ALKENES
general MF: C_nH_2n (n =2,3,..)
functional group: carbon-carbon double bond C=C, unsaturated HC
isomerism: C_nH_2n: 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, 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, 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 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


Addition of hydrogen halide, HX
reagent: conc HX (aq)
condition: room temp
equation:


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)

structural part of alkene	vigorous oxidation product
	CO2 + H2O
	, alkanoic acid
	, ketone

Addition polymerisation
alkenes or substituted alkenes >(addition polymerisation)> addition polymers (plastics)

Combustion
C_nH_2n + 1.5nO₂(g)(excess) > nCO₂(g) + nH₂O


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