Alkanes and Alkenes

Property Alkane Alkene Reactions involve heterolytic fisson Free Radical Substitution reaction Formed during cracking reactions Exist as Stereoisomers Electrophilic addition reactions take place Exist as Structural isomers Exist as Cis/trans or E/Z isomers Has 1 pi Molecular orbital Has all single bonds Decolourises bromine in absence of UV Has sigma molecular orbitals Reactions involve homolytic fisson Substitution reactions occur Addition reactions occur Burn to give carbon dioxide and water in a good supply of air Undergo polymerisation reactions

Processing Alkanes CRACKING, REFORMING, ISOMERISATION How is it carried out? C – Al2O3 and heat R – Pt/Rh and heat I – Pt separated by zeolyte catalyst and heat What are the products? C – Smaller Alkanes AND alkenes R – Cyclic alkanes and hydrogen I – Branched alkane isomers of original Why is it carried out? C – Smaller Alkanes used as fuels, alkenes used to make polymers R – Lowers bp of alkane so more vapour at same temp so better ignition (Weaker VDW as smaller molecule) I – Lowers bp of alkane so more vapour at same temp so better ignition (Weaker VDW as molecules pack together less well)

Free radical substitution chlorination of methane i.e. homolytic breaking of covalent bonds Overall reaction equation CH4 + Cl2 CH3Cl + HCl Conditions ultra violet light excess methane to reduce further substitution

Free radical substitution mechanism ultra-violet Cl2 Cl + Cl initiation step two propagation steps CH4 + Cl CH3 + HCl CH3 + Cl2 CH3Cl + Cl termination step CH3 + Cl CH3Cl CH3 + CH3 CH3CH3 minor termination step

Further free radical substitutions Overall reaction equations CH3Cl + Cl2 CH2Cl2 + HCl CH2Cl2 + Cl2 CHCl3 + HCl CHCl3 + Cl2 CCl4 + HCl Conditions ultra-violet light excess chlorine

Electrophilic addition bromine with ethene Overall reaction equation CH2=CH2 + Br2 CH2BrCH2Br 1,2-dibromoethane mechanism

Electrophilic addition mechanism bromine with ethene H C H C Br + carbocation δ+ δ- Br Br - H C Br Br 1,2-dibromoethane reaction equation

Electrophilic addition 2 Hydrogen bromide with propene Overall reaction equation CH3CH2CH2Br CH3CH=CH2 + HBr 1-bromopropane or CH3CHBrCH3 Conditions DRY HBr in Ethanol 2-bromopropane mechanism

Hydrogen bromide with Propene Electrophilic addition mechanism Hydrogen bromide with Propene H CH3 C H CH3 C + Br H δ+ δ- Br - H CH3 C Br Br H δ+ δ- 1-bromopropane reaction equation

Hydrogen bromide with Propene Electrophilic addition mechanism Hydrogen bromide with Propene H CH3 C H CH3 C + Br H δ+ δ- Br - H CH3 C Br Br H δ+ δ- 2-bromopropane reaction equation