If there is lots of chlorine available a mixture of all possible products is obtained. Explain why. Note: Trichloromethane (chloroform) was one of the first anaesthetics to be used. Tetrachloromethane (carbon tetrachloride) was widely used as a solvent e.g. in dry cleaning, but is no longer used since it is a carcinogen.