Stereo Chemistry in E2 Reaction

The stereoelectronic requirement of an E2 process is that the groups to be eliminated should be anti-periplanar (conformationally Trans to one another, with a dihedral angle of 180^o ) and not syn-periplanar (conformationally Cis to one another, with a dihedral angle of 0^o ).

The stereoelectronic requirement of anti-periplanarity exists because of the following reasons
(i)   Coplanarity of the sigma orbitals of the departing groups cause maximum overlap of the developing
       p-orbitals (minimum energy of the transition state).
(ii)  The anti geometry allows the bonding electrons of beta substituent to displace the leaving group from
       back side(back side attack is energetically favourable)
(iii) The attacking base and the leaving group remain far apart from each other(steric interactions are
       minimum).

1,2-elimination reaction

The type of elimation reaction in which the two substituents are lost from adjacent atoms with the resultant formation of a multiple bond is known as 1,2- or b-elimination