2 b alkene Alevel考试化学课件.pptx

Chapter16b

Themechanismofaddition

Thebromineandethenearenon-polarreagents.moleculeandtheapproachingπbondintheetheneinducesadipoleinthebrominemolecule.

Inthefirststageofthereaction,oneofthebromineatomsbecomesattachedtobothcarbonatoms,withthepositivechargebeingfoundonthebromineatom.Abromoniumionisformed.

Thebromoniumionisthenattackedfromthebackbyabromideionformedinanearbyreaction.

Inthismechanism,thepolarisedbrominemoleculeshasbehavedasanelectrophile.Anelectrophileisareactantwhichisattractedtoanelectron-richcentreoratom,whereitacceptsapairofelectronstoformanewcovalentbond.Thereactionisanexampleofonewhichproceedsbyanelectrophilicadditionmechanism.

Reactionsofalkeneswithmanganate(Ⅶ)ions,MnO4-Manganate(VII)ionsareastrongoxidisingagent.

Andinthefirstinstanceoxidiseethenetoethane-1,2-diol(oldname:ethyleneglycol).Thefullequationdependsontheconditions.Underacidicconditions,themanganate(VII)ionsarereducedtomanganese(II)ions.

Whenshakenwithdiluteacidifiedordilutealkalinepotassiummanganate,alkenesreactreadilyatroomtemperature.Thepurplecolourdisappearsandadiolisformed.

Whenalkenesareheatedwithanacidifiedsolutionofpotassiummanganate,anydiolformedissplitintotwofragmentswhichareoxidisedfurthertoketonesorcarboxylicacids.

CH2=CH2→HOCH2CH2OH→CHOCHO→2CO2Ethenewillgivecarbondioxide.Insummary:

PolymerisationofalkeneDuringpolymerisation,analkeneundergoesanadditionreactiontoitself.Asonemoleculejointoasecond,alongmolecularchainisbuiltup.Thereactionsareinitiatedinvariouswaysandtheinitiatormaybecomeincorporatedatthestartofthepolymerchain.Ignoringtheinitiator,theempiricalformulaofanadditionpolymeristhesameasthealkeneitcomesform.Thistypeofreactioniscalledadditionpolymerisation.

Thealkenefrom