Fix chemfig isomerization

Question

I want to represent the following isomerization in LaTeX. It is the following:

However, so far, I have this:

Thank you in advance.

My MWE is:

\documentclass[a4paper,11pt,oneside]{book}
\usepackage[utf8]{inputenc}
\usepackage{mhchem}
\usepackage{chemfig}
\usepackage{pgfplots}
\usetikzlibrary{intersections, calc}
\pgfplotsset{compat=1.14}

    \begin{document}

    \begin{figure}[h!]
    \centering
     \schemestart
    \chemfig{2-cis-butene}
    \arrow{<=>}[45]
    \chemfig{1-butene}
    \arrow{<=>}[315]
    \chemfig{2-trans-butene}
    \schemestop
    \caption{My caption}
    \label{fig:isomer}
    \end{figure}

    \end{document}

Answer 1

Sorry, I missed two arrows.

\documentclass[a4paper,11pt,oneside]{book}
\usepackage[utf8]{inputenc}
\usepackage{chemfig}

\begin{document}
 \begin{figure}[h!]
 \centering
  \schemestart
   {2-cis-butene}\arrow(aa--bb){<=>}[45]{1-butene}%
   \arrow(--cc){<=>}[-45]{2-trans-butene}%
   \arrow(--dd){<=>}[-135]{iso-butene}\arrow(@dd--@aa){<=>}%
   \arrow(@aa--@cc){<=>}\arrow(@bb--@dd){<=>}  
  \schemestop
 \caption{My caption}
 \label{fig:isomer}
 \end{figure}

\end{document}