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Organic Chemistry

Learn the toughest concepts covered in Organic Chemistry with step-by-step video tutorials and practice problems by world-class tutors.

5. Chirality

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

Specific rotation vs. observed rotation.

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So optical activity is a special feature of Cairo molecules. Alright, and what it basically means is that Cairo molecules when light is passed through them, they're able to rotate plane polarized light. Okay, and the machine that measures this is called, Let me just write this down here. A polar emitter. Polar emitter. Okay. What I'm gonna do here is I'm gonna show you how light travels through a polar emitter and show you guys where the numbers come into play. Alright, so first of all, we have some light bulb. Okay, so that light bulb is a source of light. That is the ugliest filament ever. I'm just going to raise that. So 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. Okay, then we have a polarizer. A polarizer is just the type of lens. Just like your polarized glasses. How it filters light and make sure that makes sure that it's only going in one direction. So basically the lights 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. Alright, So then it passes through the actual functional part of the polaris er and that is this tube right here. This tube is going to carry a Cairo concentration. It's gonna carry a Cairo mixture. Alright. And the interesting thing is that that Cairo mixture is going to be able to one of the cool things about Cairo molecules, is that as light passes through them, it rotates the light so that it basically changes its angle after it has passed the Cairo molecule. Alright, there's just something that scientists discovered a long time ago and it's still used today. All right now. What's going to affect what kind of equations come into play to make sure that to determine what the angle is in the rotation? Well, what's going to affect? 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 in anti amber, 100% of that molecule present. What's the maximum rotation that you could get? Okay, just, you know, specific rotation is truly a random number. It doesn't have to do with the chirality necessarily 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. Okay, just so, you know, the specific rotation could be a positive rotation or a negative rotation. We're gonna talk about that in a second as well, then the next thing is the concentration of my re agents. So the more my specific rotation and the higher the concentration in the two, obviously the more it's going to turn right the link. The last thing is the length of the tube. Okay and that just makes sense. The longer the tube is, the more time the light has to rotate as it's passing through. So all these things are going to come together to equal my observed rotation. The observed rotation is just gonna be the product of these three things combined. It's gonna be the specific rotation times the concentration times the length of the tube. Does that make sense? And that's gonna affect what I observe at the end. If I make my tube twice as long I'm gonna get twice the amount of rotation. Cool, awesome. Well it turns out that sometimes we're not always gonna solve for observed rotation a lot of times we're gonna 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 solve for specific in which case we would just flip the formula. Use a little bit of algebra and it looks like that. So that basically says that your specific rotation is your observed rotation above. Like over concentration times. Like easy stuff. Okay, it's just a little bit of solving, we're just basically taking out a variable. Alright, so now let's talk about the actual rotations. Alright, so oclock wrote wise rotation is called dextre rotary and its symbolized using a positive single symbol and that's what I was talking about. How you can have a positive rotation or negative. Okay. A counterclockwise rotation is known as level rotary and that has a negative symbol. All right. So these are just words that were given to these rotations a long time ago. Just remember that if you see dexter rotary, that's positive level rotary, that's negative. Okay, now these positive and negative names have what's this blank? Say nothing to do with the chirality of the molecule. Okay, Nada Zilch. So what that means is that a lot of people get confused and they think that positive means that you have an R. That you think that they think that positive means that you have an R. Cairo center or negative means you have an Escarole 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 um Cairo center. Okay, the clockwise thing just has to do with how we name the Cairo center, it doesn't have to actually do with what the Cairo center looks like. Okay, so what I'm trying to say is that in our let's say an R. And an timer for some molecules that could be a positive for other molecules that could be a negative. Okay, the only thing that it tells you is that it is Cairo 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!