Chiral Centers question

[soaringbirds]

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A quick question about chiral/stereogenic center:

Would the two sp2 carbons of a trans or cis alkene qualify as stereogenic or chiral centers?

Thanks

 

[BB8730]

Nope. The carbon must be sp3 and have 4 different groups attached to be a stereocenter.

 

[Singhp03]

A quick question about chiral/stereogenic center:

Would the two sp2 carbons of a trans or cis alkene qualify as stereogenic or chiral centers?

Thanks

from what I understand, sp2 carbons are never chiral, its a requirement to have at least three differing sigma attachments to a center for it to be chiral. The requirement is three, instead of four, bc nitrogen can become chiral with three differing attachments plus its lone pair(this can also happen with carbanions, but it very rare).

 

[sciencebooks]

What everyone else said. Plus, you could also think of it like this: If there's a double bond, it's connected to the SAME THING twice and that defeats the "different groups" definition of chirality. (Though yeah, the sp3 rule is a giveaway.)

 

[orgohacks]

A quick question about chiral/stereogenic center:

Would the two sp2 carbons of a trans or cis alkene qualify as stereogenic or chiral centers?

Thanks

Stereogenic and chiral centers are not necessarily the same thing.

"A stereocenter or stereogenic center is an atom, bearing groups such that an interchanging of any two groups leads to a stereoisomer.[1" - Wiki.

"A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer" - Wiki.

Sp2 carbons cannot be chiral centers, since inverting any two groups will not give you an enantiomer. [Exception you probably don't need to care about: allenes]

However they CAN be stereogenic centers. If you have a cis (Z) alkene, interchanging two groups would give you a trans (E) alkene, which will be a stereoisomer (a diastereomer, in fact).

So there ya go. - J

 

[sciencebooks]

Stereogenic and chiral centers are not necessarily the same thing.

"A stereocenter or stereogenic center is an atom, bearing groups such that an interchanging of any two groups leads to a stereoisomer.[1" - Wiki.

"A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer" - Wiki.

Sp2 carbons cannot be chiral centers, since inverting any two groups will not give you an enantiomer. [Exception you probably don't need to care about: allenes]

However they CAN be stereogenic centers. If you have a cis (Z) alkene, interchanging two groups would give you a trans (E) alkene, which will be a stereoisomer (a diastereomer, in fact).

So there ya go. - J

So even having looked it up, I'm not quite sure how allenes (with back-to-back double bonds) is capable of chirality.

 

[orgohacks]

So even having looked it up, I'm not quite sure how allenes (with back-to-back double bonds) is capable of chirality.

This is not MCAT material but allenes can have an "axis" of chirality. This video may (or may not) make it more clear - http://www.youtube.com/watch?v=HeIhlpII70o

 

[bored]

Nope. The carbon must be sp3 and have 4 different groups attached to be a stereocenter.

I agree with this answer

 

[johnnybravo19]

Cis and Trans are different configurations, and they bear handedness, so if they're not considered chiral, i'm not exactly sure what they are considered. They were considered chiral when I took ochem 2 years ago.

 

[V5RED]

Cis and Trans are different configurations, and they bear handedness, so if they're not considered chiral, i'm not exactly sure what they are considered. They were considered chiral when I took ochem 2 years ago.

Your memory must be off because cis/trans is definitely not tied directly to chirality or handedness.

Handedness refers to the arrangement of bonds on a single atom much like the arrangement of fingers on your hand. If you can rearrange the bonds to make it look different then it has handedness.

Chirality can refer to whether an atom has handedness or whether the entire molecule has a non superimposable mirror image that can be formed.

Some things that are cis or trans can also have handedness/chirality. For example in a ring structure like cis-1-bromo-2-chloro-cyclohexane, the 1 and 2 carbons both have handedness and are chiral. This is because they both have 4 different bonds attached to them, not because they are cis. The cis has nothing to do with its chirality. If the bromine was replaced with hydrogen, the carbon attached to chlorine would still be chiral. Its mirror image will be an enantiomer.

Some things that are cis or trans lack handedness/chirality. In a double bond type of cis like cis-1-bromo-2-chloro-hexene there are no chiral centers and no handedness. No carbon has 4 unique bonds attached to it. No carbon can have its arrangement of bonds changed to look different. The molecule is achiral, has nor chiral centers, and has no handedness. Its mirror image is going to be itself. The mirror image of a cis molecule is identical to the molecule.

 

[sciencebooks]

This is not MCAT material but allenes can have an "axis" of chirality. This video may (or may not) make it more clear - http://www.youtube.com/watch?v=HeIhlpII70o

Ah, that makes sense. Thanks for the link, 🙂.

 

[soaringbirds]

That helps clear it up. Thanks

Stereogenic and chiral centers are not necessarily the same thing.

"A stereocenter or stereogenic center is an atom, bearing groups such that an interchanging of any two groups leads to a stereoisomer.[1" - Wiki.

"A chiral center is a generalized extension of an asymmetric carbon atom, which is a carbon atom bonded to four different entities, such that an interchanging of any two groups gives rise to an enantiomer" - Wiki.

Sp2 carbons cannot be chiral centers, since inverting any two groups will not give you an enantiomer. [Exception you probably don't need to care about: allenes]

However they CAN be stereogenic centers. If you have a cis (Z) alkene, interchanging two groups would give you a trans (E) alkene, which will be a stereoisomer (a diastereomer, in fact).

So there ya go. - J