Transcript Reactions of alkenes
Alkenes and Alkynes Addition Reactions © E.V. Blackburn, 2011
Reactions of alkenes catalytic hydrogenation + H 2 Pt, Pd or Ni H C C H Heat of hydrogenation - the heat liberated during this reaction. H is ~125 kJ/mol for each double bond in the compound.
© E.V. Blackburn, 2011
Mechanism of alkene hydrogenation H 2 H H catalyst H H complexation of alkene to catalyst H 2 adsorbed at surface H H hydrogen insertion catalyst + H H alkane © E.V. Blackburn, 2011
Mechanism of alkene hydrogenation Hydrogenation is stereospecific.
The two hydrogens add to the same side of the double bond - a syn addition.
© E.V. Blackburn, 2011
Reactions of alkenes C C The electrons are less tightly held than the electrons. The double bond therefore acts as a source of electrons - a base – a nucleophile.
It reacts with electron deficient compounds - acids electrophiles.
© E.V. Blackburn, 2011
Electrophilic addition Electron seeking reagents are called electrophilic reagents.
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.
© E.V. Blackburn, 2011
Addition of hydrogen halides + HX H C C X HX = HCl, HBr, HI © E.V. Blackburn, 2011
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 © E.V. Blackburn, 2011
Markovnikov’s rule H H CH 3 CH 3 HCl CH 3 H 3 C C CH 3 Cl 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.
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Markovnikov’s rule CH 3 CH 2 CH=CHCH 3 + HI CH 3 CH 2 CHICH 2 CH 3 + CH 3 CH 2 CH 2 CHICH 3 Each carbon of the double bond is bonded to one H therefore both isomers are formed.
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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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HBr - the peroxide effect 1933, Kharasch and Mayo CH 3 CH 3 -C=CH 2 HBr absence of peroxides (-O-O-) H 3 CH C C Br 3 CH 3 presence of peroxides CH 3 H 3 C C CH 2 Br H © E.V. Blackburn, 2011
Addition of sulfuric acid CH 3 CH=CH 2 cold H 2 SO 4 80% CH 3 CHCH 3 OSO 3 H H 2 O CH 3 CHCH 3 OH © E.V. Blackburn, 2011
Hydration H H CH 3 + H 2 O CH 3 H + H H CH 3 C C H OH CH 3 a Markovnikov addition © E.V. Blackburn, 2011
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The mechanism of the addition C C H + + X H X 2.
C C H + + X C C H X HX = HCl, HBr, HI, H 2 SO 4 , H 3 O + © E.V. Blackburn, 2011
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H 3 C H An example H H H Cl H 3 C H H C C + H H + Cl 2.
H 3 C H H C C + H H + Cl H 3 C H H C C Cl H H © E.V. Blackburn, 2011
Orientation + CH 3 CHCH 3 CH 3 CH=CH 2 HCl X + CH 3 CH 2 CH 2 Cl Cl CH 3 CHCH 3 © E.V. Blackburn, 2011
Orientation CH 3 CH 3 C=CH 2 HCl CH 3 CH 3 CCH 3 + X CH 3 CH 3 CH=C CH 3 HCl X CH 3 + CH 3 CHCH 2 CH 3 CH 3 CH 2 CCH 3 + CH 3 + CH 3 CHCCH 3 H Cl Cl CH 3 CH 3 CCH 3 Cl CH 3 CH 3 CH 2 CCH 3 Cl © E.V. Blackburn, 2011
A more general “rule” Electrophilic addition to a carbon - carbon double bond involves the intermediate formation of the most stable carbocation.
Why? Let’s look at the transition state: + H + C C H + + C C H + © E.V. Blackburn, 2011
A more general “rule” E a > E a E CH 3 CH 2 CH 2 + CH 3 CH=CH 2 + H + CH 3 CHCH 3 + © E.V. Blackburn, 2011
Carbocation rearrangements H 3 C CH 3 CH=CH 2 CH 3 HCl H 3 C CH 3 + CH-CH 3 CH 3 H 3 C CH 3 + CH-CH 3 CH 3 H 3 C CH + 3 H CH 3 CH 3 H 3 C Cl CH + 3 H CH 3 CH 3 H 3 C CH 3 Cl H CH 3 CH 3 © E.V. Blackburn, 2011
Oxymercuration © E.V. Blackburn, 2011
Oxymercuration An anti addition via a mercurinium ion: Dissociation: Electrophilic attack: Hg(OAc) 2 CH 3 CH 3 CO 2 + + HgOCOCH 3 + HgOCOCH 3 + HgOCOCH 3 CH 3 Nucleophilic opening: + HgOCOCH 3 CH 3 H-O-H CH 3 OH Hg OCOCH 3 H © E.V. Blackburn, 2011
Oxymercuration Why do we observe Markovnikov addition?
+ HgOAc HgOAc In the mercurinium ion, the positive charge is shared between the more substituted carbon and the mercury atom.
Only a small portion of the charge resides on this carbon but it is sufficient to account for the orientation of the addition but is insufficient to allow a rearrangement to occur.
© E.V. Blackburn, 2011
Hydroboration H.C. Brown and G. Zweifel,
J. Am. Chem. Soc.,
83, 2544 (1961) + (BH 3 ) 2 diborane H B H 2 O 2 OH H OH + B(OH) 3 Brown was co-winner of the 1979 Nobel Prize in Chemistry.
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H 3 C Hydroboration 1. (BH 3 ) 2 2. H 2 O 2 /OH 3 HC H H OH trans-2-methyl cyclopentanol syn addition (CH 3 ) 3 CCH=CH 2 (CH 3 no rearrangement no carbocation!
) 3 CCH 2 CH 2 OH © E.V. Blackburn, 2011
Hydroboration (BH 3 ) 2 H 2 C=CH 2 CH 3 CH 2 BH 2 H 2 C=CH 2 (CH 3 CH 2 ) 2 BH H 2 C=CH 2 3C 2 H 5 OH + B(OH) 3 H 2 O 2 OH (CH 3 CH 2 ) 3 B © E.V. Blackburn, 2011
Hydroboration - the mechanism 1. (BH 3 ) 2 CH 3 CH=CH 2 2. H 2 O 2 /OH CH 3 CH 2 CH 2 OH CH 3 CH=CH 2 HX CH 3 CH CH 2 H X + CH 3 CHCH 3 + X © E.V. Blackburn, 2011
Hydroboration - the mechanism 1. (BH 3 ) 2 CH 3 CH=CH 2 2. H 2 O 2 /OH CH 3 CH 2 CH 2 OH CH 3 > CH H CH 2 B H H CH 3 > CH H CH B H 2 H © E.V. Blackburn, 2011
Hydroboration - the mechanism R R B R R O-OH O OH R RO RO B OR HO R O OH R R R B OR + HO 3ROH + BO 3 3 © E.V. Blackburn, 2011
Addition of halogens + X 2 X C C X X 2 = Cl 2 , Br 2 CH 3 CH=CH 2 usually iodine does not react Br 2 in CH 3 CHBrCH 2 Br CCl 4 1,2-dibromopropane © E.V. Blackburn, 2011
Mechanism of X 2 addition C C X + + X-X C C X + + X C C X X + X X polarisation + X © E.V. Blackburn, 2011
Mechanism of X 2 addition H H H Br 2 H NaCl CH 2 BrCH 2 + NaCl Br no reaction CH 2 BrCH 2 Cl CH 2 BrCH 2 Br © E.V. Blackburn, 2011
Stereospecific reactions CH 3 CH=CHCH 3 Br 2 * * CH 3 CHBrCHBrCH 3 (Z)-2-butene gives racemic 2,3-dibromobutane and no meso compound is formed. (E)-2-butene gives only meso-2,3-dibromobutane.
A reaction is stereospecific if a particular stereoisomer of the reactant produces a specific stereoisomer of the product.
© E.V. Blackburn, 2011
syn
and
anti
addition Y
syn
Z Y-Z
anti
Y Z © E.V. Blackburn, 2011
Bromine addition - an
anti
addition I. Roberts and G.E. Kimball,
J. Am. Chem. Soc.,
59, 947 (1937) 1. Br Br 2.
Br C C + + Br Br + Br C C + bromonium ion Br C C Br © E.V. Blackburn, 2011
The bromonium ion Br-Br H H CH 3 CH 3 H Br H + CH 3 CH 3 H Br Br + H H Br H Br (S,S) H Br + H Br H Br (R,R) H Br © E.V. Blackburn, 2011
Halohydrin formation + X 2 + H 2 O X 2 = Cl 2 , Br 2 C C X OH + HX © E.V. Blackburn, 2011
Halohydrin formation CH 3 -CH=CH 2 + Br H 3 CHC CH 2 H 2 O Br 2 greater + here + Br H 3 CHC CH 2 CH 3 CH-CH 2 Br H O + H CH 3 CH-CH 2 Br H O + H -H + 1-bromo-2-propanol © E.V. Blackburn, 2011
Unsymmetric electrophiles In the electrophilic addition of an unsymmetric reagent, the electrophilic part adds to the less substituted carbon of the alkene unit: CH 3 CH=CH 2 + A-B CH 3 CH-CH 2 B A © E.V. Blackburn, 2011
Free radical addition reactions + Y-Z peroxides or h c C Y Z 1. peroxides Rad chain initiation 2. Rad + HBr RadH + Br 3. Br + 4.
Br + HBr Br Br H + Br propagation © E.V. Blackburn, 2011
ionic vs radical addition CH 3 CH=CH 2 HBr X + CH 3 CHCH 3 + CH 3 CH 2 CH 2 Br CH 3 CHCH 3 Br CH 3 CH=CH 2 Br CH 3 CHCH 2 Br HBr X CH 3 CHBrCH 2 CH 3 CH 2 CH 2 Br © E.V. Blackburn, 2011
Polymerization A polymer is a long chain molecule made up of structural units (monomers) joined together.
© E.V. Blackburn, 2011
Free radical polymerization of alkenes
Initiation
peroxide Rad + CH 2 =CH G Rad Rad-CH 2 -CH G © E.V. Blackburn, 2011
Free radical polymerization of alkenes
chain propagation
Rad-CH 2 -CH + CH 2 =CH G G Rad-CH 2 -CH-CH 2 -C H G G CH 2 =CHG Rad-CH 2 -CH-CH 2 -CH-CH 2 -C H G G G etc © E.V. Blackburn, 2011
Free radical polymerization of alkenes
Chain termination
2
combination
.
Rad(CH 2 -CH) n -CH 2 -CH G G Rad(CH 2 CH) n CH 2 CHCHCH G G G 2 (CH-CH 2 ) n Rad G © E.V. Blackburn, 2011
Free radical polymerization of alkenes
Chain termination
disproportionation
.
Rad(CH 2 -CH) n -CH 2 -CH G G Rad(CH 2 CH) n CH 2 CH 2 G G + Rad(CH 2 CH) n CH=C H G G © E.V. Blackburn, 2011
Examples G monomer polymer Cl CH 2 =CHCl -CH 2 CHCl-CH 2 CHCl-CH 2 vinyl chloride polyvinyl chloride, PVC CN CH 2 =CHCN -CH 2 CHCN-CH 2 CHCN acrylonitrile polyacrylonitrile Orlon, Acrilon © E.V. Blackburn, 2011
Examples G monomer polymer Ph CH 2 =CHPh -CH 2 CHPh-CH 2 CHPh styrene polystyrene Ph = C CH 3 CO 2 CH 3 CH 3 CH 2 =C CO 2 CH 3 methyl methacrylate CH 3 CH -CH 2 -C - CH 2 - C 3 CO 2 CH 3 CO 2 CH 3 poly(methyl methacrylate) Plexiglas, Lucite © E.V. Blackburn, 2011
Polymerization The addition of other compounds can modify the polymerization: .
-CH 2 -CH + CCl 4 -CH 2 -CHCl + CCl 3 .
CCl 3 + CH 2 =CH .
Cl 3 C-CH 2 -CH styrene polymer © E.V. Blackburn, 2011
Carbenes - + H 2 C N N or h H 2 C: + N 2 diazomethane methylene RO K + + H-CCl 3 ROH + K + + Cl + Cl 2 C: dichlorocarbene CH 2 I 2 + Zn(Cu) ICH 2 ZnI a carbenoid Carbenes (and carbenoids) add to alkenes in a stereospecific manner to form cyclopropanes.
© E.V. Blackburn, 2011
Carbenes KOC(CH 3 ) 3 CHBr 3 H Br Br H H CH 2 I 2 /Zn(Cu) H © E.V. Blackburn, 2011
Hydroxylation KMnO 4 , HCO 2 OH or OsO 4 OH OH a glycol © E.V. Blackburn, 2011
Ozonolysis + O 3 O O O molozonide O + O O O O H 2 O/Zn aldehydes and ketones O O O ozonide © E.V. Blackburn, 2011
Ozonolysis CH 3 CH 2 CH=CHCH 3 1. O 3 2. Zn/H 2 O H H CH 3 CH 2 C=O + O=CCH 3 H C C CH 3 H 3 C CH 3 1. O 3 2. Zn/H 2 O CH 3 CHO + O CH 3 CH 3 © E.V. Blackburn, 2011
KMnO 4 oxidation H 3 H C CH 3 CH CH 3 C C CH 3 2 KMnO 4 CH=CH 2 CH 3 CH 3 CO 2 H + O CH 3 KMnO 4 CH 3 CH 2 CO 2 H + CO 2 © E.V. Blackburn, 2011
Addition of halogen to alkynes C C X 2 X C C X X 2 X X X X © E.V. Blackburn, 2011
Addition of HX to alkynes H 3 C C C H HCl H C C Cl H CH 3 HI H H H I Cl CH 3 Does this seem reasonable?
H Cl H C C CH 3 H H H + v Cl CH 3 This carbocation should be destabilized by the inductive effect of the Cl!
H H H + Cl CH 3 H H H Cl + CH 3 The cation is stablized by resonance! Remember this in CHEM 263 © E.V. Blackburn, 2011
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H 2 , Pd/CaCO 3 quinoline Na, NH 3 , -78 o C H H H H 2.
CH 3 CH=CH 2 1. Hg(OAc) 2 /THF-H 2 O 2. NaBH 4 CH 3 CH(OH)CH 3 CH 3 CH=CH 2 1. B 2 H 6 2. H 2 O 2 /OH CH 3 CH 2 CH 2 OH 3. CH 3 CH=CH 2 CH 3 CH=CH 2 HBr/-O-O HBr/-O-O CH CH 3 3 CH 2 CH 2 CHBrCH Br 3 © E.V. Blackburn, 2011