zoolander9
Registered: 10/24/09
Posts: 11
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Reply with quote | #1 |
I see that the highest priority groups are on opposite sides in the trans (E) and on the same in the cis (Z), the CH3 and the Cl, but i am confused because there are 2 CH3 groups? |
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NelM
Registered: 10/01/09
Posts: 15
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Reply with quote | #2 |
Hi Zoolander.. I'm not exactly sure what you mean by why there are "2 CH3 groups", but since the molecule's name name is "2-Chloro-2-Butene", the "butene" suffix implies that there are FOUR carbons in the molecule, which is why we have a chain of FOUR CH3's that are connected to each other. I attached a picture to further clarify this.
Just for more clarification to the problem:
There is also a double bond on the second carbon, which is why the molecule has a "2" in front of the butene. Also, the "2-Chloro" in front of the molecule means that there is a alkyl halide (the chlorine) attached to the SECOND carbon, thus making the prefix "2-chloro".
As for the cis (Z) and trans (E) naming, the first molecule is an "E" because the two groups of higher priority are on oppposite sides (the CH3 on the "left side" of the double bond AND the Chlorine on the "right side" of the double bond both have the higher priority). However, if the "two substituents of higher priority appear on the same side, the molecule is a Z [a cis]" (Vollhardt, Chapter 11-2. page 449). In this case, the CH3 and Chorine are now on the same side--"both are up" (Thinkbook, page 95). I also suggest reading up on chapter 11-2 as stated by the thinkbook.
Hope that helped 
Attached Images:
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zoolander9
Registered: 10/24/09
Posts: 11
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Reply with quote | #3 |
I just dont understand how the CH3 labeled 1 is different form the CH3 labeled as 4 in each image? How do you decide which is of higher priority...?
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NelM
Registered: 10/01/09
Posts: 15
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Reply with quote | #4 |
To decide which is higher CH3 is the higher priority:
After identifying the longest carbon chain (which is four carbons in this case), we need to decide which side of carbon to number first. To do this, you need to look at the function groups attached to the molecule. The functional groups should end up with the "least number possible (as there are two ways of numbering—right to left and left to right)" (http://en.wikipedia.org/wiki/IUPAC_nomenclature_of_organic_chemistry). In this case, the alkyl halide functional group (the carbon bonded to the chlorine) has the higher priority, so we start our numbering of the carbons from right to left of the chain. Does that make more sense now? I can try putting more examples.
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zoolander9
Registered: 10/24/09
Posts: 11
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Reply with quote | #5 |
Okay so I understand how to number the Cs of a simple hydrocarbon. That is not the priority I am referring to. I mean that when deciding when the two biggest groups are cis or trans, how do you know to compare the Cl with the CH3 of the 4th C and not the CH3 of the 1st C? Hope that is more clear this time...thanks for the help!
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NelM
Registered: 10/01/09
Posts: 15
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