Achem6

Achem6 ALKANES
general MF: C_nH_2n+2 (n > 0)
functional group: none (all C atoms sp³-hybridsed > saturated)
isomerism: structural- > 3 C atoms, optical- > 6 C atoms

Physical properties
-1-4 C atoms: gases, 4-17 C atoms: liquids, > 17 C atoms: gas
-mp, bp & densities ^ as Mr ^
-non-polar organic cpds, insoluble in water, soluble in organic solvents

Chemical properties
-chemically inert, don't react w/ (oxidizing / reducing agents / alkali / acid) under normal lab conditions
-typical reactions: combustion + substitution

Combustion
alkanes: hydrocarbons(H/C), in excess O₂ > CO₂ + H₂O (incomplete combustion: O₂ limited > CO + C soot)
C_nH_2n+2 + [(3n+1)/2] O₂ (g) > n CO₂ (g) + (n+1) H₂O (l: rtp / g: >100°C)

Combustion of petrol in car engine
gasoline (5-12 C) + air (78% N, 21% O, 1% rare gases) > air + fuel mixture > (combustion) >
products:
-non-pollutants: CO₂(g) + N₂(g) + rare gases + H₂O(g)
-pollutants: CO(g) + NO₂(g) + unburnt petrol + PbBr₂(s- leaded petrol only)

leaded petrol	unleaded petrol
anti-knocking agent: TEL:tetraethyl lead- Pb(CH₂CH₃)₄ TEL provides ethyl free radicals which react w/ alkanes in petrol > branched molecules which can burn more completely (lower bp) > prevents "knocking of engine" caused by preignition of residual petrol Pb(CH₂CH₃)₄ > Pb + 4CH₃CH₂ Pb can react w/ O₂ at high temp of engine > PbO(s), 1,2-dibromoehtane added to petrol > provide Br to react w/ Pb > PbBr₂ which is more volatile > vapour instead of solid	anti-knocking agent: MTBE: methyltertiarybutyl ether- CH₃OC(CH₃)₃ MTBE provides methyl free radical & tertiary butyl free radical(2-methylpropyl free radical) which react w/ alkanes in petrol > branched molecules > prevents "knocking of engine"
combustion products non-pollutants: CO₂ (g), N₂(g), H₂O(l), rare gases pollutants: CO(g), NO₂(g), H/C unused, PbBr2(from leaded petrol- 'poisons' catalytic activity of catalysts > catalytic converter useless)	combustion products non-pollutants: CO₂ (g), N₂(g), H₂O(l), rare gases pollutants: CO(g), NO₂(g), H/C unused(l) pollutants > rhodium-platinum catalyst in catalytic converter > non-pollutants -2CO(g)+ 2NO₂(g) > 2CO₂ (g) + N₂(g) -H/C(l) + N₂(g) > CO₂(g) + H₂O(l) + N₂(g)

leaded petrol

unleaded petrol

anti-knocking agent:
TEL:tetraethyl lead- Pb(CH₂CH₃)₄
TEL provides ethyl free radicals which react w/ alkanes in petrol > branched molecules which can burn more completely (lower bp) > prevents "knocking of engine" caused by preignition of residual petrol Pb(CH₂CH₃)₄ > Pb + 4CH₃CH₂

Pb can react w/ O₂ at high temp of engine > PbO(s), 1,2-dibromoehtane added to petrol > provide Br to react w/ Pb > PbBr₂ which is more volatile > vapour instead of solid

anti-knocking agent:
MTBE: methyltertiarybutyl ether- CH₃OC(CH₃)₃
MTBE provides methyl free radical & tertiary butyl free radical(2-methylpropyl free radical) which react w/ alkanes in petrol > branched molecules > prevents "knocking of engine"

combustion products
non-pollutants: CO₂ (g), N₂(g), H₂O(l), rare gases
pollutants: CO(g), NO₂(g), H/C unused, PbBr2(from leaded petrol- 'poisons' catalytic activity of catalysts > catalytic converter useless)

combustion products
non-pollutants: CO₂ (g), N₂(g), H₂O(l), rare gases
pollutants: CO(g), NO₂(g), H/C unused(l)
pollutants > rhodium-platinum catalyst in catalytic converter > non-pollutants
-2CO(g)+ 2NO₂(g) > 2CO₂ (g) + N₂(g)
-H/C(l) + N₂(g) > CO₂(g) + H₂O(l) + N₂(g)

Substitution (photochemical reaction)-H atom replaced by Cl/Br atom, occurs only in sunlight / UV rays
-Chlorination of methane:
reagent: Cl₂(g), conditions: room temp, sunlight/UV light, equation: CH₄+Cl₂ >(UV light/sunlight)> CH₃Cl + HCl
if excess Cl₂ > di-, tri-, tetra- substituted products formed (CHCl₂,CHCl₃,CCl₄)
--Mechanism: occurs through free radical (FR) substitution involving a 3 step-chain reaction
Chain initiation step: Cl₂(g): homolytic fission > Cl FR (Cl FR = Cl atoms) [Cl-Cl >(UV)> 2Cl

]
Chain propagation step: FR reacts w/ molecule > product + new FR (FR produced continues reaction)
[CH₄ + Cl

> CH₃

+ HCl => CH₃

+ Cl₂ > CH₃Cl + Cl

]
Chain termination step: 2 FR react > molecule w/ no FR > finally no more FR > reaction stops
[CH₃

+ CH₃

> CH₃CH₃, Cl

+ Cl

> Cl₂,
-bromination: same conditions as chlorination, observation: if excess org cpd used: reddish brown Br₂ (l) disappears (product = colourless)
Sources of alkanes: Crude oil (petroleum)
crude oil: mixture of alkanes of diff bp > allows separation by fractional distillation (in fractionating towers)

Fraction	C atoms	BP (°C)	Uses
petroleum gases	1 - 4	-160 - ~0.5	fuel: cooking gas
light petroleum	5 - 6	20 - 60	solvent
light naptha	6 - 7	60 - 100	solvent
gasoline (petrol)	5 - 12	40 - 200	fuel: car
kerosene	12 - 18	175 - 325	fuel: aircrafts
gas oil (diesel)	18 - 25	275 - 400	fuel: diesel engine
lubricating oil	20 - 40	non-volatile	lubricating machine
paraffin wax	25 - 40	solid	polishing waxes
bitumen (asphalt)	> 30	solid	road surfacing + prevent roof leakage

Catalytic cracking
cracking: larger org molecules broken down > smaller org molecules using temp, pressure + catalyst
large alkane molecule > smaller alkane molecule + alkene
C₁₂H₂₆ >(N₂O₃ catalyst, 400°C, 10 atm)> C₈H₁₈ + C₄H₈
larger fractions (gas oil) > more useful fraction (gasoline)

Catalytic reforming
converts straight-chained alkanes into aromatic HC (w/ benzene ring) using cyclisation & dehydrogenation
Cyclisation: alkanes of straight C chain converted to aliphatic cyclic cpd(ring cpd)
hexane >(Pt catalyst, 500°C, 10-15atm)> cyclohexane + H₂ gas
Dehydrogenation: 6 H atoms removed from cyclohexane > benzene cpd + 3H₂

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