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  •  » How many C & H signals in 4-chloro-1,1,2,2,4-pentamethylcyclopentane?

#1 2008-12-16 19:17:46


How many C & H signals in 4-chloro-1,1,2,2,4-pentamethylcyclopentane?

Basically, I have found two methods for a problem like this, one is symmetry and the other is replacement. In the upper structure, there is a plane of symmetry that passes through the CH3-C-Cl atoms and bisects the C-C bond at the back of the ring. Every atom on either side is chemically equivalent due to this symmetry. I have used colors to show the identical features.

The other method, is to replace each atom with another atom and if the two atoms or attachments give the same compound, then the atoms or attachements are identical. In the example, I have replaced one of the hydrogens with a fluorine atom. If the two structures are the same, the hydrogens are identical. While less obvious, if the hydrogens attached are identical, then the carbons to which they are attached also be identical.

I arbitrarily placed the first chlorine down. The hydrogens replaced by fluorine in the center set of structures are identical as you can see by the mirror image on the right after rotation. The others are not and their names are different. A cis or trans designation would be simpler to show this.

While the symmetry method is easier in this instance, in others, this replacement method is also very useful.


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