Dank [erik][1] von [Stackexchange][2]. Er hat mir dies hier geschrieben.
\documentclass[border=2pt]{standalone}
\usepackage{chemfig}
\usepackage{siunitx}
\newcommand\namebond[5][-1pt]{\chemmove{\path(#2)--(#3)node[midway,#4,yshift=#1,black!60]{#5};}}
\newcommand\arcbetweennodes[3]{%
\pgfmathanglebetweenpoints{\pgfpointanchor{#1}{center}}{\pgfpointanchor{#2}{center}}%
\let#3\pgfmathresult}
\newcommand\arclabel[6][red,-stealth,shorten <=1pt,shorten >=1pt]{%
\chemmove{%
\arcbetweennodes{#4}{#3}\anglestart \arcbetweennodes{#4}{#5}\angleend \draw[#1]([shift=(\anglestart:#2)]#4)arc(\anglestart:\angleend:#2); \pgfmathparse{(\anglestart+\angleend)/2}\let\anglestart\pgfmathresult \node[shift=(\anglestart:#2+1pt)#4,anchor=\anglestart+180,inner sep=0pt,
outer sep=0pt]at(#4){#6};}}
\begin{document}
\setatomsep{2cm}
\chemfig{@{hl}H-[:34]@{c}C(=[:+90]O@{ot})-[::+-56]@{ob}O-[::55]@{hr}H}
\namebond{hl}{c}{above,sloped}{$a$}
\namebond{c}{ot}{right}{$b$}
\namebond{c}{ob}{above,sloped}{$c$}
\namebond{ob}{hr}{above,sloped}{$d$}
\arclabel{0.5cm}{hl}{c}{ob}{\footnotesize \SI{112}{\degree}}
\arclabel{0.5cm}{c}{ob}{hr}{\footnotesize \SI{110}{\degree}}
\end{document}
Das Ergebnis sieht so aus.
![Ameisensäure mit Winkel.][3]
[1]: http://tex.stackexchange.com/users/24974/erik
[2]: http://tex.stackexchange.com/questions/232184/methanoic-acid-with-chemfig/232426#232426
[3]: http://texwelt.de/wissen/upfiles/dryhu.pnghttp://texwelt.de/wissen/upfiles/dryhu_1.png
