我是 chemfig 的新手，我尝试了互联网上所有可用的解决方案。但对我而言，没有一个有用。
\documentclass[12pt, a4paper]{report}
\usepackage{amsmath,chemfig}
\begin{document}
\tableofcontents
\chapter{Preparation of Haloalkanes}
\section{From Alkanes (by direct halogination)}
\schemestart
\chemfig{R-[0]H} \+ \chemfig{X-[0]X} \arrow{->[h$\nu$][]}[,2,thick] \chemfig{R-[0]X} \+ \chemfig{HX}
\schemestop
\section{From Alkenes}
\subsection{By Addition of Hydrogen Halides}
\schemestart
\chemfig{C(-[:150])(-[:210])=C(-[:30])(-[:-30])} \+ \chemfig{H-X} \arrow{->[$\text{CCl}_4$][$\Delta$]}[,2,thick] \chemfig{C(-[:150])(-[:210])(-[:90]H)=C(-[:90]X)(-[:-30])(-[:30])}
\schemestop
\subsection{Addition of HX in symmetrical Alkene}
\schemestart
\chemfig{CH_2=[:0]CH_2} \+ \chemfig{HCl} \arrow{->[$\text{CCl}_4$][$\Delta$]}[,2,thick] \chemfig{CH_3-[:0]CH_2-[:0]Cl}
\schemestop
\subsection{Addition of HX in unsymmetrical Alkene}
\schemestart
\chemfig{CH_2=[:0]CH-[:0]CH_3} \+ \chemfig{HCl} \arrow{->[$\text{CCl}_4$][$\Delta$]}[,2,thick] \chemfig{CH_3-[:0]CH(-[:90]Cl)-[:0]CH_3}
\schemestop
\end{document}
答案1
(.mid east--.mid west)在命令后添加\arrow
\arrow(.mid east--.mid west){->[$\text{CCl}_4$][$\Delta$]}[,2,thick]
答案2
您可以使用调整分子的垂直位置\arrow(--shift=<number>pt){->}，数字可以是正数或负数。
其他调整位置的命令只使用几个固定位置。使用“yshift”可以更自由地调整位置。
\subsection{By Addition of Hydrogen Halides}
\schemestart
\chemfig{C(-[:150])(-[:210])=C(-[:30])(-[:-30])} \+ \chemfig{H-X} \arrow(--[yshift=12pt]){->[$\text{CCl}_4$][$\Delta$]}[,2,thick] \chemfig{C(-[:150])(-[:210])(-[:90]H)=C(-[:90]X)(-[:-30])(-[:30])}
\schemestop
\subsection{Addition of HX in symmetrical Alkene}
\schemestart
\chemfig{CH_2=[:0]CH_2} \+ \chemfig{HCl} \arrow{->[$\text{CCl}_4$][$\Delta$]}[,2,thick] \chemfig{CH_3-[:0]CH_2-[:0]Cl}
\schemestop
\subsection{Addition of HX in unsymmetrical Alkene}
\schemestart
\chemfig{CH_2=[:0]CH-[:0]CH_3} \+ \chemfig{HCl} \arrow(--[yshift=16pt]){->[$\text{CCl}_4$][$\Delta$]}[,2,thick] \chemfig{CH_3-[:0]CH(-[:90]Cl)-[:0]CH_3}
\schemestop
