\documentclass[12pt,border=0pt,crop,multi=false,tikz,class=scrartcl]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{chemfig,chemmacros}
\usepackage{lmodern}
\usepackage{gensymb}
\usepackage{textgreek}
 
%\chemsetup[chemformula]{font-shape=sf,format=\sffamily}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setatomsep{2em}
\setdoublesep{.6ex}
\setarrowdefault{,1,thick}
\setbondstyle{thick,cap=round}

\makeatother
 
\begin{document}
\definesubmol\nobond{-[,0.25,,,draw=none]}
\schemedebug{false}
\begin{tikzpicture}
\node [inner sep=5pt]{
\schemestart
	\chemnameinit{\chemfig{\lewis{0:4:,O}=[6]-[:-30](-[6]R)-[:30](-[:-30]-[6]R)-[2]\lewis{2:4:,O}|H}}
	\chemfig{\lewis{1:3:,O}=[6]-[:-30](-[:210,1.8,,,draw=none]CH_3|@{a1}\lewis{0:2:6:,O}\mch)(-[@{a3}:210]@{a2}H)(-[6]R)-[@{a4}:30](-[2]\lewis{2:4:,O}H)-[:-30]-[6]R}
    \arrow{<=>}
    \chemfig{\lewis{1:3:,O}=[6]-[:-30](-[:-120,.25,,,draw=none]@{b1}\mch)(-[6]R)-[@{b2}:30](-[@{b3}2]@{b4}\lewis{2:4:,O}H)-[:-30]-[6]R}
    \arrow{<->>[$-$\chemfig{H\lewis{0:2:6:,O}\mch}]}
    \chemfig{\lewis{1:3:,O}=[6]-[:-30](-[6]R)=_[:30]-[:-30]-[6]R}
    \chemmove[-stealth,shorten <=3pt, shorten >=2pt, red, thick]{
    	\draw (a1.north) .. controls ++(north:2em) and ++(north:2em) .. (a2.north);
        \draw (a3.north) .. controls ++(north:2em) and ++(north:2em) .. (a4.north);
        \draw (b1.north) .. controls ++(south:1em) and ++(south:2em) .. (b2.south);
        \draw (b3.east) .. controls ++(east:1em) and ++(south east:2em) .. (b4.south east);
    }
    \chemnameinit{}
\schemestop
};
\end{tikzpicture}
\end{document}