general MF: CnH2n (n =2,3,..) functional group: carbon-carbon double bond C=C, unsaturated HC isomerism: CnH2n: cycloalkanes + alkenes (alkenes > strucral, cis/trans + optical isomers, optical: > 5 C atoms)
Formation of alkenes (1)Catalytic cracking of alkanes (2)Elimination reactions:
-(i)Dehydration of alcohol: H2O molecule removed from alcohol
reagent: conc H2SO4, conditions: heat under reflux at ~180°C
(refluxing: prevents loss of volatile organic materials)
-(ii)Dehydrohalogenation of halogenoalkane: HX molecule removed from halogenoalkane > alkene + HX, HX is acidic => HX + NaOH > H2O + NaX
reagent: ethanolic NaOH, conditions: heat under reflux Physical properties -2-4 C atoms: gases, 5-15 C atoms: liquids, > 15 C atoms: solids
-mp, bp, density ^ w/ RMM ^
-non-polar org cpds, insoluble in water, soluble in organic solvents
Chemical properties -alkenes are chemically reactive, typical reactions: addition, oxidation, polymerisation, combustion Addition: due to C=C bond, π e-s: e- source for electrophiles > electrophilic addition
-Halogenation: addition of halogen X2 to C=C double bond
Bromination of ethene reagent: Br (l) in solvent, CCl4
condition: room temp, no strong light (prevent free radical substitution)
equation: CH2=CH2 + Br2 >(room temp)> CH3BrCH3Br [ethene + bromine > dibromoethane]
observation: red Br liquid decolourised Mechanism
bromination of ethene: electophilic addition involving formation of carbocation (or carbonium ion) intermediate
-ethene molecule approachers Br molecule, π e- of C=C double bond repel shared pair of e- in Br molecule > dipole induced > electrophilic part of Br molecule(Br+) accepts pair of π electrons & adds to double-bond C atom > +ve charge carbocation intermediate
-nucleophile, :Br- adds to +ve C centre of carbocation > 1,2-dibromoethane
Existence of carbocation intermediate shown by the following reactions
-ethene + Br(aq) > 1,2-dibromoethane + 2-bromoethanol formed
formation of 2-bromoethanol: due to reaction bet water molecule & carbocation
-ethene + some NaCl(aq) + Br(aq) > 1,2-dibromoethane + 2-bromoethanol + 1-bromo-2-chloroethane formed
Morkownikoff's rule when unsymmetrical reagent AB added to unsymmetrical alkene, the more electronegative part of reagent will add to more substituted double bond C atom (C atom w/ least # of H atoms attached directly to it)
Catalytic hydration of alkene to form alcohol reagent: steam, H2O(g)
conditions: catalyst: H3PO4, absorbed in SiO2(s)
temperature: ~300°C
pressure: ~70atm
general equation:
Industrial manufacturing of ethanol
petroleum >(fractional distillation)> gas oil fraction >(cracking)> gasoline fraction + ethene
ethene >(catalytic hydration)>ethanol
Catalytic hydrogenation reagent: hydrogen gas, H2(g)
conditions: catalyst: finely divided Ni powder
temperature: ~140°C
general equation:
observation: if pressure gauge fitted to reaction vessel: pressure reading decreases
Industrial application of catalytic hydrogenation vegetable oil(l) + H2(g) >(Ni+140°C)> margarine (solid) [solid as increase in RMM > greater VDW forces]
Oxidation
Mild oxidation of alkene to form diol reagent: dil, acidified potassium manganate (VII), KMnO4/H+(aq)
condition: room temp
general equation:
observation: excess alkene > purple of KMnO4 disappears
ethane-1,2-diol = glycol: anti-freeze in car radiators, make polyester, terylene Vigorous oxidation of alkene
under vigorous oxidation: double bond cleaved > forms alkanoic acid/ ketone/ CO2, depending on reactants
reagent: conc acidified KMnO4/H+(aq)
condition: heat under reflux
equation:
(methanoic acid: only acid combustible to H2O & CO2)
, alkanoic acid
, ketone
ethane-1,2-diol = glycol: anti-freeze in car radiators, make polyester, terylene
