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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: maakii on June 22, 2007, 05:07:54 AM

Title: cahn ingold prelog rule
Post by: maakii on June 22, 2007, 05:07:54 AM
Hi everyone!

In the wikipedia article on the Cahn Ingold Prelog rule, http://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules (http://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules)I came across this example:

-CH2CH2OH outranks -CH(CH3)2

Now, is this an error? Because the first carbon in red is only a primary carbon, while the second carbon in blue is secondary, so by right the order should be the other way around? 
Title: Re: cahn ingold prelog rule
Post by: sjb on June 22, 2007, 07:19:38 AM
Whether a carbon is primary or secondary has (little or) nothing to do with priority by CIP.

-CH2CH2OH outranks -CH(CH3)2 because if you move along the chain with the highest priority in the first chain you use C-C-O, in the second C-C-H, hence the alcohol is higher than the isopropyl group.

(At least, that's how I understand it)

S
Title: Re: cahn ingold prelog rule
Post by: AWK on June 22, 2007, 08:22:04 AM
Quote from: maakii on June 22, 2007, 05:07:54 AM
Hi everyone!

In the wikipedia article on the Cahn Ingold Prelog rule, http://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules (http://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules)I came across this example:

-CH2CH2OH outranks -CH(CH3)2

Now, is this an error? Because the first carbon in red is only a primary carbon, while the second carbon in blue is secondary, so by right the order should be the other way around? 
You are right. -CH(CH3)2 has higher priority than -CH2CH2OH
Title: Re: cahn ingold prelog rule
Post by: kiwi on June 22, 2007, 09:36:11 AM
yep as AWK said, you have it right. Not sure where the wikipedia error comes from, but it looks like someone has misunderstood the example in March (pg 139). I've changed the wikipedia example to something more relevant.
Title: Re: cahn ingold prelog rule
Post by: sjb on June 23, 2007, 04:25:15 AM
OK, sits corrected, sorry :(

S