Chapter 12 Alkene Reactions
I.
Catalytic Hydrogenation
A.
Thermodynamics of addition reactions
1) C=C p-bond is weak and thus reactive
2) Addition reactions:
AB
C C
C C
A
B.
B
3)
DH = (DHop + DHoAB) – (DHoCA + DHoCB) = -DH
4)
CA and CB single s-bonds stronger than AB + p-bond
5)
Additions usually occur spontaneously and -DH is released (Table 12-1)
Hydrogenation of Alkenes
1) Addition of H2 to C=C requires a catalyst to lower Ea
2) Reaction occurs at the metal surface (Pd/C or PtO2, or Ra Ni)
3) Solvent is usually MeOH, EtOH, or HOAc
CH3
H
H2, PtO2
C C
MeOH, 25 oC
CH3
CH2CH2CH3
3)
CH3
CH3C CH CH2CH2CH3
H H
Hydrogenation is stereospecific at one face of C=C (syn addition)
CH2CH3
H2, PtO2
EtOH, 25 oC
CH3
H
H
CH3CH2
CH2CH3 +
H
H
racemic, 83%
H3C
CH3
4)
Steric Bulk may dictate which side can approach the metal surface
CH3
H
CH3
100 atm H2, PtO2
o
EtOH, 25 C
98%
H3C
H3C
CH3
CH3
II.
p-bond as Nucleophile: HX Additions
A.
A p-bond is an e- rich cloud that electrophiles can attack
1) H+ is a strong electrophile
H
+
H
C C
H X
X-
H C C
H C C
H
H
HI
H
o
0 C
2)
H
I
Low temperature reduces chance of rearrangement
B.
Markovnikov Rule
1. H+ goes to the least substituted C, and X- goes to the most substituted C
Cl H
CH3CH=CH2
2.
H
CH3
H
H
H
TS-1
HCl
CH3CHCH2
Formation of the most stable carbocation directs the reaction. Initial
protonation gives the most stable carbocation.
H
H
H
CH3
H
TS-2
III. Alcohol Synthesis bye Electrophilic Hydration
A.
Strong aqueous mineral acid gives H2O addition
1) This reaction obeys Markovnikov Rule
2) Mechanism is the reverse of the acid catalyzed alcohol dehydration
H
CH3
H
+
H
C C
H 50% H2SO4
CH3
B.
CH3
H
C C H
CH3
H
OH2
OH
H
CH3 C C H
H
CH3
OH2
Alkene Hydration vs. Alcohol Dehydration
1) All steps in the mechanism are reversible: Equilibrium
2) H+ acts as catalyst and is not consumed
3) Favor alcohol with low temperature and excess water
4) Favor alkene with concentrated acid and heat
0 oC, H2O OH
RCH=CH2 + H2O
RCHCH3
+
D, H
OH
H
CH3 C C H
H
CH3
C.
Thermodynamic Control
1. When reversible protonation can happen, an equilibrium mixture exists
2. The most stable product will be major, because minor products will be
converted back to the cation, then to the most stable product
OH
CH3CH2CHCH3
+
+
H
H2O
+
CH3CH2CHCH3
+H
-H+
+
+H
3.
CH3CH2CH=CH2
-H
major
CH3CH=CHCH3
We can use acid to interconvert alkene isomers to most stable one
+
H
IV. Halogen Addition
A.
Halogen gases (Cl2) don’t seem very electrophilic, but will add to alkenes
1) Cl2 and Br2 in CCl4 solvent at room temperature best conditions
2) F2 reacts violently; I2 doesn’t react at all (DHo = 0)
3) Disappearance of red-brown Br2 upon addition to unknown signals alkene
Br2
Br
Br
B.
H
CH3
Halogen Addition Mechanism
1. Anti addition is always observed
H
CH3
2.
Br2
CCl4
Br
H
CH3
H
racemic
Br
H
CH3
CH3
Br
racemic
Br
Br2, CCl4
CH3
H
Br2
CCl4
Br
H
CH3
CH3
H
Br
meso
Bromonium ion
a. Br—Br has a very large, polarizable s-bond
b. C=C p-bond nucleophile attacks the d+ end of Br—Br (like SN2)
c. The result is a Bromonium cation and Br- anion
3)
H
CH3
The last step is nucleophilic attack by Br- on the bromonium ion
Br
Br
Br
CH3
H
CH3
V.
H
H
CH3
Br-
Br
CH3
H
H
Br
CH3
+
racemic
Br
H3C
H
H
Br
CH3
Other Additions
A.
Halonium cation can trap other nucleophiles
1) Cl2 works just like Br2 = chloronium ion
2) Mixed products can be useful synthetic intermediates
Br
Br
Br2, CCl4
Br2, CCl4
Cl
Cl-
Cl
Cl2, CCl4
HOH
OR
ROH
NaOH, H2O, r.t.
O
OH
Oxacyclopropane
B.
Regioselectivity of Halonium ion mixed products
1. Halogen ends up on the less-substituted C. Greater d+ on more subst. C
2. Nucleophile ends up on more substituted C
3. Markovnikov-like addition because electrophile (H+, Br+) behaves same
Brd+
Br2, CCl4
CH3
ClCl
CH3
CH3
d+
4.
Br
Other reagents behave as electrophile-nucleophile pair (in that order)
a. Br—Cl
b. Br—CN
c. I—Cl
d. RS—Cl
d. XHg—X (X = acetate)
RSCl, CCl4
CH3
d+
SR
d+
CH3
SR
ClCl
CH3
C.
Oxymercuration—Demercuration
1) Oxymercuration proceeds in an anti addition, just like Br2 addition
O
O
O
H HgOCCH
CH3COHgOCCH3
3
mercuric acetate
CH3
OH
H2O
CH3
2)
Demercuration replaces the Hg with H
O
H H
H HgOCCH
3
NaBH4
OH
OH
NaOH, H2O
CH3
CH3
3)
4)
The result is Markovnikov addition just like acidic hydration reaction
The advantage is that no rearrangement can take place
O
CH3
O
CH3COHgOCCH3
mercuric acetate
H2O
H H
NaBH4
NaOH, H2O
OH
CH3