Flashcards: H2 Chemistry 11 - 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

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

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

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

eliminate H2O from ROH

reaction: elimination

r&c: excess conc. H₂SO₄, heat OR

Al₂O₃, high temp.

eliminate HX from RX

reaction: eliminationr&c: KOH, ethanol, heat(ethanolic KOH, heat)

alkene + HX -> RX

reaction: electrophilic addition

r&c: dry HX(g), rtp

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

alkene + X₂ -> dihalogenoalkane

reaction: EA

r&c: X₂ in inert organic solvent(CCl₄)

obs: decolourises

alkene + X₂ + H₂O -> halohydrin + HX

reaction: EA

r&c: X₂(aq)

obs: decolourises

alkene + H₂O -> ROH

reaction: EA

r&c: steam, H₃PO₄ catalyst, high temp, high pressure

alkene + H₂ -> alkane

reaction: reduction

r&c: H2(g), Ni catalyst, heat

reaction: mild oxidation

r&c: cold KMnO₄(aq), NaOH(aq)

obs: purple KMnO₄ turns green > formation of brown ppt. of MnO₂

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

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

test: carry out strong oxidation, pass gas evolved through limewater

obs: purple KMnO₄ decolourises, CO₂ evolved forms white ppt. w/ limewater