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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: vml on October 23, 2007, 03:21:12 PM

Title: Specific rotation
Post by: vml on October 23, 2007, 03:21:12 PM

What compound has the biggest specific rotation???
Which can be limit of specific rotation???

Title: Re: Specific rotation
Post by: CopperSmurf on October 31, 2007, 08:42:32 PM

Your question is too broad. In my experience, specific rotations are calculated based on what compound(s) you are examining and are usually determined in the lab for whatever chemicals you are using/examining.

As for the limit of specific rotation, if I interpreted your question correctly, I believe it goes from -360 to +360 degrees because less than -360 degrees and you are rotating from where you started from. Similar reason for more than 360 degrees.

Hope that helps!

Title: Re: Specific rotation
Post by: macman104 on October 31, 2007, 09:56:13 PM

Quote from: CopperSmurf on October 31, 2007, 08:42:32 PM

Your question is too broad. In my experience, specific rotations are calculated based on what compound(s) you are examining and are usually determined in the lab for whatever chemicals you are using/examining.

As for the limit of specific rotation, if I interpreted your question correctly, I believe it goes from -360 to +360 degrees because less than -360 degrees and you are rotating from where you started from. Similar reason for more than 360 degrees.

Hope that helps!

Copper, I too thought as you did.  I figured that you couldn't have a rotation more than +/- 360 degrees.  I was going to reply to this post, but I thought I'd check out what wikipedia had to say.  To my surprise it appears that there are techniques for determining rotations that are greater than the above range, interesting stuff!

Title: Re: Specific rotation
Post by: agrobert on October 31, 2007, 10:05:34 PM

Wikipedia

Specific Rotation

If a compound has a very large specific rotation or a sample is very concentrated, the actual rotation of the sample may be larger than 180°, and so a single polarimeter measurement cannot detect when this has happened (for example, the values +270° and –90° are not distinguishable, nor are the values 361° and 1°). In these cases, measuring the rotation at several different concentrations allows one to determine the true value.

Title: Re: Specific rotation
Post by: macman104 on November 01, 2007, 01:31:58 AM

Err...yea, I mean, 180, not 360...yea...

Title: Re: Specific rotation
Post by: ARGOS++ on November 01, 2007, 07:17:38 PM

Dear All,

From the Definition:    [α]^T = 100 * α / (d * c)  it follows: 
 

α = [α]^T * d * c / 100     with d in Decimeters !!!

Now its absolutely clear that only the final “total” Absorbance (losing of all Light) of the Substance will set an end of the maximal Angel, that could be measured, because you can increase and increase the “Pathlength” ( = d ) as much as you like, - Not only the Concentration  ( = c ).

So there is another real interesting Question:
            Which Substance shows us the highest [α]^T?

Good Luck!
                    ARGOS⁺⁺