TBR Error?

[MedPR]

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TBR says ALL optically active molecules are polar because "all optically active compounds must be asymmetric in order to be optically active, so they must be slightly polar."

Aren't all chiral molecules optically active?

And aren't trans allenes chiral? So aren't trans alleles optically active symmetric molecules?

 

[milski]

Nice catch, serves them right for using generalizations like 'All X are Y'.

Allenes have a chiral axis, not a chiral center. The generalization should be correct for molecules with chiral centers. Most likely. At least that's what I think.

 

[BerkReviewTeach]

TBR says ALL optically active molecules are polar because "all optically active compounds must be asymmetric in order to be optically active, so they must be slightly polar."

Aren't all chiral molecules optically active?

And aren't trans allenes chiral? So aren't trans alleles optically active symmetric molecules?

The question you need to first answer is whether "all optically active compounds" is the same subset as "all chiral molecules". That sentence you quoted focuses on molecules that rotate plane-polarized light. To rotate plane polarized light, the molecule must have an asymmetric distribution of electron density, which means the structure would also be polar (even if only slightly). It's like saying that "all people over 6'10" are tall."

What symmetry are you thinking of with trans allenes? I'm not seeing it. They have no C2 or D2 axis. Maybe I'm not visualizing things right at the moment, but I don't see rotational symmetry. They have no point symmetry. I see them as asymmetric as best I can contort them.

How does a trans allene (let's use the typical example of 1,3-dichloro) violate the rule that optically active compounds are polar? I don't think that violates the sentence in question.

Nice catch, serves them right for using generalizations like 'All X are Y'.

Allenes have a chiral axis, not a chiral center. The generalization should be correct for molecules with chiral centers. Most likely. At least that's what I think.

I think the generalization applies with allenes (chiral axis) as well as molecules with a stereogenic center. But as they worded it, they included molecules rotating plane-polarized light with can only happen if there is asymmetry of electron distribution. I don't see how it's not a valid statement.

 

[milski]

I think the generalization applies with allenes (chiral axis) as well as molecules with a stereogenic center. But as they worded it, they included molecules rotating plane-polarized light with can only happen if there is asymmetry of electron distribution. I don't see how it's not a valid statement.

You're right. I cannot even think of what my argument was when I posted half an hour ago. Which is a sure sign that I should be done with any sort of studying for today.

 

[BerkReviewTeach]

You're right. I cannot even think of what my argument was when I posted half an hour ago. Which is a sure sign that I should be done with any sort of studying for today.

It's the da*# time change I tell you. I felt fried all day today at work. I kept on drawing allenes over and over until they all started to look the same, so hopefully I'm seeing them right.

When are you taking the MCAT? I have a feeling you're getting a 40 or higher.

 

[milski]

It's the da*# time change I tell you. I felt fried all day today at work. I kept on drawing allenes over and over until they all started to look the same, so hopefully I'm seeing them right.

When are you taking the MCAT? I have a feeling you're getting a 40 or higher.

I have an ochem final in two days. Anything with C atoms in it looks the same right now. 😀

Most likely fall 2012 or January 2013 - I'm not done with the bio pre-reqs and it's my weakest subject. I'm not applying until next year, so either would work fine.

 

[MedPR]

Yea milski is definitely getting a 40+. 15 in PS for sure.

Isn't the red line a plane of symmetry? Maybe I need to go to sleep too. 😕

 

[milski]

It would be but that's not how it looks like - the bonds are "twisted" from the middle C.

 

[MedPR]

It would be but that's not how it looks like - the bonds are "twisted" from the middle C.

OH YEA! thanks, i forgot that part.