This document discusses various addition reactions to multiple bonds, specifically the stereochemical factors that determine the stereochemistry of addition. It covers topics such as the addition of hydrogen, halogens, hydroboration, hydroxylation, and epoxidation. For hydrogenation reactions, syn addition occurs which gives either erythro or threo products depending on the geometry of the alkene. Halogen addition proceeds by an anti addition mechanism. Hydroboration and epoxidation reactions retain the stereochemistry of the original alkene.

1. ADDITION TO MULTIPLE
BONDS
Mahesh waran M
I M.Sc Chemistry
S.B.K College.

2. STEREO CHEMICAL FACTORS IN ADDITON REACTIONS
 Addition of Hydrogen
 Addition of Halogen
 Hydroboration
 Hydroxylation
 Epoxidation

3. Syn Addition :
Addition of substituents to the same side of the multiple bonds.
Anti Addition :
Addition of substituents to the Opposite side of the multiple
bonds.

4. Additon of Hydrogen - catalytic Hydrogenation
 Hydrogenation of olefin in presence of catalyst Ni,Pt,Pd under
varying conditions of heat and pressure.
 Catalyst provide to form complex with olefin and promote the
activation of adsorbed hydrogen molecule.
 Molecular hydrogen splits up into two hydrogen molecule and form
new C-H bonds on the same side.
 In this type of Addition is called Syn Addition.

6.  Double Bond is Reduced more readily then triple bond. Exception of nitro
groups and acid chlorides.
 Selective Reduction of carbon-carbon Double bond.
 Addition of Hydrogen to alkene only two of the stereoisomers formed
since only Syn Addition.
 Purely depend upon the geometry of the reacting alkene.
 Cis alkene – syn addition – erythro pair of enantiomers.
 Trans alkene – syn addition – threo enantiomers.

9.  If double bond Isomerization occurs more rapidly than
hydrogenation. In this case , isomerization occurs before reduction.
 Palladium – prone to catalyze double bond isomerization
 Pt, Rh, Ir – there is some problem.

10.  Catalytic hydrogenation in Alkynes leads to Syn Addition. It can be
Stopped at alekene stage by using less reactive poisoned catalyst.
Example – Lindlar’s Catalyst ( poisoned Pd catalyst using BaSO4
Poisoned with Quinolene )

11.  Sodium metal in liquid Ammonia reduce alkynes with Anti
stereochemistry. Electrons transferred more readily to actylenes than
alkenes.
 The vinyl radical formed in course of reaction is more stable in trans
configuration.

13. Diimide Reduction
 Non Mettalic compound used to hydrogenation of C-C multiple bonds.
 Diimide – Oxidation of Hydrazine. Anti and Syn forms are generated.
 In presence of alkene – reduction occurs more rapidly.
 Only Syn form reduces the double bonds by cyclic mechanism. So the
Addition is , stereichmeically Syn Addition.
 Highly selective to reduce readily for symmetrical double bonds.
 The Unsymmetrical bonds and more polar bonds are not be reduced.

15. Homogeneous Hydrogenation – Wiliknson’s Catalyst
 Hydrogen by its incorporation sphere around the metal atom of the VIII group
elements. Ex. Rh,Ru
 Hydrogenation involved in two phases.
solid phase – catalyst containing adsorbed hydrogen
liquid phase – solution has the unsaturated reactant
 Transition metal complex – Rh[(C6H5)3P3Cl ( Wilkinson catalyst )
 Hydrogenation occurs in single phase in solution.
 This catalyst in solution exchanges reversibly on Ph3P group loosely held sovent
molecule.

16.  Catalyst reacts with hydrogen form metal hydride .
 H-H bond is cleaved and each hydrogen separately bonded to Rhodium. Then one
of its electron pairs used to oxidized to Rhodium (III) State.
 Alkene reacts with complex and replace solvent molecule.
 Alkene metal bond formation result in overlap of empty orbital of the metal with
pi cloud of th alkene.
 Then two hydrogen atoms are transferred to alkene on the same side result in Syn
Addition.

19.  Wilkinson catalyst – only alkenes and alkynes are reduced . C=O,
C=N, NO2 remain intact.
 Mono and Disubstituted alkenes are far more reactive than tri and
tetra substituted double bonds.

20. Birch Reduction
 Catalytic methods fully reduce aromatic rings .
 Electron transfer methods is highy selective and reduction in the
presence of alcohol.
 Sodium in liquid ammonia is the reducing agent . Alcohol used to trap
the anion radical rapidly.
 1,4 addition takes place to yield to Non Conjucated cyclohexadiens.
 Structure of the product determined by the site of first protonation.

22. Addition of Halogens
 Addition of electrophilic reagents in two step process.
1. formation of organic cation ( bridged cation )
2. react with Nu.
 Three membered ring was formed by the donation of Br lone pairs of the vacant P
orbital of the neighbouring carbocation.
 bromonium ion formed by the reaction of the alkene with bromine molecule and push
out the bromide ion.
 When an alkene reacts with bromine Anti Addition occurs.
 Bromonium ion formed and then attacked by the bromide ion by SN2 reaction
mechanism of the opposite side of the bromonium ion.

24.  Addition of Chlorine , chloronium ion is to be less strongly bridged.
 The attack of sulphur and selenium electrophiles have strong bridging
features.
 Its also leads to Anti stereochemistry.

25. Halohydrin formation – Markonikov’s Regiochemistry
 In the ionic addition of unsymmetrical reagent an electrophile adds to
pi bond to form bond with carbon that allows the formation of most
stable carbocation.
 In formation of bromohydrins ,
bromine – adds to least substituted carbon
hydroxyl group – adds to more substituted carbon

27. Epoxydation
 Alkenes react with peroxyacids give Epoxides.
 MCPBA – commonly used per acid . It being shock sensitive.
 Recently , MMPP as a safer substitute.

28.  Epoxidation is an Stereospecific. The configuration of the alkene to be
retained.
 The bond making and breaking steps are concerted-epoxidation, single step
transfer of the peroxy oxygen to the double bond.
 Epoxidation is an regioselective reaction. More electron rich double bond
reacts when more then one is present.

31. Hydroxylation
 Alkene converted into 1,2 diols on react with OsO4 , KMnO4.
 Formation of Osmate ester and then hydrolysed with Sodium Sulphite give Addition
product.
 The addition of two hydroxyl groups an the same side of the alkene. Syn
Stereochemistry to be occurs.
 Py complexes with Os atom as a catalyst.
 The reaction is stereospecific. E- alkenes give Syn 1,2 diols. Z- Alkenes give Anti 1,2
diols.
 OsO4 reagent together with Co-Oxidant.
 ex. NMO, Me3NO,H2O , t- BuOOH.

34. Woodward - Hydroxylation

35. Provost - Hydroxylation

36. Hydroboration
 Organo borane formed initially and further react with alkene gives
trialkyl borane R3B.
 All three hydrogens of B atom replaced by alkyl groups.
 Tri alkylboranes oxidized with basic H2O2 to give alcohols.
 The net result is Hydroboration-Oxidation.

38.  Sterically hindered alkenes reaction can be controlled and monoalkyl and
dialkyl boranes can be obtained.
 ex. 9BBN-H, Sia2BH very strongly regioslective due to steric bulk of the
reagent. Catecholborane is another one.
 9BBN-H IS Highly regioselective. The boron atom adds to less bulky of the
two substituents.
 The B-H bond is much less readily accessible than the B-H bond in BH3.

41.  On heating organoborane at 150oC an isomerization occurs.
 Internal alkene converted into primary alcohol.

42. Questions ?

44. Reference :
 “ stereochemistry mechanism and conformation “ P.S KALSI.

45. THANK YOU FOR YOUR KIND ATTENTION