ps. (solo per "archive purposes")
si poteva anche usare \newenvironment
\@definecounter{reaction}
\renewcommand\thereaction{r.\@arabic\c@reaction}
\newenvironment{reaction}{$$\refstepcounter{reaction}}{\eqno \hbox{\@reactnum}$$\@ignoretrue}
\def\@reactnum{{\normalfont \normalcolor (\thereaction)}}