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:  C 8 H 18 (g) + 12 ½ O 2 (g) → 8 CO 2 (g) + 9 H 2 O (l) exothermic (-∆H) incomplete combustion if oxygen supply is limited, the gas carbon monoxide and carbon is formed C 8 H 18 (l) + O 2 (g) → C (s) + CO (g) + CO 2 (g) + H 2 O (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 C 2 H 6 (g) + Br 2 (g) → C 2 H 5 Br (l) + HBr (g) Cl 2 + CH 4  CH 3 Cl + HCl

this happens by a process know as free radical substitution that happens in 3 steps 1. Initiation 2. Propagation 3. Termination

initiation initiated by UV light breaking a chlorine molecule into two free radicals by a process called homolytical fission (* = unpaired electron) Cl 2  Cl * + Cl * propagation keeps the chain going (radical in reactants and products) CH 4 + Cl *  CH 3 * + HCl CH 3 * + Cl 2  CH 3 Cl + Cl * termination this removes free radicals (*) from the system without replacing them by new ones Cl * + Cl *  Cl 2 CH 3 * + Cl *  CH 3 Cl CH 3 * + CH 3 *  CH 3 CH 3 each resulting atom receives one unpaired electron, known as free radicals that have lots of energy

each resulting atom receives one unpaired electron, known as free radicals

Reactions of Alkenes (Topic ): 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: C 2 H 4 + XY → CH 2 XCH 2 Y 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: C 2 H 4 + H 2 → CH 3 CH 3

Reactions of Alkenes: with halogens alkenes react readily with chlorine or bromine to create a dihalogenalkane (general name) C 2 H Cl 2 → CH 2 Cl CH 2 Cl 1,2-dichloroethane

Reactions of Alkenes (10.3.2) with hydrogen halides alkenes react readily with hydrogen halides to create a halogenalkanes C 2 H 4 + HBr → CH 3 CH 2 Br

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 H 2 O can be dissociated into H+ and OH- C 2 H 4 + H 2 O → CH 3 CH 2 OH

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 /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: 1. polyethene 2. polychloroethene 3. polypropene

Polyethene monomer: ethene CH 2 =CH 2 undergoes additions reactions with itself to make a chain n CH 2 =CH 2  [-CH 2 -CH 2 -] 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