chemfig formula in table - vertical alignment

Question

I want to create numbered chemical formula via chemfig environment.
MWE:

\documentclass[a4paper,fleqn,leqno]{article}
\usepackage{polyglossia}
\usepackage{tikz,tikz-3dplot,graphicx}
\usepackage{mathtools,breqn,xparse}
\usepackage[fontsize=12]{scrextend}
\usepackage{array,multirow}
\usepackage{chemfig,circuitikz}
\usepackage{pgfplots}
\DeclareMathSizes{12}{12}{12}{7}
\newcolumntype{C}[3]{@{\hspace*{#1}}>{\centering\arraybackslash}p{#2}@{\hspace*{#3}}}
\newcolumntype{R}[2]{>{\raggedleft\arraybackslash}p{#1}@{\hspace*{#2}}}
\newcolumntype{L}[2]{@{\hspace*{#1}}>{\raggedright\arraybackslash}p{#2}}
\newcolumntype{"}[1]{@{\vrule width #1}}
\makeatletter\newcommand{\ohline}[1]{\noalign {\ifnum 0=`}\fi\hrule height #1\futurelet\reserved@a\@xhline}\makeatletter
\newlength\savedwidth
\newcommand{\ocline}[2]{\noalign{\global\savedwidth\arrayrulewidth\global\arrayrulewidth #1}\cline{#2}\noalign{\vskip\arrayrulewidth}\noalign{\global\arrayrulewidth\savedwidth}}
\begin{document}
{\renewcommand\arraystretch{1.3}
\begin{tabular}{L{0pt}{420pt}@{}C{0pt}{50pt}{0pt}}
\schemestart\setatomsep{2em}\chemfig[line width=0.75pt]{[:18]*5((-[:230]HO)=(-[:307]OH)-(=[:20]O)-O-(-[:150](-[:90]OH)-[:205]-[:150]HO)-)}\chemsign+\chemfig{I_{\scriptscriptstyle 2}}\chemrel[]{-{Stealth[scale=1.5,width=3pt]},line width=0.75pt}\chemfig[line width=0.75pt]{[:18]*5((=[:230]O)-(=[:307]O)-(=[:20]O)-O-(-[:150](-[:90]OH)-[:205]-[:150]HO)-)}\chemsign+\chemfig{2HI}\schemestop & (4)
\end{tabular}}
\end{document}

I2, HF, (4) are not vertically centered though. They are located more on the bottom side.
I want them to be vertically centered. I tried '\arrow{0}[,0]' trick in chemfig formula with no success. I would also need help with tabular vertical alignment.

Answer 1

Why not use an equation for numbered reactions? BTW \chemsign and \chemrel are deprecated macros and have been removed in v1.3 of chemfig.

\documentclass{article}
\usepackage{chemfig}
\setbondstyle{line width=.75pt}

% \usepackage{showframe}

\newcounter{reaction}

\begin{document}

\begin{center}
\begin{tabular}{cc}
  \setatomsep{1.5em}
  \schemestart[][184]
    \chemfig{
      [:18]*5(
        (-[:230]HO)
        =(-[:307]OH)
        -(=[:20]O)
        -O-(-[:150](-[:90]OH)-[:205]-[:150]HO)
        -
      )
    }
    \arrow{0}[,0]\+
    \chemfig{I_2}
    \arrow
    \chemfig{
      [:18]*5(
        (=[:230]O)
        -(=[:307]O)
        -(=[:20]O)
        -O-(-[:150](-[:90]OH)-[:205]-[:150]HO)
        -
      )
    }
    \arrow{0}[,0]\+
    2\,HI
  \schemestop &
  \refstepcounter{reaction}(\thereaction)
\end{tabular}
\end{center}

% or:
% \begin{equation}
% \schemestart
%   ...    
% \schemestop
% \end{equation}

\end{document}