Alkanes and Alkenes Topic 10.2 Alkanes have low reactivity bond enthalpies are relatively strong low polarity only readily undergo combustion reactions with oxygen (very flammable) and substitution reactions with halogens in UV light 348 kJ mol-1 to break a C-C bond 412 kJ mol-1 to break a C-H bond Reactions of Alkanes: Combustion hydrocarbons (only contain C & H) complete combustion alkanes (CnH2n+2) burn in an excess supply of oxygen to form carbon dioxide and water: example: C8H18 (g) + 12 ½ O2 (g) → 8 CO2 (g) + 9 H2O (l) exothermic (-∆H) incomplete combustion if oxygen supply is limited, the gas carbon monoxide, carbon and water is formed C5H12 (l) + 5½ O2 (g) → + 5CO (g) + 6H2O (g) Reactions of Alkanes (methane and ethane) with Halogens (Cl and Br) alkanes do not react with halogens in the dark at room temperature, but will react in the presence UV radiation a substitution reaction will occur where some or all of the hydrogens will be replaced with a halogen C2H6 (g) + Br2 (g) → C2H5Br (l) + HBr (g) Cl2 + CH4 CH3Cl + HCl this happens by a process know as free radical substitution that happens in 3 steps 1. 2. 3. Initiation Propagation Termination initiation initiated by UV light breaking a chlorine molecule into two free radicals by a process called homolytical fission (* = unpaired electron) Cl2 + propagation Cl* Cl* each resulting atom receives one unpaired electron, known as free radicals that have lots of energy keeps the chain going (radical in reactants and products) CH4 + Cl* CH3* + HCl CH3* + Cl2 CH3Cl + Cl* termination this removes free radicals (*) from the system without replacing them by new ones Cl* + Cl* Cl2 CH3* + Cl * CH3Cl CH3* + CH3* CH3CH3 each resulting atom receives one unpaired electron, known as free radicals Reactions of Alkenes alkenes react with many substances to form a new substance often undergo a addition reactions (two molecules combine to make a single molecule) C2H4 + XY → CH2XCH2Y process occurs by breaking the double bond Reactions of Alkenes with hydrogen (hydrogenation) alkenes react with hydrogen gas to create an alkane C2H4 + H2 → CH3CH3 Reactions of Alkenes: with halogens (halogenation) alkenes react readily with chlorine or bromine to create a dihalogenalkane (general name) C2H4 + Cl2 → CH2Cl CH2Cl H H H H + Cl Cl Cl Cl H H H H 1,2-dichloroethane Reactions of Alkenes with hydrogen halides alkenes react readily with hydrogen halides to create a halogenalkanes C2H4 + HBr → CH3CH2Br H H H H + H Cl H Cl H H H H Reactions of Alkenes with water alkenes do not react readily with water if sulfuric acid is used as a catalyst, an alcohol will be created remember that H2O can be dissociated into H+ and OHH2SO4 C2H4 + H2O → CH3CH2OH H H ethene H H H H acid + H O H H O H H ethanol H Distinguish between alkanes and alkenes using bromine water bromine water (a redish liquid) tests for unsaturated hydrocarbons (alkenes) alkanes → stay redish no reaction alkenes → turn clear / colourless because of reaction with unsaturated hydrocarbon https://www.youtube.co m/watch?v=PE1CDR1 S5pk Reactions of Alkenes: Polymerization breaking alkene double bonds and then linking together into a chain naming polymers put “poly-” in front of the name of the monomer there are 3 polymerization mechanisms that you need to be familiar with: 1. 2. 3. polyethene polychloroethene polypropene Polyethene monomer: ethene CH2=CH2 undergoes addition reactions with itself to make a chain n CH2=CH2 [-CH2-CH2-]n Polychloroethene each chloroethene contains 1 chlorine therefore when the chloroethene molecules polymerize, every other carbon will bond to 1 chlorine this is PVC Polypropene + = Teflon non-stick pans