E2 Stereochemistry
E2 reactions occur most rapidly when the H-C bond and C-LG bonds involved are co-planar, most often at 180o with respect to each other. This is described as an antiperiplanar conformation. This conformation positions the σ bonds that are being broken in the correct alignment to become the π bond.
  | ||
| | ||
 | ||
also give elimination .But they are energetically unfavoured.
 | The eclipsed, synperiplanar alignment also aligns the two σ bonds that become the π bond, but is less favourable than the antiperiplanar arrangement. |  |
In the cyclic system, in order for the preferred antiperiplanar arrangement favoured by E2 reactions, the C-H and C-LG bonds both need to be axial. |
examples :
sir when we discuss the case of gauche confirmation waht will the mechanism of reaction.Will the elimination is favoured???
ReplyDeleteassuming that acidic H and leaving group are at gauche projection the leaving group and acidic hydrogen are at 60 degrees to each other .This is is energetically unfavorable condition .Repulsion's will be very very high and elimination would not
ReplyDeletebe feaseable