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Offline Ohoneo

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Question about nomenclature
« on: July 20, 2010, 04:40:56 PM »
I'm trying to draw the structure of (E)-2-bromo-2-hexene.
The way that E and Z naming was explained to me, by the professor, was that the structure was E if the "ones" (using Cohn-Ingold-Prelog priority) were on the same side, and it's Z if they were on opposite sides.
I drew the structure as such: http://tinypic.com/r/15x2tud/3
Sorry, I don't know how to link URLs.
I was wondering if that was correct, because I only have one branch off of my hexene structure so I was rather confused as to how the E stereochemistry could be assigned.

Offline plankk

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Re: Question about nomenclature
« Reply #1 on: July 20, 2010, 04:55:37 PM »
First of all you have mistaken the nomenclature of E and Z alkenes. Z arrives from German word 'zusammen' which means together and E arrives from 'entgegen' which means on opposite side. Maybe this tip will help you in remembering which is which.

Secondly draw your compound in proper way.

Offline Ohoneo

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Re: Question about nomenclature
« Reply #2 on: July 20, 2010, 05:19:41 PM »
Draw my compound in the proper way? I draw it as a skeleton structure before converting it to a line structure, as that is the easiest way for me to visualize something. Especially when I don't understand something about it.

Thanks for your help.

Offline orgopete

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Re: Question about nomenclature
« Reply #3 on: July 20, 2010, 06:04:18 PM »
I think what is being implied is the (Lewis) structure you have drawn is actually incomplete. A skeletal structure (http://en.wikipedia.org/wiki/Skeletal_formula) may include the information you need. A skeletal structure also allows you to have incomplete octets and for the viewer to understand that atoms at joints or ends have hydrogen atoms to complete the octets. Lewis structures do not allow for incomplete octets by most professors. As drawn, if inline text of CH3, CH2, and CH were added to your structure, it could be considered a condensed structure and no stereochemistry would be implied. (Probably the most common structures are skeletal/condensed structures. A skeletal structure is drawn with substitutents in condensed forms, CH3, CH2CH3, COOCH2CH3, C6H5, Ph, etc.)

Go back to your skeletal structure. Draw four bonds to the double bond, assign them CH2, CH3, Br, and H. On the ends of the alkene, designate the groups with the highest and lowest priority according to the Cahn, Ingold, Prelog protocol and you should be able to draw the correct structure.
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Offline Doc Oc

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Re: Question about nomenclature
« Reply #4 on: July 20, 2010, 06:35:59 PM »
Few things:
1) E and Z are often interchanged with trans and cis, so you can think of them that way if it helps.

2) Yes, your structure is drawn incorrectly.  Although I understood what you were doing by omitting the hydrogens, it does not show E or Z geometry around the alkene, just a non-descript branch, so it's impossible to know whether you've done it correctly.  Put all the hydrogens in and be sure to show the cis/trans relationships around the alkene.

3) You've placed the bromine on the wrong carbon in the drawing you linked.  You've also drawn 7 carbons.

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