Optical Activity

One of the special features of chiral molecules is that they are able to rotate plane-polarized light. Unfortunately, this means that now professors have an excuse to ask you math problems. Let’s see how this works. 

The Concept of Optical Activity

Concept: Specific rotation vs. observed rotation.  

Video Transcript

Now we're going to talk about one of the only topics in organic chemistry that's going to require you to use a calculator. I know that's disappointing because at the beginning of the semester I told you guys there's barely any numbers in this course, but this is one exception. There is this one topic and it's called optical activity.
Optical activity is a special feature of chiral molecules. What it basically means is that chiral molecules, when light is passed through them, they're able to rotate plane-polarized light. The machine that measures this is called – let me just write this down here – a polarimeter. What I'm going to do here is I'm going to show you how light travels through a polarimeter and show you guys where the numbers come into play.
First of all, we have some light bulb. That light bulb is a source of light. That is the ugliest filament ever. I'm just going to erase that. This light bulb is a source of multi-directional light. What do I mean by that? What I mean is that it doesn't just shoot light in one direction, it's obviously, scattering light throughout a space.
Then we have a polarizer. A polarizer is just a type of lens. Just like your polarized glasses how it filters light and makes sure that it's only going in one direction. Basically, the light's going to pass through the polarizer and it's going to turn into what's called plane-polarized light. It's only going on one plane.
Then it passes through the actual functional part of the polarimeter and that is this tube right here. This tube is going to carry a chiral concentration. It's going to carry a chiral mixture. The interesting thing is that chiral mixture is going to be able to – one of the cool things about chiral molecules is that as light passes through them, it rotates the light so that it basically changes it's angle after it has passed the chiral molecule. This is just something that a scientist discovered a long time ago and it's still used today.
Now what's going to effect, what kind of equations come into play to determine what the angle is of the rotation? What's going to effect it is a few things.
First of all, every molecule has what's called a specific rotation. The specific rotation is just a random number that has to do with the amount of rotation that you would get if you had 100% of that enantiomer or 100% of that molecule present, what's the maximum rotation that you could get.
Just so you know specific rotation is truly a random number. It doesn't have to do with the chirality necessarily. It doesn't have to do with the size of the molecule. Nothing. There's no way to predict it. You will always be given the specific rotation or you'll be given the other variables to solve for the specific rotation, but you're not supposed to know it. That's all I'm saying. Just so you know, the specific rotation could be a positive rotation or a negative rotation. We're going to talk about that in a second as well.
Then the next thing is the concentration of my reagent. The more my specific rotation and the higher the concentration in the tube, obviously, the more it's going to turn. The last thing is the length of the tube. That just makes sense. The longer the tube is, the more time that light has to rotate as it's passing through.
All these things are going to come together to equal my observed rotation. The observed rotation is just going to be the product of these three things combined. It's going to be the specific rotation times the concentration times the length of the tube. Does that make sense? That's going to effect what I observe at the end. If I make my tube twice as long, I'm going to get twice the amount of rotation. Cool? Awesome.
It turns out that sometimes we're not always going to solve for observed rotation. A lot of times we're going to be solving for specific rotation, so instead, in the problem they're going to give us the observed, the concentration and the length and then we're going to have to solve for a specific in which case we would just flip the formula, use a little bit of algebra and it looks like that. That basically says that your specific rotation is your observed rotation over concentration times length. Easy stuff. It's just a little bit of solving – we're just basically taking on a variable.
Now let's talk about the actual rotations. A clockwise rotation is called dextrorotatory. It's symbolized using a positive symbol. That's what I was talking about how you can have a positive rotation or a negative. A counterclockwise rotation is known as level levorotatory and that has a negative symbol. These are just words that were given to these rotations a long time ago. Just remember that if you see dextrorotatory that's positive, levorotatory, that's negative.
These positive and negative names have – what's this blank say? Nothing to do with the chirality of the molecule. Nada. Zilch. What that means is that a lot of people get confused and they think that positive means that you have an R. They think that positive means you have an R chiral center or a negative means you have an S chiral center because they see the clockwise and they think that it's the same thing. They're completely different. A clockwise rotation of light has nothing to do with a clockwise chiral center. The clockwise thing just has to do with how we name the chiral center. It doesn't actually have to do with what the chiral center looks like.
What I'm trying to say is that let's say an R enantiomer for some molecules that could be a positive, for other molecules that could be a negative. The only thing that it tells you is that it is chiral. But it doesn't tell you what type of chirality you have. Does that make sense? Just have to really emphasize that point. 

  • Clockwise rotation = dextrorotary (d) or (+)
  • Counterclockwise rotation = levororatory (l) or (-)

These random names/signs have nothing to do with the chirality of a molecule!

Optical Activity Additional Practice Problems

Which compound, if any will be optically active? 

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One of the following isn't true about a racemic mixture.

a) It is optically inactive

b) It has equal amounts of enantiomers

c) It doesn't rotate plane polarized light

d) Its ee is 50%

e) all are true

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The following statements describe some important properties of molecules. Choose from among the following three types of isomers and in the space provided write the letter of the one or more types of isomers for which the statemetn is TRUE.

A. A single enantiomer          B. A meso compound          C. A racemic mixture

A solution of this will rotate the plane of plane polarized light.   _________________

A solution of this will NOT rotate the plane of plane polarized light.   _____________________

Might be produced when an alkene with no chiral centers reacts with a non-chiral reagent such as H-Br or Br2.  ___________________

The LEAST desirable option when developing a new drug  ___________________

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All S enantiomers rotate the plane of polarized light anti-clockwise ( True or False 

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The specific rotation of (S)-carvone (at 20°C) is +61. A chemist prepared a mixture of (R)-carvone and its enantiomer, and this mixture had an observed rotation of –55°.

What is the specific rotation of (R)-carvone at 20°C?

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Determine whether each statements is true or false:

(a) A racemic mixture of enantiomers is optically inactive. 

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Which statement is true?

a) Compounds with R stereocenters rotate plane-polarized light clockwise. 

b) For equal concentrations and equal path lengths, solutions of (+) and (-) enantiomers rotate plane-polarized light equally, but in opposite directions. 

c) Racemic mixtures can rotate plane-polarized light either clockwise or counterclockwise.

d) Meso compounds can rotate plane-polarized light either clockwise or counterclockwise. 

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Which of the following molecules is optically active?

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A forensic chemist (chemist who deals with crime-related investigations) analyzed a sample of a poisonous alkaloid (generic term for a naturally occurring nitrogen-containing molecule) and determined that its specific rotation [α] at 25 °C using sodium D light was + 12 °. What can the chemist properly conclude? 

a) there is one enantiomer in the sample

b) the enantiomer present has the R configuration

c) the sample is optically pure

d) the butler did it

e) none of the above

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Pure (S)-2-butanol has a specific rotation of +13.5 degrees. You have made and purified a sample that has a calculated specific rotation of -6.76 degrees. What can you conclude about this sample?







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Which of the following is true about any  R enantiomer?


(1) It has a (+) rotation in a polarimeter.

(2) It has a (-) rotation in a polarimeter.

(3) It is a mirror image of the S enantiomer. 

(4) When mixed with the S enantiomer, it forms a meso compound.

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