ALKENE_REACTIONS - Alkenes and Alkynes Addition Reactions...

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© E.V. Blackburn, 2008 Alkenes and Alkynes Addition Reactions
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© E.V. Blackburn, 2008 Reactions of alkenes - catalytic hydrogenation + H 2 H C C H Pt, Pd or Ni Heat of hydrogenation - the heat liberated during this reaction. H is ~125 kJ/mol for each double bond in the compound.
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© E.V. Blackburn, 2008 Mechanism of alkene hydrogenation catalyst H 2 adsorbed at surface complexation of alkene to catalyst hydrogen insertion catalyst alkane H 2 H H H H H H + H H
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© E.V. Blackburn, 2008 Mechanism of alkene hydrogenation Hydrogenation is stereospecific. The two hydrogens add to the same side of the double bond - a syn addition.
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© E.V. Blackburn, 2008 C C π π The π electrons are less tightly held than the σ electrons. The double bond therefore acts as a source of electrons - a base – a nucleophile. It reacts with electron deficient compounds - acids - electrophiles. Reactions of alkenes
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© E.V. Blackburn, 2008 Electrophilic addition The typical reaction of alkenes is one of electrophilic addition - an acid - base reaction. C C + YZ C C Y Z Don’t forget that free radicals are electron deficient. They undergo addition reactions with alkenes. Electron seeking reagents are called electrophilic reagents.
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© E.V. Blackburn, 2008 Addition of hydrogen halides + HX H C C X HX = HCl, HBr, HI
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© E.V. Blackburn, 2008 Addition of hydrogen halides H 2 C=CH 2 + HCl CH 3 CH 2 Cl only one product is possible, chloroethane but .... CH 3 -CH=CH 2 ? CH 3 -CH=CH 2 H-Cl CH 3 CH 2 CH 2 Cl Only 2-chloropropane is formed CH 3 -CH=CH 2 CH 3 -CH-CH 3 Cl Cl-H
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© E.V. Blackburn, 2008 Markovnikov’s rule In 1869, Markovnikov proposed that in the addition of an acid to an alkene, the hydrogen of the acid bonds to the carbon which is already bonded to the greater number of hydrogens. H H CH 3 CH 3 HCl H 3 C C CH 3 Cl CH 3
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© E.V. Blackburn, 2008 CH 3 CH 2 CH=CHCH 3 + HI CH 3 CH 2 CHICH 2 CH 3 + CH 3 CH 2 CH 2 CHI CH 3 Each carbon of the double bond is bonded to one H therefore both isomers are formed. Markovnikov’s rule
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© E.V. Blackburn, 2008 Markovnikov addition - a regioselective reaction These reactions are said to be regioselective because only one of the two possible directions of addition occurs. Regioselectivity - the preferential formation of one isomer in those situations where a choice is possible.
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© E.V. Blackburn, 2008 HBr - the peroxide effect 1933, Kharasch and Mayo CH 3 -C=CH 2 CH 3 HBr H 3 C C CH 3 CH 3 Br H 3 C C CH 3 CH 2 Br H absence of peroxides (-O-O-) presence of peroxides
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© E.V. Blackburn, 2008 Addition of sulfuric acid CH 3 CHCH 3 OSO 3 H H 2 O CH 3 CHCH 3 OH CH 3 CH=CH 2 H 2 SO 4 80% cold
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© E.V. Blackburn, 2008 Hydration a Markovnikov addition CH 3 CH 3 H H + H 2 O H + C C H H H OH CH 3 CH 3
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© E.V. Blackburn, 2008
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