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Topic: How to draw cis - trans (geometric) isomers for C6H12...  (Read 31148 times)

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Offline johnboy

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How to draw cis - trans (geometric) isomers for C6H12...
« on: January 10, 2008, 03:00:28 AM »
I understand the structure of cis and trans but i'm not able to draw different isomers... for C6H12 there are 4 pairs possilble... What do i start with? and is there a procedure i need to follow to draw them?
 ???
Thnx in advance..

Offline AWK

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Re: How to draw cis - trans (geometric) isomers for C6H12...
« Reply #1 on: January 10, 2008, 05:09:56 AM »
1. draw double bond beetween C-2 and C-3 atoms (cis and trans)
2. draw double bond beetween C-3 and C-4 atoms (cis and trans)
3. Shorten carbon of chain from point 1. to 5 C-atoms (cut atom C-6) and add CH3 to the double bond at the side of ethyl group (cis and trans)
4. Move CH3 to C-4 (chain from point 1) to form isopropyl group  (cis and trans) - Note! numbering of chain will reverse after this operation.

8 isomers (4 unbranched)
« Last Edit: January 10, 2008, 05:29:29 AM by AWK »
AWK

Offline johnboy

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Re: How to draw cis - trans (geometric) isomers for C6H12...
« Reply #2 on: January 15, 2008, 11:50:51 PM »
1. draw double bond beetween C-2 and C-3 atoms (cis and trans)
2. draw double bond beetween C-3 and C-4 atoms (cis and trans)
3. Shorten carbon of chain from point 1. to 5 C-atoms (cut atom C-6) and add CH3 to the double bond at the side of ethyl group (cis and trans)
4. Move CH3 to C-4 (chain from point 1) to form isopropyl group  (cis and trans) - Note! numbering of chain will reverse after this operation.

8 isomers (4 unbranched)


thnx.... i just wanna know how do u get to this... do i need 2 practice more or u follow some sort of method?
and what is isopropyl... v just learned the tppic so we have not been taught in as much detail...

Offline Alpha-Omega

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Re: How to draw cis - trans (geometric) isomers for C6H12...
« Reply #3 on: January 16, 2008, 12:13:58 AM »
Geometric Isomers are compounds with different spatial arrangements of groups attached to the carbons of a double bond. In alkenes, the carbon-carbon double bond is rigidly fixed. Even though the attachment of atoms is the same, the geometry (the way the atoms "see" each other) is different.

When looking for geometric isomers, a guiding principle is that there MUST BE TWO DIFFERENT "GROUPS" ON EACH CARBON OF THE DOUBLE BOND. A "group" can be hydrogen, alkyls, halogens, etc.

Offline AWK

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Re: How to draw cis - trans (geometric) isomers for C6H12...
« Reply #4 on: January 16, 2008, 01:45:26 AM »

thnx.... i just wanna know how do u get to this... do i need 2 practice more or u follow some sort of method?
and what is isopropyl... v just learned the tppic so we have not been taught in as much detail...
even 9 isomers
first line - unbranched trans at carbon 2 and 3 (structures 1 and 2)
second line - structure 1 shortened from left side by one carbon atom - methyl group fit in three different position (at C-2, C-3 and C-4 , numbering of chain from right to left). This gives structures 3, 4 and 5. Structure 3 I neglected in my previous post - it is nor cis nor trans. Shortening  2 gives 3 and 4 (no new structures). Isopropyl group is left to double bond in 5.
third line - 1 and 2 converted to cis forms
fourth line - 4 and 5 converted to cis forms
AWK

Offline Kryolith

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Re: How to draw cis - trans (geometric) isomers for C6H12...
« Reply #5 on: January 17, 2008, 03:07:30 PM »
I don't agree with your number.

C6H12 has 17 acyclic isomers (4 cis/trans pairs + 9 "normal" isomers). Furthermore there are 8 monocyclic constitutional isomers (not considering stereoisomers).

To determine this, I drew the 5 isomers of hexane and tried to look for positions to put a double bond for each of them. You get 10 structures of which 4 have cis/trans-isomers.

btw.: it's a lot easier if you draw your molecules as "chains" without H-atoms (is there a name for this type of drawing?)

EDIT
grammar correction
« Last Edit: January 17, 2008, 03:20:16 PM by Kryolith »

Offline AWK

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Re: How to draw cis - trans (geometric) isomers for C6H12...
« Reply #6 on: January 18, 2008, 01:05:30 AM »

C6H12 has 17 acyclic isomers (4 cis/trans pairs + 9 "normal" isomers). Furthermore there are 8 monocyclic constitutional isomers (not considering stereoisomers).

I do not believe - show them
AWK

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