Hydrocarbons

Alkenes


Unsaturated hydrocarbons
General formula is CnH2n
Alkenes are also called as olefins.(Oil nature compounds are formed)

Preparations :

  • Controlled hydrogenation of alkynes
  • Kolbe's electrolysis
  • Elimination reaction

Controlled hydrogenation of alkynes :

Kolbe's electrolysis :

E2 elimination :
Elimination bimolecular reaction
Rate ∝ [substrate] [base]
Order = 2
One step mechanism
Intermediate = Transition state

E1 elimination :
Elimination unimolecular reaction
Rate ∝ [substrate]
Order = 1
Two steps mechanism
Intermediate ⇒ Carbocation

E1cb(Elimination unimolecular conjugate base) :

Markovnikov's rule :
In the case of unsymmetrical alkenes, the +ve part is added to double bonded carbon which has more no. of hydrogens to give stable carbocation.
Note : Negative part is added to double bonded carbon, which has less no. of hydrogens.

Anti Markovnikov's rule : Peroxide effect (or) Kharash effect.

Peroxides : H2O2

Mechanism : It involves 3 steps
H_{2}O \xrightarrow[]{h\nu} 2OH^{\cdot}

Chain propagation :

Addition of H2O :

In presence of H^\oplus  Oxymercuration - Demercuration  Hydroboration - Oxidation
 Carbocation no Carbocation no Carbocation
 Rearrangement of ring expansion is possible no rearrangement and ring expansion no rearrangement and ring expansion
 Markovnikov's product Markovnikov's product Anti Markovnikov's rule product

H2O → Alcohol
R − OH → Ethers

H_{2}O_{2}/{OH}^\ominus → Alcohols
Acids → Alkane

Oxymercuration - Demercuration :
Reagent :

Hydroboration - Oxidation :

Ozonolysis :

Hydrogenation :

Syn addition Anti Addition
1) H2/Ni X2/CCl4
2) KMnO4/OH^\ominus → Baeyer's reagent X2/H2O
OsO4 RCO3H/H+/H2O

Hydroboration and oxidation

BH3/THF/H2O2/OH

Reductive Ozonolysis :

  • Zn/H2O
  • (CH3)2S/CH2Cl2
  • p(ph)3/CH2Cl2

Oxidative Ozonolysis :

  • H2O
  • H2O2
  • RCO3H

Part1: View the Topic in this Video from 0:35 to 50:55

Part2: View the Topic in this Video from 0:06 to 21:16

Part3: View the Topic in this Video from 0:06 to 10:58

Part4: View the Topic in this Video from 0:07 to 13:41

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1. General formula of Alkenes = [CnH2n]