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Chemistry: An Introduction to Reactivity

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About this Lesson

  • Type: Video Tutorial
  • Length: 9:33
  • Media: Video/mp4
  • Use: Watch Online & Download
  • Access Period: Unrestricted
  • Download: MP4 (iPod compatible)
  • Size: 102 MB
  • Posted: 07/15/2009

This lesson is part of the following series:

Chemistry: Full Course (303 lessons, $198.00)
Chemistry: Introduction to Organic Reactions (15 lessons, $23.76)
Chemistry: Acid Strength in Organic Molecules (6 lessons, $11.88)

This lesson was selected from a broader, comprehensive course, Chemistry, taught by Professor Harman, Professor Yee, and Professor Sammakia. This course and others are available from Thinkwell, Inc. The full course can be found at http://www.thinkwell.com/student/product/chemistry. The full course covers atoms, molecules and ions, stoichiometry, reactions in aqueous solutions, gases, thermochemistry, Modern Atomic Theory, electron configurations, periodicity, chemical bonding, molecular geometry, bonding theory, oxidation-reduction reactions, condensed phases, solution properties, kinetics, acids and bases, organic reactions, thermodynamics, nuclear chemistry, metals, nonmetals, biochemistry, organic chemistry, and more.

Dean Harman is a professor of chemistry at the University of Virginia, where he has been honored with several teaching awards. He heads Harman Research Group, which specializes in the novel organic transformations made possible by electron-rich metal centers such as Os(II), RE(I), AND W(0). He holds a Ph.D. from Stanford University.

Gordon Yee is an associate professor of chemistry at Virginia Tech in Blacksburg, VA. He received his Ph.D. from Stanford University and completed postdoctoral work at DuPont. A widely published author, Professor Yee studies molecule-based magnetism.

Tarek Sammakia is a Professor of Chemistry at the University of Colorado at Boulder where he teaches organic chemistry to undergraduate and graduate students. He received his Ph.D. from Yale University and carried out postdoctoral research at Harvard University. He has received several national awards for his work in synthetic and mechanistic organic chemistry.

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Thinkwell
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Of all the aspects of chemistry that I find interesting, by far the most intriguing to me and the reason, in fact, that I went into chemistry as a career, is the ability to be able to predict a chemical reaction. Think about that for a moment. To be able to look at a molecule and say, okay, I know I've got this molecule, but to be able to say something or predict things about how that molecule would react with another molecule. What's the personality of the molecule? Is it going to be an acid? Is it going to be a base? How does it participate with other molecules? Because to be able to understand how something reacts is to explain the entire world around us. So many different processes in our world, every day processes, the drugs that we take for when we're sick, the plastics that we wrap things in or drive around in; everything around us has to do with how molecules react with other molecules. And to be able to actually have some insight into how they happen, not just memorize facts about, yes, this reaction happens when you do this or to measure all the numbers, but to be able to say something and to guess what's going to happen is truly the most exciting thing about chemistry.
Well, you are now at a point where you can start to do that. And, in fact, those of you that go on into Organic Chemistry will be spending an entire year looking at reactions. Just the same way that general chemistry has been a survey of structure of molecules, as well as the energy relationships for reactions, so organic chemistry is a focus on how molecules react with each other.
Now, unfortunately, sometimes by breaking things down the way we do into general chemistry and organic chemistry, they almost are presented as if they're separate sciences, and this is an unfortunate result because this is all the same thing. Talking about reactions of organic molecules, carbon-containing molecules, is no different than talking about reactions of any other molecule, but unfortunately with so much emphasis on carbon, it's easy to forget that. Well, why talk about carbon? Two very important reasons. One, carbon is crucially important to us. Obviously it's everywhere in organic systems. It's a crucial part of our lives. The other thing though is that from a learning standpoint, talking about carbon is a lot easier than talking about other atoms deeper in the periodic table. It's a relatively simple to understand atom and people, because it's so important, have spent many, many years really understanding it at a much more advanced level than we do with lots of the other elements. Remember, chemistry is a science that is continually evolving and the level of our understanding for carbon has reached a much higher level of understanding than many of the other elements simply because it's so important to us. So that's why the emphasis on carbon.
Well, what we want to do is start to talk about reactions. Start to give you a sense of how you can look at a molecule and predict something about what it's likely to do. Now in order to that we need to first focus on the bonds, the bonds that are going to be breaking and being made in order for a reaction to occur. So we want to be able to build a bridge for you going from general chemistry to organic without such a barrier between the two. And so I will go ahead right now and focus primarily on organic reactions simply so that it doesn't seem like such a foreign idea next year if you go on into Organic Chemistry. But please bear in mind that everything I'm talking about using examples of organic molecules pertains to any molecule, not just carbon containing molecules. The same exact principles apply.
So let's begin our discussion by reminding ourselves about bonds and polarized bonds. We've talked about bonds that are covalent, that are polar covalent and ionic and that this is entire continuum that there aren't finite classes. And that within this continuum the notion that the greater the difference in electronegativity between two atoms, their general overall desire to take on extra electron density, that depending on how different that desire is; we get more or less polarization of the bond. So it is in the essence of a polarized bond that we see 90% of all of the reactivity that we're going to be discussing. That's not to say all that happens that way. In fact, we need to make a distinction here between breaking a bond heterolytically and breaking it homolytically. And we talked about heterolytic breakage. We're talking about breaking the bond completely apart where both electrons involved in the bond go to one of the two partners. The one that needs the electrons more. Homolytic cleavage, on the other hand, is breaking a bond where one electron of the bond goes to one atom and the other electron of the bond goes to the other atom. So those are two fundamentally different types of bond breaking and they represent two fundamentally different types of reactivity as a result. I will focus right now on heterolytic bond breakage and making bonds, so we're going to be transferring electron pairs rather than individual electrons because so many reactions go by that type of mechanism.
So the more polarized the bond is, the easier it is to break in that heterolytic sense. Now there are two fundamental ways that we're going to see reactions to the primary ways that we'll focus on. Imagine a bond between an atom and this typically will be carbon or nitrogen or oxygen, but an atom and hydrogen, for instance. If this is a polarized bond where the hydrogen is slightly positively charged and the A atom is slightly negatively charged, the fundamental type of reaction is going to be an attack of that hydrogen which is missing some of its electron density and as a result, the two electrons that were shared unequally in this bond go with the atom A. Again, the bond has already been polarized to do that and, in fact, the greater the polarization of this bond, the easier it's going to be pull off the hydrogen because the electrons have already almost been taken away by A anyway. So that's the first important thing to realize. The more polarized this bond is between a hydrogen and another atom, the easier it will be as long as this is more electronegative than the hydrogen. There are a few examples that we know it's the reverse. But as long as this is the more electronegative atom, it's going to be easier to remove that hydrogen, the more polarized that bond becomes. In other words, the more electronegative A is. So this is an example of an acid-base reaction. We've seen lots of examples of acids, but we haven't spent much time yet looking at why they're acids. And so, again, that's one of the key features we're going to be looking for is that polarized bond.
Now the flip side of that is a bond between let's say carbon, again, or it could be oxygen or nitrogen or some such atom and a very electronegative atom. We'll call it X. X typically is going to be a halogen, let's say. Something that pulls electron density away such that the bond is now polarized in this direction. Again, two electrons being shared. They're held primarily over towards X. The other major type of reaction we're going to be seeing is what I'll call a base right now. Organic chemists use the word nucleophile meaning positive loving or nucleus loving, but it's essentially the same idea as a base attacking this atom, forming a new bond here and then this X coming away with the pair of electrons. So, for instance, we might form a chloride or a bromide or an iodide as the result of this reaction and we make a new bond between our base or our nucleophile and the atom A.
So really we're talking about something very similar. We have a polarized bond. We either pull off the hydrogen, in this case, from that polarized bond leaving the electrons behind. The electrons are being held tightly by A. Or if the electrons are being held tighter towards another atom than A, then the reaction we're going to be interested in is an attack on A leaving that pair of electrons. So notice again, in this case A gets the electrons of that bond. In this case, A gives up the electrons of the bond.
And those are the two fundamental types of reactions that we're going to see. As amazing as it seems, this in its most basic form describes again, 90% of all reactions you're going to encounter. This basic idea of how a polarized bond reacts.
So what we're going to do first then is focus on the reactions with respect to the above pattern here. In other words using a base to pull off a hydrogen leaving the pair of electrons behind with the carbon or the nitrogen or the oxygen or whatever else A could be. And we're going to try to predict how acidic a molecule is based on what we know about its structure. So that's where we're going to go next.
Introduction to Organic Reactions
Acid Strength in Organic Molecules
Introduction to Reactivity Page [1 of 2]

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