documentclass{article}
usepackage[version=3]{mhchem}

newcounter{rxnum}

newenvironment{reaction}{
refstepcounter{rxnum}
noindent
%???
}{
%???
parnoindent
}

begin{document}

begin{reaction}
 ce{X + Y -> XY}
end{reaction}

begin{reaction}
 ce{XX + YY -> X2Y2} \%<- newline just like in equation
 ce{A + B -> C}
end{reaction}


end{document}

begin{reaction}
ce{X + Y ->XY}descriptionforlist{XY formation reaction}\
ce{A +B -> C}
ce{G + H -> J}nonumber
    end{reaction}

R 1: XY formation reaction                6

R 2:                                      6

documentclass{article}
usepackage{chemmacros}

begin{document}

listofreactions

begin{reaction}[bla]
 $A$ ->[ H2O ] $B$
end{reaction}

begin{reaction}
 SO4^2- + Ba^2+ -> BaSO4 v
end{reaction}

end{document}

documentclass{article}
usepackage{amsmath,mhchem}
newenvironment{reaction}{begin{equation}}{end{equation}}
newenvironment{reaction*}{begin{equation*}}{end{equation*}}
begin{document}
begin{reaction}
 ce{$A$ ->[ce{+H2O}] $B$}
end{reaction}
begin{reaction}
ce{SO4^2- + Ba^2+ -> BaSO4 v}
 end{reaction}
end{document}

documentclass{article}
usepackage{amsmath,mhchem}
usepackage{tocloft}
newcommand{reactionlistname}{List of reactions}
newlistof{reactions}{rxs}{reactionlistname}

newcounter{reaction}
newcounter{tmp}
newenvironment{reaction}[1][relax]{%
setcounter{tmp}{value{equation}}
setcounter{equation}{value{reaction}}
renewcommand{theequation}{arabic{equation}}
begin{equation}
addcontentsline{rxs}{reactions}{protectnumberline{thereaction}#1}
}{%
end{equation}
setcounter{reaction}{value{equation}}
setcounter{equation}{value{tmp}}
}


begin{document}


listofreactions

begin{reaction}[bla]
 ce{$A$ ->[ce{+H2O}] $B$}
end{reaction}

begin{reaction}
ce{SO4^2- + Ba^2+ -> BaSO4 v}
 end{reaction}

end{document}