alkene reaction
TRANSCRIPT
CARBONCARBONCHEMISTRY 2CHEMISTRY 2
AlkeneAlkeneReaction and MechanismsReaction and Mechanisms
6-2
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Characteristic ReactionsCharacteristic Reactions
+
Hydrochlorination (hydrohalogenation)
CC C CH C l
HC l
+
Hydration
C C C CH OH
H2 O
6-3
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Characteristic ReactionsCharacteristic Reactions
+
Bromination (halogenation)
C C C CB r B r
B r2
+
Hydroxylation (oxidation)
C C C CHO OH
OsO4
6-4
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Characteristic ReactionsCharacteristic Reactions
Polymerization
CC C Cinitiator
nn
+
Hydrogenation (reduction)
CC C CHH
H2
6-5
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
PE DiagramsPE Diagrams
Ea
ΔH
transitionstate
A + B
C + D
Reaction Coordinate
Pote
ntia
l ene
rgy
6-6
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of HXAddition of HX• Addition is regioselective
•• MarkovnikovMarkovnikov’’s s rulerule: in the addition of HX or H2Oto an alkene, H adds to the carbon of the doublebond that has the greater number of hydrogensbonded to it
1-Chloropropane (not observed)
2-Chloropropane
Propene
+
+
ClC l
CH3 CH=CH2 HCl
CH3CH-CH2 CH3CH- CH2
H H
6-7
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
HCl HCl + 2-Butene+ 2-Butene• A two-step mechanism
• Step 1: formation of sec-butyl cation, a 2° carbocationintermediate
-+
+δ-δ+
H -C lCH3CH=CHCH3
C lsec-Butyl cation
slow, rate-limiting step
+H
CH3CH-CHCH3
6-8
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
HCl HCl + 2-Butene+ 2-Butene• Step 2: reaction of the sec-butyl cation with chloride
ion
- +
sec-Butyl cation (a Lewis acid)
+C l
C l CH3CHCH2CH3CH3CHCH2CH3 Chloride ion(a Lewis base)
fast
6-9
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
CarbocationsCarbocations•• CarbocationCarbocation: a species containing a positively
charged carbon• Carbocations are
• classified as 1°, 2°, or 3° depending on the number ofcarbons bonded to the carbon bearing the positivecharge
• electrophiles; that is, they are “electron-lovers”
6-10
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Carbocation Carbocation StructureStructure• Bond angles about the
positively charged carbon are120°
• Carbon uses sp2 hybrid orbitalsto form sigma bonds to thethree attached groups
• The unhybridized 2p orbital liesperpendicular to the sigmabond framework and containsno electrons
R CRR+
6-11
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Carbocation Carbocation StabilityStability• Relative stability
Methyl cation
(methyl)
Ethyl cation
(1°)
Isopropyl cation
(2°)
tert-Butyl cation
(3°)
Increasing carbocation stability
+ + + +CH
H
CH3 CCH3
CH3
HC
CH3
CH3CH3CH
H
H
6-12
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Carbocation Carbocation StabilityStability• Carbocations are stabilized by the electron-
withdrawing inductive effect of the positively-charged carbon• according to molecular orbital calculations:
HH
CH
H3CCH3
CCH3
+0.83
+0.06
+0.60
+0.13
6-13
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of HAddition of H22OO• Addition of water is called hydration• Acid-catalyzed hydration of an alkene is
regioselective - hydrogen adds to the lesssubstituted carbon of the double bond
Propene 2-Propanol+
OH
CH3CH=CH2 H2OH2 SO4 CH3CH-CH2
H
6-14
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of HAddition of H22OO• Step 1: proton transfer from solvent to the alkene
+
++
intermediateA 2o carbocation
+HO
H
HOHH
CH3CH=CH2
CH3CHCH3
slow, rate-limiting step
6-15
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of HAddition of H22OO• Step 2: reaction of the carbocation intermediate with
H2O to form an oxonium ion
• Step 3: proton transfer to solvent
+
++
An oxonium ion
H OHH
CH3CHCH3 O-H CH3CHCH 3fast
++
+OH HOH
H
HH O HCH3CHCH3 CH3CHCH3
OH
fast
6-16
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of ClAddition of Cl22 and Br and Br22• Carried out with either the pure reagents or in an
inert solvent such as CCl4 or CH2Cl2
2,3-Dibromobutane2-Butene+
Br BrBr2CH3CH=CHCH3 CH3CH -CHCH3CC l4
6-17
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of ClAddition of Cl22 and Br and Br22• Addition is stereoselective
•• Stereoselective Stereoselective reactionreaction: a reaction in whichone stereoisomer is formed or destroyed inpreference to all others than might be formedor destroyed
trans-1,2-Dibromo-cyclohexane
Cyclohexene
+ Br2 CCl4Br
Br
6-18
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of ClAddition of Cl22 and Br and Br22• Addition involves a two-step mechanism
• Step 1: formation of a bridged bromonium ionintermediate
C CB r
C C
B rBr
Br -
A bridged bromonium ion intermediate
+
6-19
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of ClAddition of Cl22 and Br and Br22• Step 2: Attack of halide ion from the opposite side of
the three-membered ring
Anti addition-
C C
B r
C C
B r
B r
B r
+
6-20
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Addition of ClAddition of Cl22 and Br and Br22• For a cyclohexene, anti coplanar addition
corresponds to trans-diaxial addition
trans-Diaxial(less stable)
trans-Diequatorial(more stable)
B r
B r
B rB rB r2
6-21
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Oxidation with OsOOxidation with OsO44• Oxidation by OsO4 converts an alkene to a
glycol, a compound with -OH groups on twoadjacent carbons• oxidation is syn stereoselective
H
HO
H
OH
+ ROOHOsO4
cis-Cyclopentanediol(a cis glycol)
Cyclopentene
6-22
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Oxidation with OsOOxidation with OsO44• Intermediate is a cyclic osmic ester containing a
five-membered ringH
O
H
OOs
O OA cyclic osmic ester
O OOs
O O
H
HO
H
OH
6-23
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Reduction of AlkenesReduction of Alkenes• Most alkenes react with H2 in the presence of a
transition metal catalyst to give alkanes• commonly used catalysts are Pt, Pd, Ru, and Ni
• The process is called catalytic reduction or,alternatively, catalytic hydrogenation
+ H2P t
Cyclohexene Cyclohexane
6-24
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Reduction of AlkenesReduction of Alkenes• the most common pattern is syn stereoselectivity
70% to 85%cis-1,2-Dimethyl-
cyclohexane
1,2-Dimethyl-cyclohexene
+CH3
CH3
CH3
CH3
CH3
CH3
H2/P t
30% to 15%trans-1,2-Dimethyl-
cyclohexane
6-25
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Reaction StereochemistryReaction Stereochemistry• how many stereoisomers are formed in the addition of
bromine to cis-2-butene?
C C HHH3C CH3cis--2-Butene
C CH
HH3C
CH3Br
BrC C
H
HH3C
CH3Br
Br+
A pair of enantiomers(a racemic mixture)
Br2
6-26
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Reaction StereochemistryReaction Stereochemistry• Consider the oxidation of 2-butene by OsO4
• three stereoisomers are possible for 2,3-butanediol;one pair of enantiomers and one meso compound
• which are formed in this oxidation?
CH3CH=CH2CH3 CH3CH-CHCH3
2-Butene 2,3-Butanediol
OsO4OH OH
6-27
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Reaction StereochemistryReaction Stereochemistry• syn hydroxylation of cis-2-butene produces only the
meso compound
identical;a meso compound
(2S,3R)-2,3-Butanediol
(2R,3S)-2,3-Butanediol
2
2
3
3C
HO
HO
CH
C
OH
HCH3
H3C
C
OH
H3CH
CH3H
6-28
66
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Reaction StereochemistryReaction Stereochemistry• Enantiomerically pure products can never be
formed from achiral starting materials and achiralreagents
• An enantiomerically pure product can begenerated in a reaction if at least one of thereactants is enantiomerically pure, or if thereaction is carried out in an achiral environment