Orgo question

[Halcyon32]

The answer is C and the answer explanation says that carbon 4 is the only carbon that's chiral so that's why it's the answer. I don't really understand what that means in the context of this problem and what it is meant to explain. Would anyone mind explaining?

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[Cawolf]

(+) and (-) in a compound indicate dextrorotatory and levorotatory, only possible in a chiral compound.

So to solve this question, look for the chiral carbon.

The only chiral carbon is carbon 4, so that must be the answer.

If you don't know how to determine chirality, I suggest reviewing.

 

[Halcyon32]

(+) and (-) in a compound indicate dextrorotatory and levorotatory, only possible in a chiral compound.

So to solve this question, look for the chiral carbon.

The only chiral carbon is carbon 4, so that must be the answer.

If you don't know how to determine chirality, I suggest reviewing.

I understand why carbon 4 is the only chiral carbon, it is the only carbon that has 4 different substituents, but it still doesn't make sense to me why that determines the difference btwn + and -.

 

[Cawolf]

I understand why carbon 4 is the only chiral carbon, it is the only carbon that has 4 different substituents, but it still doesn't make sense to me why that determines the difference btwn + and -.

You cannot determine optical rotation without a polarimeter.

It is extra information that only serves to inform you that you are looking for a chiral center.

 

[Cawolf]

I saw another post from you indicating you have not taken organic, so I will offer a bit more.

(+) and (-) can only be determined by physical experimentation; you cannot determine it from looking at a structure.

So your job is to know that it means you have a chiral molecule. That is the only purpose is serves.

 

[Halcyon32]

I saw another post from you indicating you have not taken organic, so I will offer a bit more.

(+) and (-) can only be determined by physical experimentation; you cannot determine it from looking at a structure.

So your job is to know that it means you have a chiral molecule. That is the only purpose is serves.

All that stuff about obtaining optical rotation with a polarimeter experimentally, I know. So since the problem is telling us that we have optical rotation it is implying that there is a chiral center. That I understand as well. I just don't understand why that information serves to inform me that I am looking for the chiral center. I'm sorry I know it may seem really obvious to you but like you said I haven't taken orgo yet and I likely need a bit more in depth of an understanding of the concept to understand this, which I probably don't have at the moment.

 

[Cawolf]

That I understand as well. I just don't understand why that information serves to inform me that I am looking for the chiral center.

Because optical rotation is a property that only occurs in chiral molecules.

 

[Halcyon32]

Because optical rotation is a property that only occurs in chiral molecules.

(-)-nicotine and (+)-nicotine are two different compounds right? Is the place where the optical rotations of + and - compounds differ also at the chiral carbon?

 

[theonlytycrane]

The chiral carbon (4) makes the molecule chiral. The chiral molecule can either have + or - optical rotation. The + and - molecules differ in stereo orientation of groups attached to carbon 4.

 

[Halcyon32]

The chiral carbon (4) makes the molecule chiral. The chiral molecule can either have + or - optical rotation. The + and - molecules differ in stereo orientation of groups attached to carbon 4.

So the molecule has a different orientation when its + than it does when its -. and this difference stems from what is attached to the chiral carbon at each orientation? Did I understand this correctly?

 

[theonlytycrane]

So the molecule has a different orientation when its + than it does when its -. and this difference stems from what is attached to the chiral carbon at each orientation? Did I understand this correctly?

Yes I would just reword it to say "the molecule has a different optical property when it's + vs. -, and this difference stems from..."

 

[Halcyon32]

Yes I would just reword it to say "the molecule has a different optical property when it's + vs. -, and this difference stems from..."

Oooook that makes more sense. Thanks a lot to all of you that helped!