Struktur-Reaktivität-Gleichungen und ihre Anwendung zur Bestimmung organischer Reaktionsmechanismen

Abstract

Several examples of the application of structure-reactivity correlations to the determination of organic reaction mechanisms are discussed.

As a first example, our results on the photochlorination of aliphatic compounds are considered. The rate limiting step of this reaction is the abstraction of a hydrogen by a chlorine (R : H + Cl⁰ —> R⁰ + H : Cl) and the kinetics actually conform to the equation: Δlog k = ρ^*σ^* + hN + Ψ according to which the hydrogen reactivity depends on inductive, hyperconjugative and resonance effects of substituents in R.

In the second place, three series of compounds, specially designed for the purpose of testing additivity of substituent effects, are examined. The first one is

X–C₆H₄–CO–CH₂–CO–NH–C₆H₄–Y,

the second one

X–C₆H₄–CO–CH(–CH₂–C₆H₄–Z)–CO–NH–C₆H₄–Y,

and the third one, R–CO–NH–C₆H₄Y. In each series the acidity constant (K_a) of the central hydrogen atom varies under the action of substituents (X, Y; X, Y, Z or R, Y). It is shown that new general correlations:  Δlog K_a = ρ_xσ_x + ρ_yσ_y;   Δlog K_a = ρ_xσ_x + ρ_yσ_y + ρ_zσ_z; Δlog K_a = ρ_yσ_y + ρ_R^*σ_R^* respectively are obtained.

Finally we show that the sign (± ) of ρ does not only depend on the type of substitution (SN₂ or SE₂), but changes also for the same type of process depending on the place of the substituent; the latter being located either in the entering group or in the substrate. Thus a given bimolecular displacement may be divided into two reaction-series, the first one with a constant entering group (N_c or E_c), the second one with different entering groups (N_v or E_v).

The following rules are then formulated: for SN₂(N_c) or SE₂(E_v) is ρ > 0 (positive); for SN₂(N_v) or SE₂(E_c) is ρ < 0 (negative).

Examples of mechanistic determinations are given for carbonylcondensations and amide basic hydrolysis.