I want to list many reactions and I created an environment (which I find nice, but if you have a better idee... do not hesitate ^^)
\newsavebox{\mybox}
\newenvironment{reactions}
{\begin{lrbox}{\mybox}
\newcommand{\reaction}[2]{ \item [- ##1] : ##2 }
\begin{minipage}{\linewidth - 2\fboxsep - 2\fboxrule}
\begin{enumdescript}}
{\end{enumdescript}
\end{minipage}
\end{lrbox}\fbox{\usebox{\mybox}}}
enumdescript is enumerate and description environment (see bellow)
Thanks to those who helped me by helping others, I wanted to vote for the answers but I've not enough reputation...
But I have many reactions. Too many and one minipage is not enough. I know I can create a second reaction environment but it's not practical and the second numerotation will start to one.
\begin{reactions}
first reactions
\end{reactions}
\begin{reactions}
last reactions
\end{reactions}
But I wonder if it's possible to do this automatically...
Thank you
Ccile
PS1 : Sorry for the mistakes, do not hesitate to correct me
PS2 : here is the entire code with fake reactions if you need.
\documentclass[a4paper]{article}
\usepackage[utf8]{inputenc}
\usepackage[T1]{fontenc}
\usepackage{multirow}
\usepackage{pdfpages}
\usepackage{array}
\usepackage{longtable}
\usepackage{enumitem}
%Liste enumerate et description
\newcounter{descriptcount}
\newlist{enumdescript}{description}{2}
\setlist[enumdescript,1]{%
before={\setcounter{descriptcount}{0}%
\renewcommand*\thedescriptcount{\arabic{descriptcount}}}
,font=\bfseries\stepcounter{descriptcount}\thedescriptcount~
}
\setlist[enumdescript,2]{%
before={\setcounter{descriptcount}{0}%
\renewcommand*\thedescriptcount{\roman{descriptcount}}}
,font=\bfseries\stepcounter{descriptcount}\thedescriptcount~
}
%Fleches rev irrev
\newcommand\irrev{\(\rightarrow \ \)}
\newcommand\rev{\(\leftrightarrow \ \)}
%Environnement reactions
\newsavebox{\mybox}
\newenvironment{reactions}
{\begin{lrbox}{\mybox}
\newcommand{\reaction}[2]{ \item [- ##1] : ##2 }
\begin{minipage}{\linewidth - 2\fboxsep - 2\fboxrule}
\begin{enumdescript}}
{\end{enumdescript}
\end{minipage}
\end{lrbox}\fbox{\usebox{\mybox}}}
\begin{document}
\section{Les réactions}
\begin{reactions}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\reaction{name}{A + B + C + D + E \irrev F + G + H + I + J}
\end{reactions}
\end{document}

mdframed? See this answer for example. – Dai Bowen Dec 14 '16 at 18:50breakabletcolorboxwhich is also shown is linked question: http://tex.stackexchange.com/a/184683/1952 – Ignasi Dec 14 '16 at 18:52