How to reduce vertical distance between compounds in chemfig

Question

When writing reaction mechanism in chemfig with subscheme there are wide vertical gaps between the two compounds as seen in the figure attached.

How can I achieve the desired position shown in the fig. Attempt so far

documentclass{book}
\usepackage[version=4,arrows=pgf-filled]{mhchem}
\usepackage{chemfig,chemmacros}
\setatomsep{1.5em}
\setbondoffset{0pt}
\begin{document}
\schemestart
\subscheme[-90]{
\chemfig{CH_3CH=[@{d}]CH_2}
\arrow{0[][][-10pt]}
\chemfig{@{e}Br-[@{f}]@{g}Br}
}
\ce{->}
\chemfig{CH_3\chemabove{C}{\scrp}HCH_2Br} \+ \chemfig{Br^{-}} 
\schemestop
\chemmove[cyan,>=latex,shorten <=2pt,shorten >=2pt]{ 
\draw[shorten >=1pt,->] (d) ..controls +(-90:1cm) and +(135:1cm) .. (e) ;} 
\chemmove[red,>=latex,shorten <=1pt,shorten >=2pt]{ 
\draw[shorten >=0pt,->] (f) ..controls +(-90:.5cm) and +(-90:1cm) .. (g) 
;}
\end{document}

Answer 1

Adjusting the dimensions of the invisible arrow between the propylene and the bromine should get you closer to the ecpected output.

I also replaced the deprecated

\setatomsep{1.5em}
\setbondoffset{0pt}

commands with

\setchemfig{atom sep=1.5em,
            bond offset=0pt}

to make the code compilable with chemfig versions newer than 1.3 (from march 2018).

Lastly, I included an alternative layout for the reaction, as reagents are usually drawn next to the reactant(s) or above the reaction arrow, instead of somewhere below the reactant.

\documentclass{book}
\usepackage[version=4,arrows=pgf-filled]{mhchem}
\usepackage{chemfig,chemmacros}

\setchemfig{atom sep=1.5em,
            bond offset=0pt}
\begin{document}
\schemestart
\subscheme[-90]{
\chemfig{CH_3CH=[@{d}]CH_2}
\arrow{0}[-90,0.25]
\chemfig{@{e}Br-[@{f}]@{g}Br}
}
\ce{->}
\chemfig{CH_3\chemabove{C}{\scrp}HCH_2Br} \+ \chemfig{Br^{-}} 
\schemestop
\chemmove[cyan,>=latex,shorten <=2pt,shorten >=2pt]{ 
\draw[shorten >=1pt,->] (d) ..controls +(-90:.75cm) and +(135:1cm) .. (e) ;} 
\chemmove[red,>=latex,shorten <=1pt,shorten >=2pt]{ 
\draw[shorten >=0pt,->] (f) ..controls +(-90:.5cm) and +(-90:1cm) .. (g) 
;}

\vspace{3cm}


\schemestart
\chemfig{CH_3CH=[@{d}]CH_2}
\arrow{->[\chemfig{@{e}Br-[@{f}]@{g}Br}]}
\chemfig{CH_3\chemabove{C}{\scrp}HCH_2Br} \+ \chemfig{Br^{-}} 
\schemestop
\chemmove[cyan,>=latex,shorten <=2pt,shorten >=2pt]{ 
\draw[shorten >=1pt,->] (d) ..controls +(90:.75cm) and +(135:.75cm) .. (e) ;} 
\chemmove[red,>=latex,shorten <=1pt,shorten >=2pt]{ 
\draw[shorten >=0pt,->] (f) ..controls +(90:.5cm) and +(90:.5cm) .. (g) 
;}
\end{document}

Answer 2

Two methods, using only commands from the "chemfig" package

1. Positioning Br2 after the end of the chemical reaction:

   \setchemfig{atom sep=1.5em,bond offset=0pt}
   
   \begin{document}
   
   \schemestart
   
   \chemfig{CH_3CH=[@{a},1.5]CH_2}
   
   \arrow{->} 
   
   \chemfig{CH_3\chemabove{C}{\scriptstyle+}HCH_2Br} 
   
   \+ \chemfig{Br^{-}}
   
   \arrow(@{a}--){0}[320,.6]\chemfig{@{b}Br-[@{c},1.5]@{d}Br}
   
   %---------------------------------------------------------------
   
   \chemmove{
   
       \draw[red,shorten <=2pt,shorten >=1pt](a).. controls +(270:8mm) and +(90:8mm)..(b);
   
       \draw[red,shorten <=1pt,shorten >=1pt](c).. controls +(90:6mm) and +(90:6mm)..(d);
   
    }
   
   \schemestop
   
   \end{document}
   

2. or placing Br2 at the beginning, but you will need to reposition the reaction arrow

    \begin{document}
   
    \schemestart
   
    \chemfig{CH_3CH=[@{a},1.5]C@{z}H_2}
   
    \arrow(@{a}--){0}[320,.6]\chemfig{@{b}Br-[@{c},1.5]@{d}Br}
   
    \arrow(@{z}--){->} 
   
    \chemfig{CH_3\chemabove{C}{\scriptstyle+}HCH_2Br}
   
    \+ \chemfig{Br^{-}}
   
    %---------------------------------------------------------------
   
    \chemmove{
   
    \draw[red,shorten <=2pt,shorten >=1pt](a).. controls +(270:8mm) and +(90:8mm)..(b); 
   
    \draw[red,shorten <=1pt,shorten >=1pt](c).. controls +(90:6mm) and +(90:6mm)..(d);
   
    }
   
    \schemestop
   
    \end{document}
   

Both have a very similar result.

With bromine at angle 270

\begin{document}
    \schemestart
    %
    \chemfig{CH_3CH=[@{a},1.5]C@{z}H_2}
    %
    \arrow(@{a}--){0}[320,.6]\chemfig{@{b}Br-[@{c},1.5]@{d}Br}
    %
    \arrow(@{z}--){->}
    %
    \chemfig{CH_3@{y}\chemabove{C}{\scriptstyle+}HCH_2Br}
    %
    \arrow(@{y}--){0}[-90,.3]\+ \chemfig{Br^{-}}
    %
    %---------------------------------------------------------------
    \chemmove{
        \draw[red,shorten <=2pt,shorten >=1pt](a).. controls +(270:8mm) and +(90:8mm)..(b);
        \draw[red,shorten <=1pt,shorten >=1pt](c).. controls +(90:6mm) and +(90:6mm)..(d);  
    }
\schemestop

\end{document}