Preparation of alkenes

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Preparation of alkenes 1. Elimination reactions
E2 is a non-selective reaction. However
CH3CHC(CH3)3 HB Br- 2-methyl-2-butene
(69) More stable 2-methyl-1-butene
(31) CH3CH2CCH2 HB Br-
CH3
H H
CH2 B- CH3CH-C-Br CH3
Why?? Because the transition
state has some double bond
character
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Zaitsevs rule
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Stereochemistry (antiperiplanar attack)
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A consequence
cis
trans
anti-periplanar
syn-periplanar
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2. Alcohol dehydration (again elimination)
-C-C-
CC H2O
H OH
Strong acid conditions required (rather than
basic as during eliminations)

• Two observations
• 30 alcohol gt 20 alcohol gt 10
  alcohol
• some primary and secondary alcohols
  undergo skeleton
• rearrangement

CH3
CH3 CH3
CH3 CH3 CH3-C --- CH-CH3
CH3-C C-CH3
CH2C ---CHCH3 CH3 OH
80 20
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Mechanism (E1 obviously)
fast
Step 1
slow
Step 2
fast
Step 3
That explains

• 30 alcohol gt 20 alcohol gt 10 alcohol

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Stability of carbocations
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Dehydration of primary alcohols (E2 reaction)
fast
Step 1
slow
Step 2
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Carbocation rearrangements
Dehydration of secondary alcohols
10
CH3
CH3 CH3
CH3 CH3 CH3-C --- CH-CH3
CH3-C C-CH3
CH2C ---CHCH3 CH3 OH
80 20
H
1,2-shift
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Can we have carbocation rearrangement with
primary alcohols?
Yes but using a completely different mechanism
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Skeleton rearrangement during dehydration of
primary alcohols
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Vicinal Debromination
vicinal (vic)
geminal (gem)
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E2 mechanism
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Reactivity (ch 8) (Addition reactions)
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Syn and Anti additions to the double and triple
bonds
Syn addition
A very hexothermic reaction DH -120
kJ/mol but It needs a catalyst
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Movie
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Hydrogenation of alkynes
Syn addition for the formation of cis-alkenes
Anti addition for the formation of trans-alkenes
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Anti addition the mechanism
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Additions to Alkenes
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Additions to Alkenes
Reactions usually hexothermic
Electron-rich region
Susceptible of electrophilic attack
Electron-rich region
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Electrophiles are Lewis Acids
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Markovnikovs rule
Observation
C atom with the greater of H
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Markovnikovs rule
Explanation
t-butyl bromide !!
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General formulation In the ionic addition of an
unsymmetrical reagent to a double bond, the
positive portion of the adding reagent attaches
in the way that generates the most stable
carbocation.
These reactions are called regioselective
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Stereochemistry of the addition reaction
Addition of H2SO4 to form the alkyl hydrogen
sulphate X HSO4 Addition of H2O to form
the corresponding alcohol X OH (it
needs acid as a catalyst. Carbocation
transpositions are possible with 10 and 20
carbocations)
A valuable reaction
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Addition of halogens
Fast, high yield reactions forming vic-dihalides
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Mechanism
anti addition a stereospecific reaction
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Stereospecificity
The reaction forms only one steroisomeric form
(trans) but is not enantiospecific
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Stereospecificity
(MODELS)
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Stereospecificity
R
S
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Formation of halohydrins (addition of X2 and H2O)
Use excess of water to favor halohydrin
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Formation of halohydrins (addition of X2 and H2O)
mechanism
anti attack
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Cyclopropanation reactions (addition of carbenes)
diazomethane
CH2 N2
HCCl3 RO- RO-H
CCl2 Cl-
Carbenes are highly unstable (in situ generated)
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Cyclopropanation reactions (addition of carbenes)
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Alkene Oxidations
Syn-hydroxylation
OH-, H2O
CH2CH2 KMnO4 H2C CH2

OH OH
Two methods
Na2SO3/H2O
CH2CH2 OsO4
H2C CH2

OH OH
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Mechanism
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Oxidative cleavage
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Ozonolysis
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Synthesis of alkynes
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Synthesis of alkynes
Prepare the alkene (large variety of preparation
methods enables a variety of molecular
architectures) Destroy the double bond by
adding Br2 thus generating a vic-dibromide Doubl
e elimination reaction performed with NaNH2
generates the alkyne No stereochemical
implications gem-dihalides may be used as well
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The acidity of terminal alkynes
sp2
sp
sp3
pKa 25
44 50
More s character, more stable anion, weaker
conjugated base, better acid
Acidity CH3CH3 lt CH2CH2
lt HCCH Basicity CH3CH2 gt
CH2CH gt HCC
OH lt OR lt CCR lt NH2 lt CHCH2 lt CH2-CH3
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Alkynides as Good Nucleophiles for
SN2 Good Base for E2
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Alkynes on the same line as alkenes
Addition of Br2 and Cl2 Addition of
HX Oxidative cleavage
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Hydrogenation of alkynes
Syn addition for the formation of cis-alkenes
Anti addition for the formation of trans-alkenes
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Anti addition the mechanism