An undergraduate experiment that illustrates the phenomenon of perichromism-the sensitivity of a dye to its microenvironment, as assessed by color changes of its solutions-is described. An easily prepared perichromic imine is synthesized and characterized, and its solvatochromism, thermochromism, halochromism, and preferential solvation in binary solvent mixtures are demonstrated by visual inspection of its solutions. The results are rationalized by invoking solute - solvent interactions in the various media.

Uma equação de tipo Hammett, relacionando as eletrofilicidades teóricas globais e de substituinte, foi derivada para etilenos e ácidos benzóicos substituídos, permitindo o cálculo de constantes eletrofílicas para substituintes σω. As constantes teóricas foram correlacionadas com parâmetros cinéticos ou termodinâmicos para vários processos químicos da literatura. Embora formalmente semelhantes às constantes de Hammett, as constantes eletrofílicas para substituintes são obtidas a partir de postulados teóricos inteiramente distintos. As semelhanças e diferenças entre os dois conjuntos de constantes são discutidas e racionalizadas em termos da definição do poder eletrofílico de uma espécie química.

A Hammett-like equation, relating theoretical global and substituent electrophilicities, is derived for substituted ethylenes and benzoic acids, allowing the calculation of electrophilic substituent constants σω. Correlations are given between the theoretical σω constants and kinetic or thermodynamic parameters of various chemical processes from the literature. Though electrophilic substituent constants are formally similar to the Hammett constants, they are derived from entirely different theoretical postulates. The similarities and differences between the two sets of constants are discussed and rationalized in terms of the definition of the electrophilic power of a chemical species.

An undergraduate organic lab experiment is described based on the preparation of two readily accessible hydrazones. The UV-visible spectra of these N-H acids and of their conjugate bases are employed to illustrate the importance of through-conjugation in determining their acid strength and their internal charge-transfer-band transitions.

O efeito da anelação em fragmentos aromáticos doadores e/ou aceptores em merocianinas aparentadas é discutido com auxílio de um modelo teórico baseado em cálculos semi-empíricos utilizando o método AM1. As previsões teóricas são validadas com exemplos de oito corantes solvatocrômicos descritos na literatura.

The effect of annulation of the donor and/or the acceptor ring fragments of related merocyanines is discussed with the aid of a theoretical model based on semiempirical calculations performed with the AM1 method. The theoretical expectations are validated with examples of eight solvatochromic dyes described in the literature.