Download Reactions of Alkenes

Reactions of Alkenes
H
C
+ HCl
C
C
+
C
C
C
Cl
hyd roch lorination
(h yd rohalogenation )
C
OH
hydration
H
H+
H2 O
C
H
+
H
Br
C
H
C
C
C
H
C
C
C
+ Br 2
H+
+
CCl4
OsO4
O3
C
C
(CH3 ) 2 S
C
O
H
O
C
H
H
H
C
C
H
H
C
+
H
1. BH3
2. HO , H2O2, H2O
O3
(CH3 ) 2 S
H
redu ction
(h yd rogenation)
C
Br
C
addition of radicals
C
H oxymercuration/demercuration
OCH3
C
H
C OH
H
Br
O
H
oxidation
H
HO
H
Ozonolysis
1. Hg(OAc)2, CH3OH
-
Br
H2 O
Addition of HOX; X: Cl, Br
peroxide
2. NaBH4
C
H
Br2
H
bromination
(h alogenation)
OH OH+ (hydrogenation)
Ozonolysis
+
H2
+ HBr
C
C
H
C
C
H H
H
hydroboration-oxidation
H
O
H
+
H
H
Reaction Mechanism
• A reaction mechanism describes how a
reaction occurs
– which bonds are broken and which new ones are
formed
– the order in which bond-breaking and bondforming steps take place
– the role of the catalyst (if any is present)
– the energy of the entire system during the
reaction
Energy Diagram
• Energy diagram: a graph showing the changes in
energy that occur during a chemical reaction
• Reaction coordinate: a measure of the change in
position of atoms during a reaction
• Heat of reaction, H: the difference in energy
between reactants and products
– exothermic: products are lower in energy than
reactants; heat is released
– endothermic: products are higher in energy than
reactants; heat is absorbed
Energy Diagram
• Transition state: an energy maximum
– represents an unstable species formed during the
course of a reaction
• Activation Energy, Ea: the difference in energy
between reactants and the transition state
– determines the rate of reaction
– if Ea is large, only a few molecular collisions occur with
sufficient energy to reach the transition state, and the
reaction is slow
– if Ea is small, many collisions generate sufficient
energy to reach the transition state, and the reaction
is fast
Activation Energy
• Activation energy
– Is the difference in energy between the reactants
and the transition state.
– Is the minimum energy required to a reaction to
occur or happen
– This will determine the rate of the reaction
• How fast the reaction occurs
Energy Diagram
– an energy diagram for a two-step reaction
involving formation of an intermediate
• Nucleophile (nucleus loving): An electron rich
species that seeks a region of low electron
density.
• Electrophile (electron loving): A low electrondensity species that seeks a region of high
electron density.
Addition of HX (X=halogens)
Hydrohalogenation
• Carried out with pure reagents or in a polar solvent
such as acetic acid
CH2 =CH2 + HCl
Ethylene
H
Cl
CH2 -CH2
Chloroeth ane
• Addition is regioselective
– regioselective reaction: a reaction in which one
direction of bond-forming or bond-breaking occurs in
preference to all other directions
CH3 CH=CH2 + HCl
Prop ene
Cl H
H Cl
CH3 CH-CH2 + CH3 CH-CH2
2-Chloropropane 1-Ch loroprop ane
(not ob served )
– Markovnikov’s rule: in additions of HX, H adds to the
carbon with the greater number of hydrogens
HCl + 2-Butene
• A two-step mechanism
– Step 1: formation of the sec-butyl cation, a 2° carbocation
intermediate
+
CH3 CH-CHCH3
+
: Cl :
:
CH3 CH=CHCH3
H
:
: :
slow , rate
+ determinin g
+ H-Cl :
sec-Butyl cation
(a 2° carbocation
intermediate)
– Step 2: reaction of the sec-butyl cation (an electrophile) with chloride
ion (a nucleophile) completes the reaction
:
+
:
: Cl :
+
:
CH3 CHCH 2 CH 3
Ch loride ion sec-Butyl cation
(a nucleop hile) (an electrop hile)
fast
:Cl :
CH3 CHCH2 CH3
2-Chlorobutan e
Carbocations
• Carbocation: a species in which a carbon atom has only six
electrons in its valence shell and bears positive charge
• Carbocations are:
– classified as 1°, 2°, or 3° depending on the number
of carbons bonded to the carbon bearing the
positive charge
– electrophiles; that is, they are electron-loving
– Lewis acids; that is, they are electron-pair
acceptors
Carbocations
– a 3° carbocation is more stable than a 2°
carbocation, and requires a lower activation
energy for its formation
– a 2° carbocation is, in turn, more stable than a 1°
carbocation, and requires a lower activation
energy for its formation
– methyl and 1° carbocations are so unstable that
they are never observed in solution
H
H C+
H
Meth yl cation
(methyl)
CH3
H
C+
CH3
CH3
C+
CH3
CH3
C+
H
H
CH3
Ethyl cation
Is op ropyl cation tert -Butyl cation
(1°)
(2°)
(3°)
Increas ing carb ocation stability
Addition of H2O
• Addition of H2O is called hydration
– acid-catalyzed hydration of an alkene is
regioselective; hydrogen adds preferentially to the
more substituted carbon of the double bond
CH3 CH=CH2 + H2 O
Prop ene
CH3
CH3 C=CH2 + H2 O
2-Methylprop ene
H2 SO4
H2 SO4
OH H
CH3 CH-CH2
2-Prop anol
CH3
CH3 C-CH2
HO H
2-Methyl-2-propanol
Step 1: proton transfer to the alkene gives a carbocation
+
CH3 CH=CH2 + H O H
slow , rate
determining
+
CH3 CHCH3
A 2o carb ocation
intermediate
H
+
O H
H
Step 2: a Lewis acid/base reaction gives an oxonium ion
+
CH3 CHCH3 + O-H
fast
CH3 CHCH3
O+
H
H
H
An oxonium ion
Step 3: proton transfer to solvent gives the alcohol
CH3 CHCH3 + H-O-H
+
O
H
H
fas t
+
CH3 CHCH3 + H-O-H
O
H
H
Carbon Rearrangements
+
+
HCl
Cl
expected
product;
40%
Cl
rearrangement
product; 60%
• Rearrangement: change in connectivity of atoms in
a product compared to starting material
• 1,2-shift
• Hydride ion is migrating group
• More stable carbocation is formed
HCl
slow,
rate determining
+
CH
H
H
+
fast
CH
+
C
H
H
+
C
Cl
H
-
fast
Cl
+
Cl
-
Addition of Cl2 and Br2
• Carried out with either the pure reagents or in
an inert solvent such as CH2Cl2
Br Br
CH3 CH=CHCH3
2-Butene
+
Br2
CH2 Cl2
CH3 CH-CHCH3
2,3-Dibromobutane
Addition of Cl2 and Br2
– addition is stereoselective
– stereoselective reaction: a reaction in which one
stereoisomer is formed or destroyed in preference
to all others that might be formed or destroyed
– addition to a cycloalkene, for example, gives only a
trans product
Br
+ Br2
Cyclohexen e
CH2 Cl2
Br
t rans -1,2-D ib romocycloh exane
Addition of Cl2 and Br2
– Step 1: formation of a bromonium ion intermediate
Br
Br
C
Br
C
C
Br
C
C
C
+
A bridged b romon ium
ion intermediate
– Step 2: halide ion opens the three-membered ring
:
:
: Br : +
: Br :
:Br :
:
C
C
C
C
:
: Br :
A nti (cop lan ar) orien tation
of ad ded b romin e atoms
Br
-
Addition of Cl2 and Br2
• Anti coplanar addition to a cyclohexene
corresponds to trans-diaxial addition
Br
+ Br2
Br
Br
tran s diaxial
(les s stable)
Br
tran s diequ atorial
(more stable)
Ozonolysis
• Cleavage of a Carbon-Carbon double bond
• Two Carbonyl groups in place
1. O3
2. (CH3)2S
O
O
+
H
Reduction of Alkenes
• Alkenes react with H2 in the presence of a
transition metal catalyst to give alkanes
– the most commonly used catalysts are Pd, Pt, and
Ni
– the reaction is called catalytic reduction or
catalytic hydrogenation
+ H2
Cyclohexene
Pd
25°C, 3 atm
Cyclohexane
Reduction of Alkenes
– the most common pattern is syn addition of
hydrogens; both hydrogens add to the same side
of the double bond
CH3
+ H2
CH3
1,2-D imeth ylcycloh exene
Pt
CH3
CH3
cis-1,2-D imeth ylcyclohexane