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Thermolysis converts 1 to (E,E) geometric isomer 2, but 3 to (E,Z) isomer 4.. The Woodward–Hoffmann rules (or the pericyclic selection rules) [1] are a set of rules devised by Robert Burns Woodward and Roald Hoffmann to rationalize or predict certain aspects of the stereochemistry and activation energy of pericyclic reactions, an important class of reactions in organic chemistry.
The best-known example is benzene (C 6 H 6) with a conjugated system of six π electrons, which equals 4n + 2 for n = 1. The molecule undergoes substitution reactions which preserve the six π electron system rather than addition reactions which would destroy it. The stability of this π electron system is referred to as aromaticity. Still, in ...
The cyclopentadienyl anion is a planar, cyclic, regular-pentagonal ion; it has 6 π-electrons (4n + 2, where n = 1), which fulfills Hückel's rule of aromaticity. Each double bond and lone pair provides 2 π-electrons, which are delocalized into the ring. [4] Cyclopentadiene has a pKa of about 16. It is acidic relative to many carbon acids.
In organic chemistry, an electrocyclic reaction is a type of pericyclic, rearrangement reaction where the net result is one pi bond being converted into one sigma bond or vice versa. [1] These reactions are usually categorized by the following criteria:
The table in Figure 2 summarizes the Möbius–Hückel concept. The columns specify whether one has a Möbius or a Hückel structure and the rows specify whether 4n + 2 electrons or 4n electrons are present. Depending on which is present, a Möbius or a Hückel system, one selects the first or the second column.
To summarize, we are assuming that: (1) the energy of an electron in an isolated C(2p z) orbital is =; (2) the energy of interaction between C(2p z) orbitals on adjacent carbons i and j (i.e., i and j are connected by a σ-bond) is =; (3) orbitals on carbons not joined in this way are assumed not to interact, so = for nonadjacent i and j; and ...
A cycloaddition is a reaction that simultaneously forms at least two new bonds, and in doing so, converts two or more open-chain molecules into rings. [3] The transition states for these reactions typically involve the electrons of the molecules moving in continuous rings, making it a pericyclic reaction.
The quantum mechanical basis for its formulation was first worked out by physical chemist N. Colin Baird at the University of Western Ontario in 1972. [1] [2] The lowest triplet state of an annulene is, according to Baird's rule, aromatic when it has 4n π-electrons and antiaromatic when the π-electron count is 4n + 2, where n is any positive ...