\documentclass[12pt,border=1pt,crop,multi=false,tikz]{standalone}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{chemfig,chemmacros}
\usepackage{lmodern}
\usetikzlibrary{decorations.pathmorphing}
\chemsetup[chemformula]{font-shape=sf,format=\sffamily}
\renewcommand*{\familydefault}{\sfdefault}
\renewcommand*\printatom[1]{\ensuremath{\mathsf{#1}}}
\setatomsep{3em}
\setdoublesep{.6ex}
\setarrowdefault{,1.5,thick}
\setbondstyle{thick}
\makeatletter
\begin{document}
\definesubmol\nobond{-[,0.25,,,draw=none]}
\schemestart[][north]
\chemleft.\subscheme{
\chemname
{\chemfig{R-[:30]C(=[2]O)-[:-30]CH_3}}
{ENOL mode}
\arrow{<=>[catalytic \ch{H+}]}
\chemname
{\chemfig{R-[:30]C(-[2]OH)=[:-30]CH_2}}
{enol}
\arrow{<=>[{\chemfig[blue][scale=0.75]{H-[:30]C(=[2]O)-[:-30]R'}}][\parbox{2.25cm}{\footnotesize(reacts in protonated form)}]}
\chemname
{\chemfig{R-[:30]C(=[2]O)-[:-30]CH_2-[:30]
\textcolor{blue}{C}(-[2,,,,blue]\textcolor{blue}{O}(-[:150]H))(-[6,.75,,,blue]\textcolor{blue}{H})-[:-30,,,,blue]\textcolor{blue}{R'}}}
{aldol addition product}
\arrow(@c4--){0}[-90,.75]
\chemfig{R-[:30]C(=[2]O)-[:-30]CH_2-[:30]
\textcolor{blue}{C}(-[2,,,1,blue]\textcolor{blue}{O}^\ominus(-[,.5,,,draw=none]M^\oplus))(-[6,.75,,,blue]\textcolor{blue}{H})-[:-30,,,,blue]\textcolor{blue}{R'}}
\arrow{<=>[{\chemfig[blue][scale=0.75]{H-[:30]C(=[2]O)-[:-30]R'}}]}[-180]
\chemname*
{\chemfig{R-[:30]C(-[2,,,1]O^\ominus(-[,.5,,,draw=none]M^\oplus))=[:-30]CH_2}}
{enolate}
\arrow{<=>[base]}[180]
\chemname*
{\chemfig{R-[:30]C(=[2]O)-[:-30]CH_3}}
{ENOLATE mode}
}
\chemright\}
\arrow{->[$-$\ch{H2O}]}
\chemname
{\chemfig{R-[:30]C(=[2]O)-[:-30]C(-[6,.25,,,draw=none] H)=[:30]
\textcolor{blue}{C}(-[2,,,,blue]\textcolor{blue}{H})-[:-30,,,,blue]\textcolor{blue}{R'}}}
{aldol condensation product}
\schemestop
\end{document}