Alkanes and Alkenes

Alkanes and Alkenes Topic 10.2 and 10.3

Alkanes • have low reactivity • bond enthalpies are relatively strong • 348 kJ mol-1 to break a C-C bond • 412 kJ mol-1 to break a C-H bond • low polarity • only readily undergo combustion reactions with oxygen (very flammable) and substitution reactions with halogens in UV light

Reactions of Alkanes: Combustion • hydrocarbons (only contain C & H) • complete combustion • alkanes 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 and carbon is formed C8H18 (l) + O2 (g) →C (s) + CO (g) + CO2 (g) + H2O (l) (notice left over carbon (black soot) and dangerous CO)

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 of sunlight (UV) • 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 • Initiation • Propagation • Termination

each resulting atom receives one unpaired electron, known as free radicals that have lots of energy • initiation • initiated by UV light breaking a chlorine molecule into two free radicals by a process called homolytical fission (* = unpaired electron) Cl2 Cl* + Cl* • propagation • 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 (Topic 10.3.1):the general mechanism • alkenes react with many substances to form a new substance • catalysts, acids or other substances may be required to complete the reaction: C2H4 + XY → CH2XCH2Y • process occurs by breaking the double bond.

Reactions of Alkenes (10.3.2) : with hydrogen • alkenes react with hydrogen gas to create an alkane, using nickel as a catalyst at 150ºC: C2H4 + H2→ CH3CH3

Reactions of Alkenes: with halogens • alkenes react readily with chlorine or bromine to create a dihalogenalkane(general name) C2H4 + 2 Cl2→ CH2Cl CH2Cl 1,2-dichloroethane

Reactions of Alkenes (10.3.2)with hydrogen halides • alkenes react readily with hydrogenhalides to create a halogenalkanes C2H4 + HBr→ CH3CH2Br

Reactions of Alkenes with water • alkenes do not react readily with water. • if sulfuric acid is used as a catalyst, an alcohol to be created • remember that H2O can be dissociated into H+ and OH- C2H4 + H2O → CH3CH2OH

Distinguish between alkanes and alkenes using bromine water • bromine water (a red liquid) tests for unsaturated hydrocarbons (alkenes) • alkanes → stay yellow/orange • no reaction • alkenes → turn clear / colourless • because of reaction with unsaturated hydrocarbon http://www.youtube.com/watch?v=NjIuBvod2eM

Reactions of Alkenes: Polymerization • naming polymers • put “poly-” in front of the name of the monomer • there are 3 polymerization mechanisms that you need to be familiar with: • polyethene • polychloroethene • polypropene

Polyethene • monomer: ethene CH2=CH2 • undergoes additions 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

Outline the economic importance of the reactions of alkenes (10.3.5) • making margarine • hydrogenation (addition of H) of vegetable oils (alkenes)

making ethanol • ethene + water • drink it or use as fuel

making plastics • Teflon • tetrafluoroethene • PVC • polychloroethen

Alkanes and Alkenes