Alkenes and Alkynes II Addition Reactions. Introduction: Additions to Alkenes An addition to a C-C...
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Transcript of Alkenes and Alkynes II Addition Reactions. Introduction: Additions to Alkenes An addition to a C-C...
![Page 1: Alkenes and Alkynes II Addition Reactions. Introduction: Additions to Alkenes An addition to a C-C double bond.](https://reader036.fdocuments.net/reader036/viewer/2022062300/56649e705503460f94b6e913/html5/thumbnails/1.jpg)
Alkenes and Alkynes IIAddition Reactions
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Introduction: Additions to AlkenesAn addition to a C-C double bond
C C
H X
H OSO3H
H OH
HA (cat)
X X
H C C X
H C C OSO3H
H C C OH
X C X
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H+
carbocation
X H X+
HX
Addition Products
HX
H+
carbocation
+ X
alkene
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X HC
H
C
Electrophile Nucleophile
+ X
C
H
C
electrophile
+ X
nucleophile
C C
H X
Electrophiles include postive reagents such as proton (H+), neutral reagent such as bromine and the Lewis acids such as BH3, BF3 and AlCl3Metal ions that contain vacant orbitals: Ag+, Hg2+ or Pt2+
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CH2 CHCH
H X
H2C CHCH3
H X
Markovnikov addition product
carbon atomwith greater number of hydrogen atoms
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+ HBr
Br
Br+
littlemajor
+ HBrBr
Br
major minor
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The Rate Determining stepStep 1 is the rate determined step
Formation of carbocation
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CH3CH CH2
HBr
slow
CH3CH2CH2
1o
Br-CH3CH2CH2 Br
CH3CHCH3Br-
2o
CH3CHCH3
Br
1-bromopropanelittle formed
2-bromopropanemain product
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A mechanism for the reactionPredict the products and show the full
mechanism
+ HCl
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Modern Statement of markovnikov’s RuleIn the ionic addition of an unsymmetrical
reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon of the double bond so as to yield the more stable carbocation as intermediate
H3C
C
HC
CH2 + I Cl CH2
I
ClCl I
H
H
2-methylpropene 2-chloro-1-iodo-2-methylpropane
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Regioselective ReactionsWhen a reaction that can potentially yield
two or more constutional isomers actually produces only one (or a predominance of one), the reaction is said to be regioselectiveHX + unsymetrical alkene more than
product
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An exception of Markovnikov’s RuleWhen alkenes are treated with HBr in
presence of peroxide, an anti-Markovnikov addition
CH3CH CH2 + HBr
ROORCH3CH2CH2Br
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8.3 Stereochemistry of the Ionic Addition to an AlkeneThe carbocation is formed in the first step of
the addition is trigonal planarachiral
C2H5 CH
CH2
H X
C2H5
H
CH2
H
+ X
a
b
C2H5
X
HCH3
(S)-2-halobutane 50%
b
a
C2H5
HCH
X(R)-2-halobutane50%
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8.4 Addition of Sulfuric Acid AlkenesWhen alkenes are treated with cold
concentrated sulfuric acid, they dissolve because they react with electrophilic addition to form alkyl hydrogen sulfate
H O
O
O
O
H
Alkene sulfuric acid
H
O
O
O
O
H
OSO3H
H
carbocation hydrogensulfate ion
alkyl hydrogen sulfate
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Alcohols from Alkyl Hydrogen SulfateAlkyl hydrogen sulfates can be easily
hydrolyzed to alcohols by heating them with waterMarkovnikov addition of H- and -OH
CH3CH CH2cold
H2SO4
CH3CHCH3
OSO3H
CH3CHCH3
OH
H2O, heat
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exampleProvide mechanistic explanation for the
following observationThe addition of hydrogen chloride to 3-methyl-
1-butene produces two products: 2-chloro-3-methylbutane and 2-chloro-2-methylbutane
Give the structure and name of the product that would be obatained from ionic addition of IBr to propene
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exampleIn one industrial synthesis of ethanol, ethene
is first dissolved in 95% sulfuric acid. In a second step of water is added and the mixture is heated. Out the reaction involved
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8.5 Addition of Water to Alkene:Acid-Catalyzed Hydration Method for preparation of low molecular
weight alcohols Usually regioselective
+ HOHH3O
H OH
2-methylpropene
+ HOHH3O
25oH
HO
2-methyl-2-propanol
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+ H O
H
H
slow
step 1
H
O
H
H
alkene donates an electron pair to a proton to form the more stable 3o carbocation
step 2
O
H
H
fast
O
H
H
The carbocation reacts with a molecule of water to form a protonated alcohol
step 3
O
H
H O
H
Hfast
O H H O
H
H+
A transfer of a proton to a molecule of water leads to the product
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Rate determining step
However, formation of 1o carbocation does not take place
+ H O
H
H
slow
step 1
H
O
H
H
alkene donates an electron pair to a proton to form the more stable 3o carbocation
+ H O
H
H
slow
step 1
O
H
H
alkene donates an electron pair to a proton to form the more stable 1o carbocation
H
HH
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RearrangementThe carbocation formed initially invariable
rearranges to a more stable one if possibleProvide the full mechanism
H2SO4
H2OHO
3,3-dimethyl-1-butene 2,3-dimethyl-2-butanolmajor