Numbering/Order

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Derek Li--Dis1a
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Numbering/Order

Postby Derek Li--Dis1a » Fri Feb 26, 2016 2:59 pm

For the first Quiz 3 preparation...question 5, there is a cycloalkane. It has a bromine attached to one of the carbons, and iodine attached to another carbon. From the diagram, I figured out it would be 1-Bromo-3-Iodocyclohexane. However, the answer on the back says it is a 3-bromo-1iodocyclohexane. I thought the first substituent gets the lowest number, so would it be 1-Bromo-3-Iodocyclohexane?? Or are they the same molecule; thus, both are accepted answers?

Ronald Yang 2F
Posts: 86
Joined: Fri Sep 25, 2015 3:00 am

Re: Numbering/Order

Postby Ronald Yang 2F » Fri Feb 26, 2016 5:35 pm

First of all, not sure how you determined that bromine was the "first substituent," but, if you refer to the Ochem textbook, for functional groups like halogens (Cl, Br, I), priority is based on the atomic number of the atom, so the greater the atomic mass, the greater is its priority. Thus, the priority in this case is given to the Iodine, since it has a greater molar mass than Bromine does. Thus, it should be 3-bromo-1-iodocyclohexane. Hope this helps! :)

Derek Li--Dis1a
Posts: 22
Joined: Fri Sep 25, 2015 3:00 am

Re: Numbering/Order

Postby Derek Li--Dis1a » Fri Feb 26, 2016 8:24 pm

Oh okay that makes sense! I determined bromine as the 1st substituent because of alphabetic order.

Ronald Yang 2F
Posts: 86
Joined: Fri Sep 25, 2015 3:00 am

Re: Numbering/Order

Postby Ronald Yang 2F » Mon Feb 29, 2016 1:58 pm

Hey Derek,

I made a mistake. The priority I was talking about only applies to alkenes in determining "priority" for whether a molecule is cis or trans. So sorry about that. You are correct, and the workbook has it wrong.


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