uncover chemfig molecule (in beamer)

Question

I teach chemistry and wish to demonstrate drawing and naming of chemical structures I use chemfig to draw my molecules and I can do that fine. However, in for example the molecule 4-ethyl-3,3-dimethylhexane there are a number of components to the molecule which I want to uncover bit by bit to illustrate the naming/drawing of such a molecule.

We have a hexane root - draw this

add an ethyl group on carbon 4 - highlight this addition

add two methyl groups on carbon 3 - highlight these additions

Below is the code to build the molecule bit by bit as individual figures but I would like to be able to uncover on one molecule and highlight the pieces as they are added.

I had tried to use \onslide<n->in the parts I wanted to uncover but just got goobleydegook. I would be grateful if anyone has any ideas...

Below is fine but I would prefer to add to the parent molecule bit by bit...

\documentclass[t]{beamer}
\usepackage{chemfig}
\begin{document}

\begin{frame}{}
4-ethyl-3,3-dimethylhexane
\chemfig[][scale=0.5]{CH_3%6 these numbers are the carbon positions in the chain for ease of reading
-[::30]%5
-[::-60]%4
(-[::-60]-[::60]CH_3)%ethyl
-[::60]%3
(-[::30]CH_3)
(-[::90]CH_3)%methyl
-[::-60]%2
-[::60]CH_3%1
}
\begin{itemize}
\item root is hexane; hex=6; 6 carbon alkane
\onslide<2->{
\chemfig[][scale=0.5]{CH_3%6 these numbers are the carbon positions in the chain for ease of reading
-[::30]%5
-[::-60]%4
%(-[::-60]-[::60]CH_3)%ethyl
-[::60]%3
%(-[::30]CH_3)%methyl
%(-[::90]CH_3)%methyl
-[::-60]%2
-[::60]CH_3%1
}
}
\onslide<3->{\item 4-ethyl: ethyl substituent = 2 carbon alkyl group; on carbon 4}
\onslide<4->{
\chemfig[][scale=0.5]{CH_3%6 these numbers are the carbon positions in the chain for ease of reading
-[::30]%5
-[::-60]%4
(-[::-60]-[::60]CH_3)%ethyl}
-[::60]%3
%(-[::30]CH_3)%methyl
%(-[::90]CH_3)%methyl
-[::-60]%2
-[::60]CH_3%1
}
}
\onslide<5->{\item 3,3-dimethyl: di= 2, methyl substituents = 1 carbon alkyl group; on carbons 3 and 3}

\onslide<6->{\chemfig[][scale=0.5]{CH_3%6 these numbers are the carbon positions in the chain for ease of reading
-[::30]%5
-[::-60]%4
(-[::-60]-[::60]CH_3)%ethyl
-[::60]%3
(-[::30]CH_3)%methyl
(-[::90]CH_3)%methyl
-[::-60]%2
-[::60]CH_3%1
 }
}
\end{itemize}
\end{frame}
\end{document}

Answer 1

We can set the opacity of the bonds we do want to hide to zero and make them visible again with \pgfkeysalso.

Sadly, I have not found a simple way to pass TikZ options to the nodes (the atoms like C and H_3) otherwise it would work like the same with visible on=<slides>.

I propose two solutions:

1. Redefinition of \printatom with checks a set of integers against the current node number.

   To see the numbers one has to use \qrrDebugNodes which needs to enclosed in { } if it shouldn’t be global or one reset at the end of the to analyzed \chemfig with

   \qrrDebugNodes[false]
   

   The bonds can still be placed on different atoms (whatever the syntax).

2. A simply \unc macro that is built upon \uncover which hides its argument on the specified slides. This has the disadvantages that CH_3 is seen as one atom and not as two (C and H_3), which makes the placements of the bonds defunct, even if we use more than one \unc (for every atom one).

Solution with a re-defined \printatom

Code

\documentclass[t]{beamer}
\usepackage{chemfig,mathtools}
\tikzset{
    invisible/.style={opacity=0,text opacity=0},
    visible on/.style={alt=#1{}{invisible}},
    alt/.code args={<#1>#2#3}{%
      \alt<#1>{\pgfkeysalso{#2}}{\pgfkeysalso{#3}} 
    },
}
\makeatletter
\newcounter{qrr@nodes}
\newif\if@qrr@debug
\newcommand*{\setNodes}[1]{%
    \def\qrr@nodes{#1}%
    \setcounter{qrr@nodes}{0}% if \setNodes is used inside a frame the reset happens too!
}
\newcommand*{\qrrDebugNodes}[1][true]{
    \csname @qrr@debug#1\endcsname
}
\renewcommand*{\printatom}[1]{%
    \def\qrr@cmd{}%
    \def\qrr@slides@{}%
    \foreach \qrr@number/\qrr@slides in \qrr@nodes {%
        \foreach \qrr@number@ in \qrr@number {%
            \ifnum\value{qrr@nodes}=\qrr@number@\relax
                \global\let\qrr@cmd\uncover
                \xdef\qrr@slides@{<\qrr@slides>}%
                \breakforeach
            \fi
        }%
    }%
    \ensuremath{%
        \expandafter\qrr@cmd\qrr@slides@{\mathrm{#1}}%
        \if@qrr@debug
            \smash{\mathllap{\mathop{}\limits_{\textnormal{\tiny (\the\value{qrr@nodes})}}}}%
        \fi
    }%
    \stepcounter{qrr@nodes}%
}
\makeatother
\begin{document}
\begin{frame}
\setNodes{{5,6}/2-,{8,9}/3-,{10,11}/4-}
%\qrrDebugNodes
\chemfig[][scale=1.5]{CH_3%6 these numbers are the carbon positions in the chain for ease of reading
-[::30]%5
-[::-60]%4
(-[::-60,,,,visible on=<2->]-[::60,,,,visible on=<2->]CH_3)%ethyl
-[::60]%3
(-[::30,,,,visible on=<3->]CH_3)%methyl
(-[::90,,,,visible on=<4->]CH_3)%methyl
-[::-60]%2
-[::60]CH_3%1
}
\end{frame}
\end{document}

Output

Output with debug settings

Solution with \unc

The following solution with \unc makes chemfig see CH_3 as one atom and can not split it into C and H_3 to place bonds correctly, even if one uses two \unc.

The upper CH_3 are misplaced compared to the normal output.

Code

\documentclass[t]{beamer}
\usepackage{chemfig}
\tikzset{
    invisible/.style={opacity=0,text opacity=0},
    visible on/.style={alt=#1{}{invisible}},
    alt/.code args={<#1>#2#3}{%
      \alt<#1>{\pgfkeysalso{#2}}{\pgfkeysalso{#3}} 
    },
}
\newcommand*{\unc}[2]{%
    \uncover<#1>{#2}
}
\begin{document}
\begin{frame}
\chemfig[][scale=1.5]{CH_3%6 these numbers are the carbon positions in the chain for ease of reading
-[::30]%5
-[::-60]%4
(-[::-60,,,,visible on=<2->]-[::60,,,,visible on=<2->]\unc{2-}{CH_3})%ethyl
-[::60]%3
(-[::30,,,,visible on=<3->]\unc{3-}{CH_3})%methyl
(-[::90,,,,visible on=<4->]{\unc{4-}{C}}{\unc{4-}{H_3}})%methyl
-[::-60]%2
-[::60]CH_3%1
}
\end{frame}
\end{document}

Output

Answer 2

You may cheat a little with white color :-)

Try this, for example:

\parbox[c][.5\textheight][c]{\columnwidth}{%
    \centering\mbox{}%
    \def\test{white}%
    \only<2>{\def\test{red}}%
    \only<3->{\def\test{black}}%
    \chemfig[][scale=0.5]{CH_3%6 
    -[::30]%5
    -[::-60]%4
    (-[::-60,,,,\test]-[::60,,,,\test]\textcolor{\test}{CH_3})%ethyl
    -[::60]%3
    (-[::90]CH_3)%methyl
    -[::-60]%2
    -[::60]CH_3%1
    }
}

The above code will display a branch in red at slide 2, then in black.