Content Review Video: Isomers I, Final Quiz Q#2

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BusyBeeh
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Content Review Video: Isomers I, Final Quiz Q#2

Postby BusyBeeh » Sat Jun 10, 2017 12:19 pm

I ran into the following issue with Q#2 of the Final Quiz (Slide 17):

Rank the following substituents from highest to lowest priority:
-C=O
-CH2OH
-CH2CH2CH2Br
-CH2CH2CH3


The video ranks them as the carbonyl group having the highest priority despite it having a lower molecular weight than -CH2CH2CH2Br (which obviously has the highest molecular weight). I would think that despite that, the -CH2CH2CH2Br would be the highest priority of them all because of its molecular weight. Next, the 2nd highest priority is given to the methanol group rather than the -CH2CH2CH2Br group. How is this possible if the methanol group doesn't have any double or triple bonds like carbonyl?

I guess my question is: How is -CH2CH2CH2Br a third priority to the carbonyl and methanol group in this case?
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NS_Tutor_Andrew
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Re: Content Review Video: Isomers I, Final Quiz Q#2

Postby NS_Tutor_Andrew » Sun Jun 11, 2017 11:45 pm

Hi BusyBeeh,

The trick here is that for priority rules, you don't you evaluate the MW of the whole substituent. Instead, you start with the atoms that are immediately connected to the stereocenter and then move to successively more distant positions to resolve ties. In this example, the groups immediately next to the stereocenter are 3 CH2 groups and a =O group. The double-bonded oxygen wins. Now we have to break the tie among the other 3 groups, which start with CH2. We move to the next position, where we have OH, CH2, and CH2. The OH wins....and so on.

Hope this helps, & definitely check out the Wikipedia page on the priority rules for more info if needed: https://en.wikipedia.org/wiki/Cahn%E2%80%93Ingold%E2%80%93Prelog_priority_rules
Andrew D.
Content Manager, Next Step Test Prep.

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