Transcript Physical Organic Chemistry

Islamic University in Madinah
Department of Chemistry
Physical Organic Chemistry
CH-5
Addition & Rearrangement reactions
Prepared By
Dr. Khalid Ahmad Shadid
Addition to double bond
 Alkene double bond contain sigma and pi, the bond is more reactive thane in alkane.
 Can react through electrophilic addition.
 Electrophilic addition
 Bromine and chlorine can react with alkene, while Iodine doesn't react. Florin react very
fast but no product.
 General Mechanism:
R2C=CR2 + X2 ——> R2CX-CR2X
Brominating Mechanism
 Exclusively Trans Addition to alkene. Even when alkene contain bulky group
like tertiary butyl.
 Br+ adds to an alkene producing a cyclic ion
 Bromonium ion, bromine shares charge with carbon
 Gives trans addition
 Electrophilic addition of bromine to give a cation is followed by cyclization to give a
bromonium ion
 This bromonium ion is a reactive electrophile and bromide ion is a good nucleophile
 Stereospecific anti addition
Addition of strong Acids
 Addition of proton to a double bond (rate determining step), then fast
nucleophilic attack.
Addition of Hydrogen Halide to alkene
 Addition of HX to alkene. Can cause carbocation rearrangement.
 Carbocation rearrangement from secondary to more stable tertiary.
Addition of Hypohalous Acids to
Alkenes: Halohydrin Formation
 This is formally the addition of HO-X to an alkene to give a 1,2-halo alcohol, called a halohydrin
 The actual reagent is the dihalogen (Br2 or Cl2 in water in an organic solvent)
 (HO-X), X: CL or Br is electrophile, its less electronegative than Oxygen
Mechanism of Formation of a Bromohydrin
 Br2 forms bromonium ion, then water adds
Orientation toward stable C+ species
Aromatic rings do not react
Addition sulfonyl chloride
Here electrophile is a cation RS+ . Chlorine more electronegative than sulfur
(CH3)2C=CH2 + C6H5SCl ——> (CH3)2CCl-CH2SC6H5
Addition of Water to Alkenes
 Hydration of an alkene is the addition of H-OH to to give an alcohol
 Acid catalysts are used in high temperature industrial processes: ethylene is
converted to ethanol
Oxymercuration Intermediates
 For laboratory-scale hydration of an alkene
 Use mercuric acetate in THF followed by sodium borohydride
 Markovnikov orientation
via mercurinium ion
Addition of Water to Alkenes: Hydroboration
 Herbert Brown (HB) invented hydroboration (HB)
 Borane (BH3) is electron deficient and is a Lewis acid
 Borane adds to an alkene to give an organoborane
Orientation in Hydration via
Hydroboration
 Regiochemistry is opposite to Markovnikov orientation
 OH is added to carbon with most H’s
 H and OH add with syn stereochemistry, to the same face of the alkene (opposite of anti addition)
 STEREOSPECIFIC
Mechanism of Hydroboration
 Borane is a Lewis acid
 Alkene is Lewis base
 Transition state involves anionic development on B
 The components of BH3 are added across C=C
 More stable carbocation is also consistent with steric preferences
Halogen Addition
 Mixed Halogens are polarized: X+- X more electronegative halogen will carry partial negative charge
 Rate of addition:
BrCl > Br2 > ICl > IBr > I2
 Morkovinikov addition
Oxidation of Alkenes: Epoxidation and Hydroxylation
 Oxidation is addition of O, or loss of H
 Epoxidation results in a cyclic ether with an oxygen atom
 Stereochemistry of addition is syn
 MCPBA in CH2Cl2 are the usual conditions
 Addition of acid results in a trans-1,2-diol
 Treatment of the epoxide with aqueous acid give a trans diol
Osmium Tetroxide Catalyzed Formation of Diols
 Hydroxylation - converts to syn-diol
 Osmium tetroxide, then sodium bisulfate
 Via cyclic osmate di-ester
 Osmium is toxic, so catalytic amount and NMO are used
What is Rearrangement Reactions?

The term of “rearrangements” is used to describe organic reactions which involve the
migration of an H atom or of a larger molecular fragment.

Nucleophilic Rearrangements

Electrophilic rearrangements

Radical rearrangements
1. Nucleophilic Rearrangements
1
 [1,2]-Rearrangements
1'
1
1'
1
R(H)
C
C+
2'
+
C
1'
C
2'
1
R(H)
C
R(H)
+
N
2'
+
C
1'
R(H)
N2'
Wagner-Meerwein rearrangements
 Wagner-Meerwein Rearrangements are [1,2]-rearrangements of H atoms
or alkyl groups in carbenium ions that do not contain any heteroatoms
attached to the valence-unsaturated center C-1 or to the valencesaturated center C-2.
1
1'
1
R(H)
C
C+
2'
+
C
1'
R(H)
C
2'
CH3
CH3 C CH2OH
CH3
CH3
CH3 C CH2
CH3
H+
CH3
CH3 C CH2OH2
CH3
Cl-
CH3 C CH2CH3
CH3
H+
H2O
Cl
CH3 C CH2CH3
CH3
CH3C=CHCH3
CH3
Wagner-Meerwein rearrangements
Carbocation
CH3+ < CH3CH2+ < (CH3)2CH+ < CH2=CH-CH2+ < C6H5CH2+
Stability
 Carbenium ions: 1 °→2 °，1 °→3 °
2 °→3 °
 Reactions include Wagner-Meerwein rearrangement step:
1. Electrophilic additions of alkenes
2. Nucleophilic substitutions (SN1)
3. E1 elimination
4. Friedel-Crafts alkylation reactions, etc
 Example: Friedel-Crafts Alkylation
 1-Bromopropane isomerizes quantitatively to 2-bromopropane under FriedelCrafts conditions. The [1,2]-shift A→B involved in this reaction again is an Hatom shift.
CH3
cat. AlCl3 in
H
H
H
+
H
Br
NO2
CH3
H
-
AlBr4
A
+
H
H
H
AlBr4
B
- AlBr3
Br
 Example:
Wagner-Meerwein rearrangement as part of an isomerizing E1
elimination
OH conc. H SO
2
4
+
OH2
H
+
_H O
2
H
Methyl shift
H H
_
H+
+
 Example: Nucleophilic Substitution
CH3
CH2 NH2
H3 C
HNO2
CH3
OH
CH3 C CH2CH3
CH3
Methyl shift
Mechanism
CH3
CH2 NH2
H3 C
CH3
CH3 +
C CH2
CH3
CH3
OH
C CH2CH3
CH3
HNO2
CH3
CH3
+
C CH2CH3 H2O
H+
CH3
 Example: E1 and Nucleophilic Substitution
CH2NH2
HNO 2
OH
CH2OH
+
+
+
Mechanism
CH2NH2
HNO 2
+CH
CH2OH
+
+
2
OH
+
GOOD LUCK