Optical vs. specific rotation?

[SaintJude]

Read EK & Kaplan, but still don't understand the differences b/w the 2 really.

Is one a book value?

If you increase the concentration of your enantiomer, the specific rotation will decrease.
But if you increase the concentration of your enantiomer, the observed rotation will increase.

---

Members don't see this ad.

 

[MedPR]

Read EK & Kaplan, but still don't understand the differences b/w the 2 really.

Is one a book value?

If you increase the concentration of your enantiomer, the specific rotation will decrease.
But if you increase the concentration of your enantiomer, the observed rotation will increase.

The specific rotation is the observed rotation under specific conditions, kind of like how 1mol of any gas has a volume of 22.4L at STP, but ONLY at STP.

You measure the observed rotation.

 

[SaintJude]

The specific rotation is the observed rotation under specific conditions, kind of like how 1mol of any gas has a volume of 22.4L at STP, but ONLY at STP.

You measure the observed rotation.

Alright, can you explain this: What would happen to the specific and observation rotation if only part of a certain reaction mixture was transferred to the volumentric flask?

 

[MedPR]

Alright, can you explain this: What would happen to the specific and observation rotation if only part of a certain reaction mixture was transferred to the volumentric flask?

If I encountered this as a discrete I would pick the answer that said no change would occur, but judging from the way you presented it, I'm assuming something changes. I have no idea.

If the mixture is homogenous and you just take some out to put into a polarimeter, it shouldn't be any different than if you had a huge polarimeter big enough to accommodate a greater volume of the solution.

 

[hellocubed]

Alright, can you explain this: What would happen to the specific and observation rotation if only part of a certain reaction mixture was transferred to the volumentric flask?

It depends on if
1.) the solution was diluted or not
2.) has the path of light through the volume decreased/increased (A increases with light path length)

More information is needed

 

[SaintJude]

Ok read Kaplan and a few other sources: will pass on their words. Like MedPr said, the specific rotation of a pure material is an intrinsic property of that material at a given wavelength and temperature. Whether you change the tube or change the concentration of an optical compound, its specific rotation will remain the same as long as temp and wavelength of light is same. So 0.1 M of (+) mandelic acid will have the same specific rotation as 6.0 M of (+) mandelic acid, so you can look this value up in a book.

The observed rotation, is literally what a orgo student schmuck like me would observe in the lab. Depending on my technical lab skills and personal procedure, the observed rotation for mandelic acid could change from one day to the next.

The cardinal equation is specific rotation = observed rotation / (L x c)
L = length of tube & c = concentration of solution
So what would happen when a pre-med schmuck spilled some of the reaction mixture as the solution was transferred to the volumetric flask ?

The observed rotation would decrease. Since specific rotation is a constant, if concentration decrease the observed rotation would decrease to compensate for the changes.

Also, I recommend anyone interested on rotation expand the left window of this webpage. Given that we talk about rotation of pure enatiomers a lot, I wouldn't be surprised if there was an MCAT passage that actually dealt with the science behind it. Helped me a lot.
http://eduweblabs.com/Database/Lab_FoldersP/SRotation/SRotation.html

 

[MedPR]

Ok read Kaplan and a few other sources: will pass on their words. Like MedPr said, the specific rotation of a pure material is an intrinsic property of that material at a given wavelength and temperature. Whether you change the tube or change the concentration of an optical compound, its specific rotation will remain the same as long as temp and wavelength of light is same. So 0.1 M of (+) mandelic acid will have the same specific rotation as 6.0 M of (+) mandelic acid, so you can look this value up in a book.

The observed rotation, is literally what a orgo student schmuck like me would observe in the lab. Depending on my technical lab skills and personal procedure, the observed rotation for mandelic acid could change from one day to the next.

The cardinal equation is specific rotation = observed rotation / (L x c)
L = length of tube & c = concentration of solution
So what would happen when a pre-med schmuck spilled some of the reaction mixture as the solution was transferred to the volumetric flask ?

The observed rotation would decrease. Since specific rotation is a constant, if concentration decrease the observed rotation would decrease to compensate for the changes.

Also, I recommend anyone interested on rotation expand the left window of this webpage. Given that we talk about rotation of pure enatiomers a lot, I wouldn't be surprised if there was an MCAT passage that actually dealt with the science behind it. Helped me a lot.
http://eduweblabs.com/Database/Lab_FoldersP/SRotation/SRotation.html

But spilling part of a homogenous mixture does not change the concentration.

 

[SaintJude]

Really? Kaplan says:

Partial transfer of the reaction mixture into the volumetric flask would result in a lower concentration of products.

Ok, maybe they meant that the concentration of the products will be smaller due to a the smaller mixture that undergoes reaction. Would that make a difference?

Here is the passage:

Pure sucrose, distilled water, and concentrated hydrochloric acid were heated to reflux for approximately 2 hours. (The optical rotation of sucrose was measured using a polarimeter prior to heating.) The reaction mixture was then cooled to room temperature, transferred to a volumetric flask, and diluted with distilled water. The optical rotation of the product mixture was then determined.

 

[MedPR]

Really? Kaplan says:



Ok, maybe they meant that the concentration of the products will be smaller due to a the smaller mixture that undergoes reaction. Would that make a difference?

Here is the passage:

Pure sucrose, distilled water, and concentrated hydrochloric acid were heated to reflux for approximately 2 hours. (The optical rotation of sucrose was measured using a polarimeter prior to heating.) The reaction mixture was then cooled to room temperature, transferred to a volumetric flask, and diluted with distilled water. The optical rotation of the product mixture was then determined.

You forgot to mention previously that the reaction mixture was subsequently diluted 🙂 If you think you put in 5mol of sucrose+hcl but really only put in 2mol, you will come out with a really strange observed rotation because you will calculate your concentration using 5mol instead of what you really have (2mol).

So yea, if you a smaller concentration of optically active particles, it should make sense that the light won't be rotated as much as if you have more optically active particles.

 

[SaintJude]

Oh, sorry. You know I didn't even realize that was a big deal. But it makes sense. The dilution is adding to the measurement of the solvent. Thanks MedPr!

 

[MedPR]

Oh, sorry. You know I didn't even realize that was a big deal. But it makes sense. The dilution is adding to the measurement of the solvent. Thanks MedPr!

I know, I didn't mean it that way. We all know how easy it is to overlook the simple things when we are trying to figure out something that seems completely independent.

 

[SaintJude]

One last thing: So one can say that observed rotation is directly proportional to length of tube (depth of sample tube) or concentration of optical active compound in solution. 👍 ?

 

[MedPR]

One last thing: So one can say that observed rotation is directly proportional to length of tube (depth of sample tube) or concentration of sugar in solution. 👍 ?

Yes because observed=specific*L*c

 

[chiddler]

Yes because observed=specific*L*c

where did you find this formula?

 

[SaintJude]

It's in Kaplan--not in their review books (as usual) but in their online review material when you do questions. And EK talks about difference b/w specific and observed. But "observed rotation" isn't on AAMC list, so dunno if it's a common topic...

 

[MedPR]

where did you find this formula?

my orgo 1 class