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

Which group gets higher priority and why?


$\ce{R-CH\_2OCH\_3}$


$\ce{R-CH=O}$




As mentioned by Yomen , the double bond counts for two atoms with a single bond between them . They are called ghost atoms . This makes aldehyde group at higher priority than the ether .


Wikedia has a very nice explanation on it :



> 
> If an atom A is double-bonded to an atom B, A is treated as being singly bonded to two atoms: B and a ghost atom that has the same atomic number as B but is not attached to anything except A. In turn, when B is replaced with a list of attached atoms, A itself is excluded in accordance with the general principle of not doubling back along a bond that has just been followed, but a ghost atom for A is included so that the double bond is properly represented from both ends.
> 
> 
> 


A triple bond is handled in a similar manner . 


Wikipedia link : [Cahn–Ingold–Prelog priority rules](http://en.m.wikipedia.org/wiki/Cahn%E2%80%93Ingold%E2%80%93Prelog_priority_rules)


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$\ce{R-CH=O}$ 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.


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