Chapter 6: Reactions of Alkenes: Addition Reactions

13 Pages·2009·1.48 MB·English

by  Carmelo Rizzo

Chapter 6: Reactions of Alkenes: Addition Reactions
6.1: Hydrogenation of Alkenes – addition of H-H (H ) to the
2
π-bond of alkenes to afford an alkane. The reaction must be
catalyzed by metals such as Pd, Pt, Rh, and Ni.
H H H H
Pd/C ΔH° = -136 KJ/mol
C C + H H EtOH H C C H hydrogenation
H H H H
C-C π-bond H-H C-H
= 243 KJ/mol = 435 KJ/mol = 2 x -410 KJ/mol = -142 KJ/mol
• The catalysts is not soluble in the reaction media, thus this
process is referred to as a heterogenous catalysis.
• The catalyst assists in breaking the π-bond of the alkene and
the H-H σ-bond.
• The reaction takes places on the surface of the catalyst. Thus,
the rate of the reaction is proportional to the surface area
of the catalyst.
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• Carbon-carbon π-bond of alkenes and alkynes can be reduced
to the corresponding saturated C-C bond. Other π-bond bond
such as C=O (carbonyl) and C≡N are not easily reduced by
catalytic hydrogenation. The C=C bonds of aryl rings are not
easily reduced.
O O
H2, PtO2
ethanol
O
C5H11 OH H2, Pd/C CH3(CH2)16CO2H
Linoleic Acid (unsaturated fatty acid) Steric Acid (saturated fatty acid)
O
O
OCH3 H2, Pd/C OCH3
ethanol
C H2, Pd/C C
N N
ethanol
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6.2: Heats of Hydrogenation -an be used to measure relative
stability of isomeric alkenes
H H H CH3
trans isomer is ~3 KJ/mol
H3C CH3 H3C H more stable than the
cis isomer
cis-2-butene trans-2-butene
ΔH° : -2710 KJ/mol -2707 KJ/mol
combustion
H H H2, Pd H2, Pd H CH3
CH3CH2CH2CH3
H3C CH3 H3C H
cis-2-butene trans-2-butene
ΔH° : -119 KJ/mol -115 KJ/mol
hydrogenation
trans isomer is ~4 KJ/mol more stable than the cis isomer
The greater release
of heat, the less
stable the reactant.
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Table 6.1 (pg 228): Heats of Hydrogenation of Some Alkenes
Alkene !H° (KJ/mol)
H2C=CH2 136
H H
monosubstituted
125 - 126
H3C H
H H
117 - 119
H3C CH3
H CH3
disubstituted 114 - 115
H3C H
H3C H
116 - 117
H3C H
H3C H
trisubstituted 112
H3C CH3
H3C CH3
tetrasubstituted
110
H3C CH3 113300
6.3: Stereochemistry of Alkene Hydrogenation
Mechanism:
H H
H2C CH2 H H H2C CH2
H2C CH2
H2
H H
H H
H H C C
H HCCHH H H
The addition of H across the π-bond is syn, i.e., from the
2
same face of the double bond
H CH3
CH3 H2, Pd/C CH3 H
CH3 EtOH H CH3 H CH3
syn addition
of H2 Not observed
113311
6.4: Electrophilic Addition of Hydrogen Halides to Alkenes
C-C σ-bond: ΔH°= 368 KJ/mol
C-C π-bond: ΔH°= 243 KJ/mol
π-bond of an alkene can
act as a nucleophile!!
Electrophilic addition reaction
H H Br H
C C + H-Br C C
H H
H H H H
nucleophile electrophile
Bonds broken Bonds formed
C=C π-bond 243 KJ/mol H C-H C–H -410 KJ/mol
3 2
H–Br 366 KJ/mol H C-H C–Br -283 KJ/mol
3 2
calc. ΔH° = -84 KJ/mol
expt. ΔH°= -84 KJ/mol 113322
Reactivity of HX correlates with acidity:
slowest HF << HCl < HBr < HI fastest
6.5: Regioselectivity of Hydrogen Halide Addition:
Markovnikov's Rule
H
R CCH H-Br R BCrHC H + R HC BCrH
H H H H H
none of this
R
R CCH H-Br R BCrHC H + R HC BCrH
H R H R H
none of this
R
BrH H Br
R CCH H-Br R C C R + R C C R
R R H R H
none of this
R
BrH H Br
HCCH H-Br R C C R + R C C R'
R' H H H H
Both products observed
For the electrophilic addition of HX across a C=C bond, the H (of
HX) will add to the carbon of the double bond with the most H’s
(the least substitutent carbon) and the X will add to the carbon of
the double bond that has the most alkyl groups. 11 33 33
Mechanism of electrophilic addition of HX to alkenes
6.6: Mechanistic Basis for Markovnikov's Rule:
Markovnikov’s rule can be explained by comparing the
stability of the intermediate carbocations 113344
For the electrophilic addition of HX to an unsymmetrically
substituted alkene:
• The more highly substituted carbocation intermediate is
formed.
• More highly substituted carbocations are more stable than
less substituted carbocations. (hyperconjugation)
• The more highly substituted carbocation is formed faster
than the less substituted carbocation. Once formed, the
more highly substituted carbocation goes on to the final
product more rapidly as well.
113355
6.7: Carbocation Rearrangements in Hydrogen Halide
Addition to Alkenes - In reactions involving carbocation
intermediates, the carbocation may sometimes rearrange if a
more stable carbocation can be formed by the rearrangement.
These involve hydride and methyl shifts..
H
Cl H
H H
H3C C C C H H-Cl H3C C C C H + H3C C H
C C
H3C H H H3C H H H H3C Cl H H
~ 50% ~ 50%
expected product
H
Cl H
H3C H
H3C C C C H H-Cl H3C C C C H + H3C C H
C C
H3C CH3 H H3C CH3 H H H3C Cl H H
Note that the shifting atom or group moves with its electron pair.
A MORE STABLE CARBOCATION IS FORMED.
113366
6.8: Free-radical Addition of HBr to Alkenes
H
C H H-Br BrH + H Br Polar mechanism
H3CH2C C H3CH2C C C H H3CH2C C C H (Markovnikov addition)
H H H H H
none of this
H
C H H-Br BrH + H Br Radical mechanism
H3CH2C C H3CH2C C C H H3CH2C C C H (Anti-Markovnikov addition)
H peroxides H H H H
(RO-OR)
none of this
H H-Br BrH H Br
C H R C C H + R C C H
R C ROOR H H H H The regiochemistry of
H (peroxides) none of this HBr addition is reversed
R H-Br BrH H Br in the presence of
R C HC H ROOR R RC CH H + R RC CH H peroxides.
none of this
R
BrH H Br Peroxides are radical
R C C H H-Br R C C R + R C C R
initiators - change in
R ROOR R H R H
none of this mechanism
R
BrH H Br
C H H-Br
H C R C C R + R C C R'
R' ROOR H H H H 113377
Both products observed
The regiochemistry of free radical addition of H-Br to alkenes
reflects the stability of the radical intermediate.
H H R
R C• R C• R C•
H R R
Primary (1°) < Secondary (2°) < Tertiary (3°)
138
6.9: Addition of Sulfuric Acid to Alkenes (please read)
6.10: Acid-Catalyzed Hydration of Alkenes - addition of water
(H-OH) across the π-bond of an alkene to give an alcohol;
opposite of dehydration
H3C H2SO4, H2O H3C
C CH2 C OH
H3C
H3C H3C
This addition reaction follows Markovnikov’s rule The more
highly substituted alcohol is the product and is derived from
The most stable carbocation intermediate.
Reactions works best for the preparation of 3° alcohols
139
Mechanism is the reverse of the acid-catalyzed dehydration
of alcohols: PPrriinncciippllee ooff MMiiccrroossccooppiicc RReevveerrssiibbiilliittyy
140
66..1111:: TThheerrmmooddyynnaammiiccss ooff AAddddiittiioonn--EElliimmiinnaattiioonn EEqquulliibbrriiaa
H3C H2SO4 H3C
C CH2 + H2O H3C C OH
H3C H3C
Bonds broken Bonds formed
C=C π-bond 243 KJ/mol H C-H C–H -410 KJ/mol
3 2
H–OH 497 KJ/mol (H C) C–OH -380 KJ/mol
3 3
calc. ΔH° = -50 KJ/mol
ΔG° = -5.4 KJ/mol ΔH° = -52.7 KJ/mol ΔS° = -0.16 KJ/mol
How is the position of the equilibrium controlled?
Le Chatelier’s Principle - an equilibrium will adjusts to any stress
TThhee hhyyddrraattiioonn--ddeehhyyddrraattiioonn eeqquuiilliibbrriiaa iiss ppuusshheedd ttoowwaarrdd hhyyddrraattiioonn
((aallccoohhooll)) bbyy aaddddiinngg wwaatteerr aanndd ttoowwaarrdd aallkkeennee ((ddeehhyyddrraattiioonn)) bbyy
rreemmoovviinngg wwaatteerr 141
The acid catalyzed hydration is not a good or general method for
the hydration of an alkene.
Oxymercuration: a general (2-step) method for the Markovnokov
hydration of alkenes
H H OH H OH
C H 1) Hg(OAc)2, H2O C Hg(OAc) 2) NaBH4 C H
C4H9 C C4H9 C C4H9 C
H H H H H
O NaBH reduces the C-Hg
4
Ac= acetate = C bond to a C-H bond
H3C O
142
6.12: Hydroboration-Oxidation of Alkenes - Anti-Markovnikov
addition of H-OH; syn addition of H-OH
1) BH, THF H
CH3 2) H2O6, NaOH, H O CH3
2 2 2
H
HO
6.13: Stereochemistry of Hydroboration-Oxidation
6.14: Mechanism of Hydroboration-Oxidation -
Step 1: syn addition of the H B–H bond to the same face of the
2
π-bond in an anti-Markovnikov sense; step 2: oxidation of the
B–C bond by basic H O to a C–OH bond, with retention of
2 2
stereochemistry
143
6.15: Addition of Halogens to Alkenes
X = Cl and Br
2 2 2
X2 X X
C C C C (vicinal dihalide)
alkene 1,2-dihalide
6.16: Stereochemistry of Halogen Addition - 1,2-dibromide
has the anti stereochemistry
Br Br
+ Br2 +
Br Br
not observed
Br
CH CH
3 Br 3
2
Br
H
144
6.17: Mechanism of Halogen Addition to Alkenes:
Halonium Ions - Bromonium ion intermediate explains the
stereochemistry of Br addition
2
145
6.18: Conversion of Alkenes to Vicinal Halohydrins
"X-OH" X OH
C C C C
alkene halohydrin
X, H O X
2 2
+ HX
OH
anti
stereochemistry
Mechanism involves a halonium ion intermediate
146