# Which has higher priority (Cahn-Ingold-Prelog)?

Which group gets higher priority and why?

$\ce{R-CH_2OCH_3}$

$\ce{R-CH=O}$

• Aldehyde over ether, oxygen in the $\ce{C=O}$ double bond counts twice. – Klaus-Dieter Warzecha Feb 20 '15 at 20:47

$\ce{R-CH=O}$ has higher priority over $\ce{R-CH_2OCH_3}$. Because, if $\ce{R}$ group is the same, then we have to compare between the next substituents: For $\ce{R-CH=O}$ you have one carbon atom connected to 2 oxygen atoms (a double bond is equivalent to two simple bonds) and one hydrogen atom. For $\ce{R-CH_2OCH_3}$, you have one carbon atom connected to one oxygen atom and two hydrogen atoms. So, $\ce{R-CH=O}$ has higher priority.