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Physical Organic Chemistry CH-5 Addition & Rearrangement reactions Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry.

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Presentation on theme: "Physical Organic Chemistry CH-5 Addition & Rearrangement reactions Prepared By Dr. Khalid Ahmad Shadid Islamic University in Madinah Department of Chemistry."— Presentation transcript:

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.

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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


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