H2 Chemistry 13 - Arenes

each benzene C sp² hybridised >

each C use 2 sp² orbitals form 2 σ bonds w/ 2 other C atoms(head-on overlap) >

remaining sp² form σ bond w/ 1s orbital of H >

each C unhybridised p overlap sideways w/ 2 adjacent p orbitals >

forms 2 continuous e^- clouds -> π e^- cloud

delocalised π e^- cloud confers extra stability >

generally unreactive to electrophiles >

catalyst required to generate stronger electrophile(full +ve charge)

each benzene C sp² hybridised >

each C use 2 sp² orbitals form 2 σ bonds w/ 2 other C atoms(head-on overlap) >

remaining sp² form σ bond w/ 1s orbital of H >

each C unhybridised p overlap sideways w/ 2 adjacent p orbitals >

forms 2 continuous e^- clouds -> π e^- cloud

delocalised π e^- cloud confers extra stability >

addition makes C sp3 hybridised, lose π e^- system >

to preserve stability, undergoes substitution

Electrophilic substitution

r&c: X₂, anhydrous AlX₃/FeX₃, heat

G; X₂ + AlX₃/FeX₃ -> X⁺ + AlX₄^-/FeX₄^-

R; HX formed, AlX₃/FeX₃ regenerated

catalyst anhydrous; if (aq), will hydrolyse in H₂O, catalyst destroyed

accept lp of e^- from X₂ >

form stronger electrophile X⁺

Electrophilic substitution

r&c: conc HNO₃, conc H₂SO₄, 50^oC

mono-nitration @ 50^oC, further subn > temp

G; HNO₃ + H₂SO₄ -> NO₂⁺ + HSO₄^- + H₂O

F; arrow directed to N, where +ve charge is

acts as Bronsted-Lowry acid >

protonate HNO₃ >

generate stronger electrophile NO₂⁺

Electrophilic substitution(Friedel-Crafts Alkylation)

r&c: RX, anhydrous AlX₃, heat

higher temp, favour polysubbed pdt

G; RX + AlX₃ -> R⁺ + AlX₄^-

F; arrow directed to C, where +ve charge is

R; HX formed

act as Lewis acid >

accept e^- pair from RX >

generate stronger electrophile R⁺

GFR, Green Fucking Ranger

Generate stronger electrophile

rxt + catalyst -> electrophile + reacted catalyst

Form arenium cation

benzene + electrophile -> arenium ion

*slow, + within ring, ring cut @ area where sp² orbital lost, -ve e- cloud attacks electrophile(arrow)

Restore aromaticity, Regenerate catalyst

arenium ion + reacted catalyst -> pdt + catalyst

*fast, to restore aromaticity, e^- donated from H to ring(arrow), catalyst regenerated

FRS

r&c: limited X₂(g), UV light

Oxidation

r&c: KMnO₄(aq), H₂SO₄(aq), heat

alkylbenzene + 3[O] -> benzoic acid + H₂O

obs: purple KMnO₄ decolourises, white

OR

r&c: KMnO₄(aq), NaOH(aq), heat

alkylbenzene + 3[O] -> sodium benzoate + 2H₂O

obs: purple KMnO₄ turns green then forms brown ppt

FRS; side-chain when limited X₂(g), UV light

electrophilic substitution; aromatic nucleus when X₂(g), anhydrous FeX₃/AlX₃, heat

EDG(2,4 directing) - ring-activating, donate e- to π e^- cloud >

↑ e^- density in ring >

benzene ring > e^--rich, > reactive to electrophilic attack >

requires milder conditions, i.e.

mono-nitration of EDG-mono-subbed arenes, requires temp <30^oC

EWG(1,3 directing) - ring-deactivating, take e^- away from π e^- cloud >

↓ e^- density in ring >

benzene ring < e^--rich, < reactive to electrophilic attack >

requires harsher conditions, i.e.

mono-nitration of EWG-mono-subbed arenes, requires temp >50^oC

CO; prevents transport of O₂ around body

NO, NO₂; acidic gas, forms acid rain causing corrosion,causes respiratory problems in humans, interfere w/ plant metabolism, in strong sunlight forms photochemical fog causing lung damage

N₂O; enhanced greenhouse effect

SO₂, SO₃; acidic gas, forms acid rain causing corrosion

unburnt hydrocarbons; in strong sunlight forms photochemical smog causing lung damage, enhanced greenhouse effect

Greenhouse gases trap infrared radiation from earth's surface >

trap heat in atmosphere, increases temp >

deforestation, burning of fossil fuels lead to enhanced greenhouse effect, result in global warming >

melts ice caps, rising sea levels; affects wildlife, environment

fossil fuel; formed when dead organisms buried underground subjected to high temp, pressure >

takes millions of years to form, consumed faster than replaced >

hence finite, non-renewable resource

thus, recycling of plastic important;

plastic require large quantities of non-renewable fossil fuels >

non-biodegradable, takes up scarce land spaces(landfills), incineration releases toxic gases >

recycling reuses plastic waste as feedstock for new plastic products >

lowers environmental impact

Electrophilic substitution(Friedel-Crafts Acylation)

r&c: RCOCl, anhydrous AlCl₃, heat

G; RCOCl + AlCl₃ -> RCO⁺ + AlCl₄^-

R; HCl formed

test: add few drops of KMnO₄(aq), H₂SO₄(aq) to each sample separately, heat

obs: purple KMnO₄ decolourises, white ppt of benzoic acid formed upon cooling

NaOH(aq) used; purple KMnO₄ turns green, then brown ppt forms

test: add few drops of KMnO₄(aq), H₂SO₄(aq) to each sample separately, heat;

pass any gas evolved thru limewater.

obs: for both, purple KMnO4 decolourises, white ppt of benzoic acid formed upon cooling;

ethylbenzene; CO₂ gas evolved forms white ppt w/ limewater