15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule

  Hydrocarbons

    1.0 Introduction
    2.0 Alkanes
    3.0 Methods of Preparation Alkanes
    4.0 Physical Proparties
    5.0 Chemical Properties
    6.0 Alkenes
    7.0 Methods of Preparation Alkenes
    8.0 Physical Proparties
    9.0 Chemical Properties
    10.0 Mechanism Of Some Important Reaction Of Alkenes
    11.0 Alkynes
    12.0 Methods of Preparation Alkynes
    13.0 Physical Properties
    14.0 Chemical Properteis
    15.0 Modern Concept
    16.0 Properteis
    17.0 Mechanism of Electrophilic Substitution Reactions
    18.0 Toluene
    19.0 Alkenyl Benzene

---

15.2 Huckel’s rule or $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electron rule

3 The rule states that in a conjugated, planar, cyclic system if the number of delocalized $\pi $-electrons is $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)$where n is an integer, i.e., 1, 2, 3 etc. Benzene, naphthalene, anthracene and phenanthrene are aromatic as they contain $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ electrons i.e. 6, 10, 14, $\pi $ electrons in a conjugated cyclic system. The cyclopentadiene and cyclooctatetraene are non-aromatic as instead of $\left( {{\bf{4n}}{\text{ }} + {\text{ }}{\bf{2}}} \right)\pi $ e? these have $4n\pi {e^ - }.$ Moreover, they are non-planar.

Benzene was first isolated by Faraday from cylinders of compressed illuminating gas obtained from the pyrolysis of whale oil.

Benzene was first synthesised by Berthelot by passing acetylene through ared hot tube.