Title: Reactivity ch 8
1Reactivity (ch 8) (Addition reactions)
2Syn 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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5Movie
6Hydrogenation of alkynes
Syn addition for the formation of cis-alkenes
Anti addition for the formation of trans-alkenes
7Anti addition the mechanism
8Additions to Alkenes
9Additions to Alkenes
Reactions usually hexothermic
Electron-rich region
Susceptible of electrophilic attack
Electron-rich region
10Electrophiles are Lewis Acids
11Markovnikovs rule
Observation
C atom with the greater of H
12Markovnikovs rule
Explanation
t-butyl bromide !!
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14General 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
15Stereochemistry 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
16Addition of halogens
Fast, high yield reactions forming vic-dihalides
17Mechanism
anti addition a stereospecific reaction
18Stereospecificity
The reaction forms only one steroisomeric form
(trans) but is not enantiospecific
19Stereospecificity
(MODELS)
20Stereospecificity
R
S
21Formation of halohydrins (addition of X2 and H2O)
Use excess of water to favor halohydrin
22Formation of halohydrins (addition of X2 and H2O)
mechanism
anti attack
23Cyclopropanation reactions (addition of carbenes)
diazomethane
CH2 N2
HCCl3 RO- RO-H
CCl2 Cl-
Carbenes are highly unstable (in situ generated)
24Cyclopropanation reactions (addition of carbenes)
25Alkene Oxidations
Syn-hydroxylation
OH-, H2O
CH2CH2 KMnO4 H2C CH2
OH OH
Two methods
Na2SO3/H2O
CH2CH2 OsO4
H2C CH2
OH OH
26Mechanism
27Oxidative cleavage
28Ozonolysis
29Synthesis of alkynes
30Synthesis 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
31The 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
32Alkynides as Good Nucleophiles for
SN2 Good Base for E2
33Alkynes on the same line as alkenes
Addition of Br2 and Cl2 Addition of
HX Oxidative cleavage
34Hydrogenation of alkynes
Syn addition for the formation of cis-alkenes
Anti addition for the formation of trans-alkenes
35Anti addition the mechanism