Question about Determining Chirality

[HockeyDr09]

---

Members don't see this ad.

Thanks for anyone who can help me with this.
When looking at ring structures, or long chains, how far down are you suppose to count if the carbon has identical substituents in both directions?
Example, cholesterol

http://foodspace.files.wordpress.com/2008/05/cholesterol.gif

A question asks how many chiral centers there are present in cholesterol.
If I named the carbon attached to OH the first carbon, how far along the ring would I count to see if its chiral.
I see it's attached to 1 OH, 1H, and then 2CH2-C... which lead to the rest of the structure. Do you stop there and say it is achiral or do you have to continue counting where I have listed the ...? Thanks

 

[Phantastic]

Unless the structure was meso(?) it wouldn't be achiral when forming a ring structure, I believe.

The left path and the right path are certainly different, and have their own "handedness" to them. As you said, you keep going, but qualitatively you should be able to view the molecule as either being symmetrical or not in its ring structure.

 

[orgohacks]

Thanks for anyone who can help me with this.
When looking at ring structures, or long chains, how far down are you suppose to count if the carbon has identical substituents in both directions?
Example, cholesterol

http://foodspace.files.wordpress.com/2008/05/cholesterol.gif

A question asks how many chiral centers there are present in cholesterol.
If I named the carbon attached to OH the first carbon, how far along the ring would I count to see if its chiral.
I see it's attached to 1 OH, 1H, and then 2CH2-C... which lead to the rest of the structure. Do you stop there and say it is achiral or do you have to continue counting where I have listed the ...? Thanks

You continue counting until you find a difference. If you reach the end and find no difference, it's achiral.

To take an extreme example, something like CH3(CH2)100CH(CH2)101CH3 would be chiral.

 

[Didntdoit]

You continue counting until you find a difference. If you reach the end and find no difference, it's achiral.

To take an extreme example, something like CH3(CH2)100CH(CH2)101CH3 would be chiral.

This guy is right.