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Alkenes and Alkynes IIAddition Reactions
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
H+
carbocation
X H X+
HX
Addition Products
HX
H+
carbocation
+ X
alkene
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+
CH2 CHCH
H X
H2C CHCH3
H X
Markovnikov addition product
carbon atomwith greater number of hydrogen atoms
+ HBr
Br
Br+
littlemajor
+ HBrBr
Br
major minor
The Rate Determining stepStep 1 is the rate determined step
Formation of carbocation
CH3CH CH2
HBr
slow
CH3CH2CH2
1o
Br-CH3CH2CH2 Br
CH3CHCH3Br-
2o
CH3CHCH3
Br
1-bromopropanelittle formed
2-bromopropanemain product
A mechanism for the reactionPredict the products and show the full
mechanism
+ HCl
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
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
An exception of Markovnikov’s RuleWhen alkenes are treated with HBr in
presence of peroxide, an anti-Markovnikov addition
CH3CH CH2 + HBr
ROORCH3CH2CH2Br
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%
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
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
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
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
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
+ 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
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
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