Chirality in Cyclic Compounds

[AmirTimur]

---

Members don't see this ad.

Please see the attached picture. (And pardon my horrendous art skillz)

According to TPR, Cyclopentane with two cis OH groups is a chiral molecule, and those are the two chiral centers. Cyclohexane with OH and CH3 however, does not have any chiral centers.

Q: Why? Why aren't the carbons that hold CH3 and OH groups chiral? They have 4 different substituents, so they must be chiral, no? Don't we see the same thing in cyclopentane, which is chiral?

 

[hiaips]

Please see the attached picture. (And pardon my horrendous art skillz)

According to TPR, Cyclopentane with two cis OH groups is a chiral molecule, and those are the two chiral centers. Cyclohexane with OH and CH3 however, does not have any chiral centers.

Q: Why? Why aren't the carbons that hold CH3 and OH groups chiral? They have 4 different substituents, so they must be chiral, no? Don't we see the same thing in cyclopentane, which is chiral?

The -OH and -CH3 groups are placed symmetrically. Hence, two of the groups bound to the corresponding carbons are the same.

 

[AmirTimur]

The -OH and -CH3 groups are placed symmetrically. Hence, two of the groups bound to the corresponding carbons are the same.

Thanks for the explanation. What do you mean by "OH and CH3 are placed symmetrically?" what makes the two cis OHs assymetric? Wouldn't the two groups bound to corresponing carbons on cyclopentane molecule also be the same?

Thanks a lot. Sorry but I am very confused 🙁

 

[hiaips]

Thanks for the explanation. What do you mean by "OH and CH3 are placed symmetrically?" what makes the two cis OHs assymetric? Wouldn't the two groups bound to corresponing carbons on cyclopentane molecule also be the same?

Thanks a lot. Sorry but I am very confused 🙁

No problem. "Symmetrically" was probably a poor word to use there. 🙂

In the cyclohexane molecule, the two substituents are on exact opposite sides of the molecule, four carbons away from each other *in both directions* (cw and ccw, if you want to think about it that way). This means that both R groups attached to each of the two carbons in question are the same. (In your diagram, the R groups attached to the lower C are both -CH2-CH2-CHOH-; on the upper C, they are both -CH2-CH2-CHCH3-.) Make sense?

 

[AmirTimur]

No problem. "Symmetrically" was probably a poor word to use there. 🙂

In the cyclohexane molecule, the two substituents are on exact opposite sides of the molecule, four carbons away from each other *in both directions* (cw and ccw, if you want to think about it that way). This means that both R groups attached to each of the two carbons in question are the same. (In your diagram, the R groups attached to the lower C are both -CH2-CH2-CHOH-; on the upper C, they are both -CH2-CH2-CHCH3-.) Make sense?

Ok, this makes sense now. Thanks for the lightbulb ( ) moment 🙂

 

[CodeRedDew]

Please see the attached picture. (And pardon my horrendous art skillz)

According to TPR, Cyclopentane with two cis OH groups is a chiral molecule, and those are the two chiral centers. Cyclohexane with OH and CH3 however, does not have any chiral centers.

Q: Why? Why aren't the carbons that hold CH3 and OH groups chiral? They have 4 different substituents, so they must be chiral, no? Don't we see the same thing in cyclopentane, which is chiral?

I don't agree with TPR's explanation in regards to the cyclopentane being chiral. It is an achiral molecule because it's considered to be a meso-compound; however, it DOES have chiral centers, but the overall molecule is in fact, achiral. A meso-compound is a molecule that contains chiral centers, but has a line of symmetry which makes the overall molecule achiral.

The cyclohexane does contain a line of symmetry; however, there are no chiral centers in the molecule to begin with so it doesn't classify as a meso-compound.