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Chemistry: Strengths of Organic Bases


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  • Type: Video Tutorial
  • Length: 10:04
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  • Size: 108 MB
  • Posted: 07/14/2009

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: Base Strength in Organic Molecules (3 lessons, $4.95)

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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If our task is to look at a molecule and to predict or determine the ability of that molecule to act as a base, in other words a proton acceptor, then our first task is to consider the stability of the resulting conjugate acid of that base.
Let's look at an example. Let's suppose we're considering the base strength of an amine, like in this case trimethylamine or water. In this case, the fundamental difference between these guys. They both have lone pairs so we have a place to put the proton. Actually, our first task is to identify where the proton is going to go. In this case it's going to be pretty straight forward, but what we do need to evaluate is what's the difference between nitrogen's ability to accept a proton and oxygen's ability to accept a proton.
I remind you that when we compared a series of carbon, nitrogen, oxygen, and fluorine, going across the periodic table, we said that as electronegativity increases the ability of these guys to accept an additional proton decreases. Again, fluorine being not as good of a base as oxygen or nitrogen. All else being equal simply because fluorine is more electronegative, it's holding on to those lone pairs much more tightly than in the case of oxygen, or in the case of ammonia. So our prediction would have to be that the amine would be better at accepting a proton than water would be. Indeed the numbers bear that out.
Let me remind you of the relationship between K[a] and K[b, ]or the pK[a] and pK[b ]of conjugate acid base pairs. Very often you're going to find tables describing bases in terms of their K[a]s. That seems a little confusing, but what's being described there is not the K[a] of the base, but the K[a] of the conjugate acid from that base. Imbedded in that is the information about the base strength.
For instance, let me remind you K[a] x K[b] is 10^-14, if we're in water and if we're at room temperature. That means that the pK[a] + pK[b] must be equal to 14. Again, this only pertains to room temperature to 20 degrees Celsius, but that's normally where we're going to be operating.
If I look up the pK[a] of the conjugate acid to trimethylamine - trimethylammonium - so if I put an H there - that would have a pK[a] of 9.8. That's considerably higher, that number, than the pKa for the conjugate acid of water. In other words, H[3]0^+. That has a pK[a] of 0 because after all the equilibrium between H[3]0^+ forming H[3]0^+ is 1 because that's the same thing on either side of the equation. So the pK[a] for H[3]0^+ is 0 and that means then that if we look at what the pK[b]s are for these guys, the pK[b] of the amine is 4.2, whereas the pK[b] for water would be 14. What does that tell us? Just like what happens with pK[a]s, the larger the number is the weaker the base strength. The fact that this has a pK[b] of 14, and this has got a pK[b]of 4, tells us that the amine is roughly 10 orders of magnitude more basic than the water is. Remember again that these are log units now. We're talking about 10 pK[b] units difference or 10^10 difference between the base strength of amine and water. Once again to remind you, that the lower the value of pK[b] the stronger the base is. So our prediction certainly is going to be correct here.
You're familiar probably with other examples of amines acting as bases. In fact, there's certainly a big enough difference here that in your body all the amines at a pH of about 7.5 actually are in their protonated forms, not in their neutral forms like this. It's basic enough that in your blood at least, these things are all protonated. As we mentioned before, that's crucial in holding your proteins together and various biological functions occurring for you. It's a good thing that that's a lot more basic.
Just as an aside, we are so lucky that we've got the element of nitrogen because so much of the structure happening in our body is based on the ability of nitrogen to easily pick up another proton, but also easily lose it again. It just is this perfect balance that nitrogen has, unlike any other element, that allows in our body which is buffered at a certain pH to be able to have this property.
Let's look at a more difficult example. We're going to have to rely on one of the other factors that we identified with acids. Suppose you're given the task of identifying which of these three organic molecules is the best base, an amine characterized by this nitrogen with three bonds and a lone pair, an imine characterized by the carbon double bond nitrogen but still with a lone pair on it, and then a nitrile - a nitrile characterized by a C triple bond N. A C on this particular molecule is acetyl nitrile, a very common solvent used for organic reaction. These three guys are all potentially bases in that the nitrogen on all of them has a lone pair so we have a place to put the proton. What would our prediction be about which of these three things is indeed the most basic?
Our strategy is going to be - let's go ahead and consider the conjugate acids of these three things and ask ourselves is there something we can predict about the relative acid strengths. After all, we're now experts on looking at a molecule and predicting the strength of acids. What we'll do is protonate all of these things and make, in this case, the ammonium or the iminium, or the nitrilium conjugate acids. What I'm going to point out to you is the big difference between these three things is that the bond between the nitrogen and the proton that we just put on is made up of an sp^3 hybrid from the nitrogen rather than sp^2 or sp. We saw last time when we were talking about acids how the more s-character we have, the tighter the electrons are held to the atom, the lower the energy they are in an orbital. The tighter they're held, the easier it's going to be for the proton to come off. Of these three things, we have the most s-character in the nitrile because that's an sp^3 hybrid. Having more s-character than let's say the ammonium bond here, those electrons between the nitrogen and the hydrogen are pulled in closer to the nitrogen making it much easier for us to remove the hydrogen.
If we were given these three molecules and asked to predict their acid strength, we would have to conclude that the nitrile is much more acidic than the iminium which is much more acidic than the ammonium. Just like acetalines are more acidic than ethylenes, or alkenes are more acidic than alkanes. If you want to review that at this point, this is a good time to do that.
Our conclusion would have to be that since this is much more acidic than this, in fact, look at the difference here, protonated nitrile has a pK[a] of 10^-10. That's much stronger than sulfuric acid. This is an incredibly powerful acid, whereas ammonium is not a particularly strong acid. We know from that pK[a] that this is relatively speaking a very weak acid. So huge differences in acidity between the two ends here.
What does that mean about the base? Once again, think about that relationship. If this is the most acidic acid and this is the weakest acid, then it must be true that if I look at the conjugates of these molecules, this would have to be the weakest base and this would have to be the strongest base, once we remove the proton.
Coming back then to this, our conclusion would have to be that the imine of these three is the strongest base, followed by amine, and followed then by the nitrile, being the very weakest base. That is in fact correct for these molecules. There's a huge difference, about 20 orders of magnitude difference between the ability of an imine to act as a base, and in your body imines are protonated, and a nitrile to act as a base. You can't protonate nitriles even with sulphuric acid - at least without some difficulty. So, enormous differences even though all of these compounds contain nitrogen with the lone pair.
Now, let's do what we did with acids and leave the idea of molecules. You've got a set of tools available to evaluate acid strength and you're going to use those same set of tools to evaluate base strength. Just with the relationship that we've been talking about between acids and their conjugate bases. We don't need to do any more examples here of the other effects that would contribute to base strength.
Let's go ahead and do one other thing and that is to look at the effect of solvent and how the environment plays a role in determining base strength.
Introduction to Organic Reactions
Base Strength in Organic Molecules
Strengths of Organic Bases Page [1 of 2]

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