Download presentation
Presentation is loading. Please wait.
Published byTabitha Elliott Modified over 8 years ago
1
Physical Organic Chemistry CH-5 Addition & Rearrangement reactions Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry
2
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: R 2 C=CR 2 + X 2 ——> R 2 CX-CR 2 X
3
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
4
Addition of strong Acids Addition of proton to a double bond (rate determining step), then fast nucleophilic attack.
5
Addition of Hydrogen Halide to alkene Addition of HX to alkene. Can cause carbocation rearrangement. Carbocation rearrangement from secondary to more stable tertiary.
6
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 (Br 2 or Cl 2 in water in an organic solvent) (HO-X), X: CL or Br is electrophile, its less electronegative than Oxygen
7
Mechanism of Formation of a Bromohydrin Br 2 forms bromonium ion, then water adds Orientation toward stable C + species Aromatic rings do not react
8
Addition sulfonyl chloride Here electrophile is a cation RS +. Chlorine more electronegative than sulfur ( CH 3 ) 2 C=CH 2 + C 6 H 5 SCl ——> (CH 3 ) 2 CCl-CH 2 SC 6 H 5
9
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
10
Oxymercuration Intermediates For laboratory-scale hydration of an alkene Use mercuric acetate in THF followed by sodium borohydride Markovnikov orientation via mercurinium ion
11
Addition of Water to Alkenes: Hydroboration Herbert Brown (HB) invented hydroboration (HB) Borane (BH 3 ) is electron deficient and is a Lewis acid Borane adds to an alkene to give an organoborane
12
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
13
Mechanism of Hydroboration Borane is a Lewis acid Alkene is Lewis base Transition state involves anionic development on B The components of BH 3 are added across C=C More stable carbocation is also consistent with steric preferences
14
Halogen Addition Mixed Halogens are polarized: X + - X - more electronegative halogen will carry partial negative charge Rate of addition: BrCl > Br 2 > ICl > IBr > I 2 Morkovinikov addition
15
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 CH 2 Cl 2 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
16
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
17
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 [1,2]-Rearrangements
18
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 valence- saturated center C-2.
21
Wagner-Meerwein rearrangements 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 Carbocation Stability CH 3 + <CH 3 CH 2 + <(CH 3 ) 2 CH + <CH 2 =CH-CH 2 + <C 6 H 5 CH 2 +
22
Example: Friedel-Crafts Alkylation 1-Bromopropane isomerizes quantitatively to 2-bromopropane under Friedel- Crafts conditions. The [1,2]-shift A→B involved in this reaction again is an H- atom shift.
23
Wagner-Meerwein rearrangement as part of an isomerizing E1 elimination Methyl shift Example:
24
CH 3 C CH 3 CH 3 CH 2 CH 3 C CH 3 CH 2 CH 3 H + H 2 O HNO 2 Mechanism Methyl shift + + Example: Nucleophilic Substitution
25
Mechanism Example: E1 and Nucleophilic Substitution
26
GOOD LUCK
Similar presentations
© 2025 SlidePlayer.com. Inc.
All rights reserved.