Stereocenter vs chiral center

[deleted388502]

Hi guys,

Just wanted to clarify the difference between chiral centers and stereo centers?
I was under the impression that double bond carbons were stereo centers, but I just did an ochem passage that suggested otherwise so I wanted to clear it up.

Thanks!!

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[Dipraman]

They are both the same thing

 

[gettheleadout]

They are both the same thing

They are not.

Stereocenters are merely atoms wherein the spatial arrangement of the bonding groups is such that exchanging any two groups results in the formation of a different stereoisomer.

Chirality itself refers to the superposability of mirror images; for a chiral center, the mirror images of a particular spatial arrangement of bonds around an atom are not superposable. This requires then, that a chiral carbon have four substituents. (Source: http://goldbook.iupac.org/C01060.html)

Interestingly, you can have molecules that exhibit chirality while possessing no chiral centers, only stereocenters (e.g. allenes).

 

[deleted388502]

They are not.

Stereocenters are merely atoms wherein the spatial arrangement of the bonding groups is such that exchanging any two groups results in the formation of a different stereoisomer.

Chirality itself refers to the superposability of mirror images; for a chiral center, the mirror images of a particular spatial arrangement of bonds around an atom are not superposable. This requires then, that a chiral carbon have four substituents. (Source: http://goldbook.iupac.org/C01060.html)

Interestingly, you can have molecules that exhibit chirality while possessing no chiral centers, only stereocenters (e.g. allenes).

Gotcha, so alkenes carbons WOULD be stereo centers, correct? The Ochem passage I did in TBR suggested that they were not so I was confused under what circumstances they would be considered stereo centers.

 

[gettheleadout]

Gotcha, so alkenes carbons WOULD be stereo centers, correct? The Ochem passage I did in TBR suggested that they were not so I was confused under what circumstances they would be considered stereo centers.

Depends on the alkene! Ethene? No. 2-Butene? Yes. Can you see why?

 

[deleted388502]

Yes. Makes perfect sense. I should probably bring up the question I'm referring to.

#8, in the image attached. I got this right just because none of the answers made sense, but I really want to say that the molecule does have a stereogenic center. The explanation says that "All of the carbons have at least two identical substituents or sp2 hybridization, so there are no stereogenic centers."

Am I missing something?

 

[gettheleadout]

Yes. Makes perfect sense. I should probably bring up the question I'm referring to.

#8, in the image attached. I got this right just because none of the answers made sense, but I really want to say that the molecule does have a stereogenic center. The explanation says that "All of the carbons have at least two identical substituents or sp2 hybridization, so there are no stereogenic centers."

Am I missing something?

So TBR kind of dropped the ball on this question. The question is poorly written as the stem includes a negative ("EXCEPT") and so does answer choice C. That alone makes the question difficult to evaluate, and this is an issue of very basic test design. Let's look at the answer choices anyway:

"(a) one stereogenic center" - the molecule in fact contains four stereogenic centers; TBR is presumably using the term incorrectly to mean chiral centers (definition here). This answer cannot be correct.
"(b) one cis double bond" - the molecule does contain one cis double bond. This answer choice cannot be correct.
"(c) no tertiary carbons" - the molecule does not contain tertiary carbons, but the question asks for the characteristic not present (hence the "EXCEPT"). Since there are no tertiary carbons, the feature of lacking tertiary carbons (which is the feature given in this answer choice) is present. This answer choice cannot be correct.
"(d) conjugation" - the double bonds in the molecule are indeed conjugated. This answer choice cannot be correct.

So we're left with...nothing. Honestly the question should be thrown out or ignored because of the issue with the wording of answer choice (c).

 

[deleted388502]

Thank you!! That was kind of my thought process on this, but I wanted to make sure that I wasn't mixing up the definitions, as I had in fact read earlier in TBR that they meant different things. 🙂