Rotation of Plane polarised Light

[liambob1]

In our instrumentation lab we have been using a polarimeter on salicylic acid tp measure the degree it rotates plane polarised light, I understand that it does this and the applications of this but Im confused about HOW it does this, I know it has something to do the vibration of molecules. If someone could help me clear this up I would appreciate It.

[swansont]

You can view a linear polarization as the superposition of circular polarization in opposite directions, with the phase between them dictating the direction of the linear polarization. The material has a different index for the two circular polarizations, so it induces an additional phase difference between them as the light travels through. This results in a different direction of the linear polarization.

 

There has to be some asymmetry in the molecular structure that makes it preferentially interact with one polarization.

[John Cuthber]

Salicylic acid doesn't rotate the plane of polarised light. It can't It has a mirror symmetry plane.

[liambob1]

We did the test of various substances, salacylic was one, we used methyl orange among others

[mississippichem]

We did the test of various substances, salacylic was one, we used methyl orange among others

 

2-hydroxybenzoic acid, right?

 

John Cuthber is correct. Salicyclic acid is not a chiral compound. It is superposable on its own mirror image, and more formally it displays a [math] \sigma[/math]-mirror plane in the "plane" of the benzene ring. Try it for yourself. Try to draw a pair of enantiomers for salicyclic acid, you'll find that it can't be done.

[John Cuthber]

For the record, methyl orange isn't chiral either.

[liambob1]

I checked back in my reports to the experiment and the solution we tested was a sucrose solution, I sought help not to get my errors thrown in my face John

[mississippichem]

I checked back in my reports to the experiment and the solution we tested was a sucrose solution, I sought help not to get my errors thrown in my face John

 

Any questions about the sucrose solution? I doubt John Cuthber was trying to throw something in your face though. He may be terse, but he just wants the correct information to propagate through the forum as it it his responsibility as a resident expert; and he knows what he is talking about. No need to take offense, everyone gets corrected sometimes, even experts by other experts.

[liambob1]

As part of the experiment we used an acid to dehydrate the sucrose and see did this effect the rotary power , the result was inconclusive, what should have happened?

And I know he wasnt 'twas silly of me

[John Cuthber]

You might want to have a look at this.

http://en.wikipedia.org/wiki/Inverted_sugar_syrup#Chemical_reaction_of_the_inversion

[liambob1]

Thank you very much, it's greatly appreciated