LiroxtronIV (LiroxtronIV)-
state criteria for cis-trans isomerism in alkenes
1. restricted rotation about C=C db due to presence of π bond
2. 2 diff. groups attached to each C of C=C db
cis-isomer -> identical groups are on same side of db
trans-isomer -> identical groups are on opp. sides of db
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why b.p/m.p increase as no. of C atoms in alkenes increase?
no. of e- per molecule to be polarised increases as no. of C increases >
increasing amount of energy is required to overcome increasing strength of id-id attractions b/w molecules
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why b.p of branched alkenes decreases as branching increases?
branched molecules > spherical in shape, smaller SA of contact b/w branched molecules >
< energy required to overcome < extensive id-id attraction b/w branched molecules
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why alkenes undergo electrophilic addition?
C=C db is e- rich, attracts electrophiles >
π bond weaker than σ bond, alkenes undergo addition involving breaking of π bond >
alkenes react using π e- to form bonds, σ bond left intact
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prepare C=C from ROH
eliminate H2O from ROH
reaction: elimination
r&c: excess conc. H2SO4, heat OR
Al2O3, high temp.
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prepare C=C from RX
eliminate HX from RX
reaction: eliminationr&c: KOH, ethanol, heat(ethanolic KOH, heat)
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C=C react with HX
alkene + HX -> RX
reaction: electrophilic addition
r&c: dry HX(g), rtp
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describe mechanism of electrophilic addition(EA>GO)
Green Ogres
Generation of carbocation, step 1(SLOW)
< electronegative atom, partially +ve, > electronegative atom, partially -ve >
π e- from e--rich C=C attacks e--deficient atom, forming bond >
leaves other C atom w/ full +ve charge, forming carbocation >
2 e- move to partially -ve atom forming anion
Opposite charges react, step 2(FAST)
e--rich anion from step 1 attacks e--deficient C+ of carbocation by donating e- pair >
forms bond, yielding electrically neutral pdt
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C=C react with X2
alkene + X2 -> dihalogenoalkane
reaction: EA
r&c: X2 in inert organic solvent(CCl4)
obs: decolourises
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C=C react with X2(aq)
alkene + X2 + H2O -> halohydrin + HX
reaction: EA
r&c: X2(aq)
obs: decolourises
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C=C react with H2O(g)
alkene + H2O -> ROH
reaction: EA
r&c: steam, H3PO4 catalyst, high temp, high pressure
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reduction of C=C to form alkanes
alkene + H2 -> alkane
reaction: reduction
r&c: H2(g), Ni catalyst, heat
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alkenes oxidised to form diol
reaction: mild oxidation
r&c: cold KMnO4(aq), NaOH(aq)
obs: purple KMnO4 turns green > formation of brown ppt. of MnO2
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alkenes undergo strong oxidation
reaction: strong oxidation
r&c: KMnO4(aq), dilute H2SO4(aq), heat
obs: purple KMnO4 decolourises, if CO2 evolved, gas evolved forms white ppt. w/ limewater
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identify C=C bond
test: add few drops of Br2(aq) to each sample separately.
obs: orange Br2 decolourises
OR
test: add few drops of KMnO4(aq), H2SO4(aq) to each sample separately & heat
obs: purple KMnO4 decolourises
OR
test: add few drops of cold, KMnO4(aq), NaOH(aq) to each sample separately
obs: purple KMnO4 turns green, brown ppt formed
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identify =CH2 group
test: carry out strong oxidation, pass gas evolved through limewater
obs: purple KMnO4 decolourises, CO2 evolved forms white ppt. w/ limewater
