Alkenes and alkynes (II) 9.1 Introduction: Additions to alkenes We shall study other examples of additions to the double bonds of alkenes. We begin with the additions of hydrogen halides, sulfuric acid, water (in the presence of an acid catalyst), C C + A-B A C C B A ddition
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Alkenes and alkynes (II) 9.1 Introduction: Additions to alkenes We shall study other examples of additions to the double bonds of alkenes. We begin with.
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Alkenes and alkynes (II)
9.1 Introduction: Additions to alkenes
We shall study other examples of additions to the double bonds of alkenes. We begin with the additions of hydrogen halides, sulfuric
acid, water (in the presence of an acid catalyst), and halogens.
C C + A-B A C C BAddition
An Addition reaction
C C
H C C X
Alkene
H X
HOSO2OHH C C OSO2OH
H C C OH
HOH
Alkyl halide
Alkyl hydrogen sulfate
Alcohol
X C C X
X - X
Dihaloalkane
1. An addition reaction results in the conversion of one pi π -bond
and one sigma-bond into two sigma bonds
C C X C C YX - Y+
Bonds broken
2 - Bonds formed
sp2
sp3
Addition
2. The electrons of the pi-bond are exposed. Because the pi-
bond results from overlapping p orbital, the pi electrons lie above
and below the plane of the double bond:
C C
sp2
Electrophiles Ç×µçÊÔ¼Á
H+, X+,Lewis acids; BF3 and AlCl3Metal ions; Ag+, Hg2+, Pt2+,
9.2 Addition of Hydrogen Halides to Alkenes: Markovnikov’
s Rule ( 马氏规则)
What is it Mar’s Rule?-----The addition of HX to an alkenes, the hydrogen atom adds to the carbon atom of the double bond that already has the greater number of hydrogen atoms.
H2C CHCH3 + H+X- CH2CHCH3
H X
Markovnikov addition productCarbon atom with the greater number of hydrogen atoms
Markovnikov’s Rule ( 马氏规则)
H2C CCH3 + HCl CH2C(CH3)2
H Cl
(not ClCH2CH(CH3)2
CH3
tert-Butyl chloride2-Methylpropene
CH3
+ HCl
Cl
CH3
H
Methylcyclohexene 1-Chloro-1-Methylcyclohexane
A mechanism for addition of a hydrogen halide to an alkene
(two steps;)
Step 1 + HXSlow
H
+
rate-determining step
Step 2
H
+ + X-fast
H X
9.2A Theoretical explanation of Markovnikov’s Rule
CH
CH2 + H-Br CH3CHCH2H
Br
Markovnikov addition product
(not BrCH2CHCH3)H3C
H
Methyl push electron
1. Electronic Effects ( 电子效应)
H2C C + HCl CH2C(CH3)2
H Cl
(not ClCH2CH(CH3)2
CH3
tert-Butyl chloride2-Methylpropene
CH3
CH3
+ HCl
Cl
CH3
H
Methylcyclohexene 1-Chloro-1-Methylcyclohexane
2. The Stabilities of carbocation
(碳阳离子的稳定性解释马氏规则)
CH
CH2 + H-Cl
CH3CHCH2H
Markovnikov addition product
H3C
+b
a
CH3CHCH2
+
Cl-CH3CHClCH3
2o C+
Cl-CH3CH2CH2Cl
Anti-Mar's Rule1o C+
Stable
Unstable
2o C+ 1o C+
a
bH
The Stabilities of carbocation
C CH2+ H-Cl
CH3CCH2H
Markovnikov addition product
H3C
+b
a
CH3CCH2
+
Cl-CH3CClCH3
3o C+
Cl-CH3CHCH2Cl
Anti-Mar's Rule
1o C+
Stable
Unstable
3o C+ 1o C+
a
bH
CH3
CH3
CH3
CH3
CH3
Mar’s Rule help us to predict the outcome of the addition
reactionH2C C + HCl CH2C(CH3)2
H Cl
(not ClCH2CH(CH3)2
CH3
tert-Butyl chloride2-Methylpropene
CH3
Major Product
H2C C + I-Cl CH2C(CH3)2
I Cl
(not ClCH2CI(CH3)2
CH3
2-Chloro-1-iodo-2-methylpropane2-Methylpropene
CH3
(Major product)
9.2D An exception to Markovnikov’s Rule
( 马氏规则例外)The addition of HBr to alkenes, it is Only when HBr is used in the presence of peroxides (ROOR), it is an exception to Mar’ R
ule.
CH3CH=CH2 + HBr
ROORCH3CH2CH2Br
Major product
CH3CHBrCH3
Mar's product
Ionic reaction
Free radical reaction
9.3 Stereochemistry of the Ionic Addition to an alkene
CH3CH2CH=CH2 + HCl CH3CH2CHCH3*
Cl
(± )-2-Chlorobutane
We got a racemic form? Do you know why?
CH3CH2CH=CH2 + HCl CH3CH2CHCH3*
Cl
(± )-2-Chlorobutane
H+
CH2H
C+
H CH2CH3
Cl-H3C
Cl
H3CH2C
HCl-
CH3
ClH
CH2CH3
Achiral, trigonalPlanar carbocation( S )-2-Chlorobutane
If the halogenation of an alkene is carried out in aqueous solution (rather than in carbon tetrachloride), the major product of the overall reaction is not a vic-dihalide, but rather it is a
halo alcohol called a halohydrin.
+ X2 + H2OC
X
C
OH
+ C
X
C
X
Halohydrin vic-Dihalide
Major product Minor product
Halohydrin formation can be explained by the following mechan
ism:
+ X H2OX
+Step1.C C
X+
Halonium ion
Step2. C C
X+
Halonium ion
H2O X-
Anti-addition
C
X
C
OH
+ C
X
C
X
Halohydrin vic-Dihalide
Major product Minor product
Isobutene reacts with bromine in water
C CH2
H3C
H3C
Br2
CH3
CH3C CH2
Br+
:OH2
H3C
CH3
+OH2
CH2Br
- H+
H3C
CH3
OH
CH2Br
(73%)
9.9 Radical addition to alkenes: The anti-Markovnikov addition o
f hydrogen bromideR O O R
An organic peroxideÓлú¹ýÑõ»¯Îï
R O O H
An organic hydroperoxideÓлú¹ý Ñõ»¯Çâ
HC CH2
HBrH3C H3C CHCH3
Br
No peroxideÎÞ¹ýÑõ»¯Îï
Markovnikov addition·ûºÏÂí¶û¿Æ·òÄá¿Æ·ò¼Ó³É
HC CH2
HBrH3C H3C CH2CH2
Br organic peroxide Óлú¹ýÑõ»¯Îï
Anti-Markovnikov addition·´Âí¶û¿Æ·òÄá¿Æ·ò¼Ó³ÉR O O R
Do you know why?
HC CH2
HBrH3C H3C CH2CH2
Br organic peroxide Óлú¹ýÑõ»¯Îï
Anti-Markovnikov addition·´Âí¶û¿Æ·òÄá¿Æ·ò¼Ó³ÉR O O R
9.11 Oxidations of alkenes ( 烯烃的氧化) : Syn hydroxyl
ation (顺式 - 羟基化作用)Alkenes undergo a number of reactions in which the carbon-carbon double bond is oxidized. Potassium permanganate (KMnO4) or osmiun tetroxide ( OsO4), for example, can be used to oxidize alkenes to 1,2-diols