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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: rleung on June 30, 2005, 05:09:12 PM

Title: R and S configuration
Post by: rleung on June 30, 2005, 05:09:12 PM

Hey,

I am not sure why the following molecule is considered to be in S conformation (the book says it is).  To me, it seems to be in R config.  I assigned (CH3O)2CH as #1 since the connected C is connected to 2 O's, and both of the O's are connected to methyl groups.  

I assigned CHO as #2 since the connected C is connected to 2 O's (one is imaginary, and therefore, stands alone).

I assigned CH(CH3)2 as #3 since it is connected to 1 H and 2 methyl groups.  I assigned CH(double bond)CH2 as #4 since it too is connected to 1 H and two C's (one imaginary) but since the imaginary C is a lone C without any attached H's, it gets last priority.  Why am I wrong?  Thanks.

Ryan

Title: Re:R and S configuration
Post by: movies on June 30, 2005, 07:00:40 PM

You have the priorities mixed up between the third and fourth ones.  When counting the vinyl group, the first carbon has three bonds to carbon and one to hydrogen, the next carbon out has two bonds to carbon and two bonds to hydrogen, right?

On the isopropyl group, the first carbon has 3 bonds to carbon and one to hydrogen (the same as with the vinyl group).  Following to the next carbon, however, there is only one bond to carbon and three to hydrogen, so the isopropyl group actually has the lowest priority.

Title: Re:R and S configuration
Post by: rleung on July 01, 2005, 01:31:27 PM

Thanks.

Doesn't that mean the molecule is in R conformation and not S conformation?  The books says S, but I think that is a mistake.

Ryan

Title: Re:R and S configuration
Post by: movies on July 01, 2005, 03:46:50 PM

No, it's S.  The methoxy acetal is 1, the aldehyde is 2, and the vinyl group is 3.  Then place the isopropyl group in the back and it rotates counter clockwise.