Abstract
Replacement of a carbon atom of an aromatic compound by a more electronegative heteroatom gives rise to suppression of reactivities of the remaining carbon atoms in electrophilic substitution (SE2) reactions. To prove this experiment-based rule, the reactivities are related to the relevant extents of the intermolecular charge transfer. General algebraic expressions are derived and analyzed for these extents for various directions of the attack of electrophile. The reactant is considered as a perturbed alternant hydrocarbon, the perturbation coinciding with the alteration in the Coulomb parameter of the carbon atom undergoing the replacement, whereas the reagent is represented explicitly by an external vacant orbital. Changes in the intermolecular charge transfer due to introduction of heteroatom are shown to consist of two components, viz. of an initial-population-dependent component contributing increments of alternating signs for shifting positions of electrophile along the hydrocarbon chain and of an additional negative contribution originating from the increased interval between diagonal Hamiltonian matrix elements for orbitals participating in the charge transfer. For soft electrophiles, the negative contribution is shown to predominate over the alternating one. This implies the charge transfer between heteroaromatic reactants and electrophiles to be suppressed vs. the relevant values for hydrocarbons-containing system. (doi: 10.5562/cca1794)
Highlights
Relative reactivities of related chemical compounds towards a certain reagent usually exhibit more or less regular behaviour
In the simple Hückel model, the 2n -dimensional basis set {χ} of any even alternant hydrocarbons (AHs) consisting of 2pz AOs of carbon atoms is known to be divisible into two n-dimensional subsets {χ } and {χo} so that the intrasubset resonance parameters take zero values. 47 50,59 Given that the Coulomb parameters are assumed to be uniform and the equality αC = 0 is accepted for convenience, the model Hamiltonian matrices of AHs acquire a common form containing non-zero submatrices in its off-diagonal positions only as exhibited below in Eq (1)
The above study gives algebraic expressions for matrices of charge transfer (CT) abilities of systems participating in the SE2 reactions
Summary
Relative reactivities of related chemical compounds towards a certain reagent usually exhibit more or less regular behaviour. The majority of the relevant parent systems, in turn, belong to the so-called alternant hydrocarbons (AHs)[5,6,11,12,45−48] that are representable by bipartite graphs of their C-skeletons.[49−51] Just this implies the validity of the rule of alternating polarity[45−47] being interpretable in graphtheoretical terms too.[46] As a result, populations of AOs of the heteroatom-containing subset of any derivative are predicted to be increased, whilst those of the remaining subset are expected to be reduced vs uniform occupation numbers of all AOs of an AH equal to unity[49,50] (The well-known partition of the basis set of 2pz AOs of carbon atoms of an AH into two subsets[48−51] is invoked here) On this basis, enhanced relative reactivities are anticipated for all positions of the derivative represented by AOs of the first subset in direct contradiction to the above-discussed experimental facts. The final sections are devoted to analysis of the results and to their illustration
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