\documentclass[12pt,border=1pt]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{chemfig,chemmacros}
\usepackage{lmodern}

\chemsetup[chemformula]{font-shape=sf,format=\sffamily}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setatomsep{2.5em}
\setdoublesep{.6ex}
\setarrowdefault{,1.25}
\setbondstyle{thick}
\renewcommand{\familydefault}{\sfdefault}
\begin{document}
\schemestart
	\chemfig{R-[:-30]=[:30]@{a1}O}
    \arrow{<=>[\chemfig{@{a2}H-[,.8]NH_3^{\scriptstyle+}}]}
    \subscheme{
        \chemfig{\qquad@{b1}NH_3}
        \arrow{0}[-90,0.3]
        \chemfig{R-[:30]@{b22}=[@{b2}:-30]@{b3}O^+-[:30]H}
      }
    \arrow{->}
    \chemfig{R-[:30](-[::60]NH_3^{\scriptstyle+})-[:-30]OH}
    \arrow{<=>[][*{0}\parbox{1.5cm}{\small Proton transfer}]}[-90,.9]
    \chemfig{R-[:30](-[@{c2}::60]@{c1}NH_2)-[@{c3}:-30]@{c4}OH_2^{\scriptstyle+}}
    \arrow{->[$-$\chemfig{H_2O}]}[180]
    \chemfig{R-[:-30]@{d1}=[@{d2}:30]@{d3}NH_2^{\scriptstyle+}}
    \arrow{->[\chemfig{@{d0}\chemabove{C}{\scriptstyle\ominus}N}]}[180,.8]
    \chemfig{R-[:30](-[::60]NH_2)-[:-30]~[::0]N}
    \chemmove[->,red, shorten <=2pt, shorten >=2pt,-stealth]{
    	\draw (a1) .. controls +(north:8mm) and +(north west:5mm) .. (a2);
        \draw (b1) ..controls +(west:5mm) and +(north:5mm) .. (b22);
    	\draw (b2) .. controls +(south west:8mm) and +(south west:8mm) .. (b3);
        \draw (c1) .. controls +(west:5mm) and +(west:5mm) .. (c2);
        \draw (c3) .. controls +(north:5mm) and +(north:5mm) .. (c4);
        \draw (d2) .. controls +(south:5mm) and +(south:5mm) .. (d3);
        \draw [shorten <=8pt] (d0) .. controls +(north:15mm) and +(north:15mm) .. (d1);
    }
\schemestop
\end{document}