Pericyclic Reactions, Exams of Organic Chemistry

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Chapter No. 2 Pericyclic Reactions 2.1 Introduction (Peri mean around and cyclic mean circle) therefore, Pericyclic mean ground the circle, It may | be defined as: “A pericyclic reaction is a concerted reaction that proceeds: through a cyclic tramation state” 2.2 Characteristics of pericyelic reactions i. Pericyclic reactions are a single step. ii. ‘These reactions have conjugated cyclic transition state. iii, These reactions are internally self-sufficient (no extra reagent or catailyst is required) and they are initinted thermally or photochemically. iv. ‘These are sterenspecific in nature (only one stereoisomer product is formed). ¥, They show an equilibrium state (reactants and products have the ability to interchange with each other). 2.3 Types of pericyelic reactions There are following types of pericyclic reactions 1) Electrocyclie reactions 2) (Cycloaddition reactions 3) Sigmatropic rearrangement 4) Group Transfer reactions i Electrocyelic reactions “An electrocyelic reaction i8 a reversible reaction that involves a ring closure or ic reaction is the concerted interconversion ‘of a conjugated polyene and a ring-opening reaction.” An electrocycli reactions are induced either thermally or photochemically. Fl —/7\ cyeloalkene, These ¢ reactions are reversible reactions and open-chain system in the reaction is always & Electrocycli 1Ons conjugated system whereas cyclic one may or may not be a conjugated system. In electrocyclic react either a ring is formed with the generation of a'new ¢ bond and the loss of az bond or vice versa. 4X LI kw electrons (k =2) welectrons + one & bond k= 4 (loss of'one x bond and gain of one o bond) 63