Junctive Sites

A junctive site is a reacting atom at which a change in directed bonding occurs. In Figure 8.2 (p. 3), all junctive sites are marked with arrows. A junctive simplex is the smallest portion of a molecule/molecular species that encompasses the reacting atomic sites involved in a junctive process. Jimctive simplexes are of two types - fundamental and topological. Processes are derived from fimdamental simplexes, or topological ones, or both (vide infra). 

Chlorine atom 1 (in l+2- [3] ) and the carbenic carbon in 18a (18a+18b- [19] ) are atomic junctive simplexes (type 26 and 27 (Figure 8.3), respectively) for each of them, aj=l. In the latter case, it is solely the central carbon that constitutes the junctive atom the rest of the molecule is not part of the simplex. Bond junctive simplexes are exemplified by the O-H bond (type 28) in 24a for the transformation 24a+24b- 25), and the C-C bond of 5 (type 29) in transformation 5+6- 7. In these cases, the junctive sites are the terminal atoms of the a-bond thus, a =2. Lastly, n-junctive simplexes are typified by ketone 9 (type 30, Figure 8.3) during the course of its reactions with an alkyllithium reagent (9+10- ll), and 12 (type 31, Figure 8.3 3 =2), in its Diels-Alder reactions (12+13- 14) in the latter, the junctive sites are the terminal carbons of the 7i-system. Hence, for each of 30 and 31, a,=2. 

The formation of a directed bond between two (or more) reactive atomic sites (belonging to one, two or more reactants/reactant moieties) en route to fransifion state, intermediate or product(s) (Figure 8.2) is, ipso facto, a junctive process. Such a process manifests itself in the formation of (a) incipient a bonds (l+2- [3] , 5+6- [7]), (b) full-fledged a bonds (l+2- 4, 5+6- 8, 9+l0- ll, 12+13- 14), (c) partial bonds due to dipole-dipole, dipole-ion (in forming host-guest complexes ... 

In describing junctive/disjunctive processes, with increasing numbers of interacting molecular entities, and increasing number of interactions beween different reactive sites, the denotation can... 

Fundamental simplex junctivity, jg f is the sum of atom junctivities (ja. ) of all reacting atomic sites in a fundamental simplex s, and is given by Equation 8.2 ... 

A bijunctive molecule may have two monojunctive atomic sites a tetrajunctive molecule may have two bijunctive simplexes. The j , value of M depends on the process it undergoes. For that molecule, in the specified transformation, j , is the sum of the js/j values of all of its reacting junctive simplexes - fundamental and/or topological - taking part in the transformation (Equation 8.5) ... 

The fundamental processes at atomic sites are junclive (associative, additive or complexative), disjunctive (dissociative, eliminative or decomplexative), or synchronous (substitutive, simultaneously junctive and disjunctive). The concept of junctivity/disjimctivity was discussed in Chapter 8. 

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