Many of you are probably starting your second semester of organic chemistry. You may also be realizing that there is a reason it is called part two and that your instructor was right, you can’t forget everything you learned last semester. Before you panic about how much material there is to review, Alchemie is here to help you!
Research on learning has shown that in order for new concepts to stick, students must be able to connect the new information to an existing framework. Our goal with this series of blog posts is to help you identify and review key pieces of the framework you learned first semester that are essential to understanding second semester material.
For this post, we will be reviewing electronegativity and resonance. At the end of the post you will find a link with some guided practice problems as well. See the next post for a review on equilibrium and acid-base chemistry.
And of course always remember electrons flow from electron-rich to electron-poor!
Electronegativity - the ability of an atomic nucleus to attract electrons in a covalent bond (i.e. sigma and pi bonds). Stated differently, the more electronegative atom will have a higher electron density. You could memorize the electronegativity values for H, C, N, O, F, Cl, Br but what we will really be concerned about is relative electronegativities. To simplify things, you can just memorize that 1) fluorine is the most electronegative atom and as you go down the periodic table or to the left electronegativity decreases and 2) hydrogen is less electronegative than carbon. Electronegativity is useful for identifying areas of high and low electron density within a single molecule.
For example, let’s take a look at Acid Base 2. Here we have hydronium (H3O+) and hydroxide (OH-). The oxygen atom in both molecules has a formal charge, +1 for hydronium and -1 for hydroxide. So from our saying “electrons flow from electron rich to electron poor” you might think that electrons will flow from the oxygen of hydroxide to the oxygen of hydronium.
If you try that move in the app though it does not work. But why? Let’s apply the concept of electronegativity. Oxygen is more electronegative than hydrogen so electron density of the oxygen hydrogen sigma bond will be higher by oxygen. Sometimes we use a dipole to show this. Therefore, actually it is hydrogen atoms of hydronium that are the most electron poor. Indeed, in the correct move the electrons flow from hydroxide to a hydrogen atom of hydronium.
For this post, we are going to review what useful information resonance structures can reveal about the reactivity of the molecule.
Tip: If you are looking for a refresher on how to draw resonance structures watch the videos associated with Structure 2 - 5 and play through the puzzles within the Structure pack.
Part 1 - The resonance hybrid and finding partial charges
Remember, the structure of a molecule is a hybrid of all the possible resonance structures.
Let’s use a non-chemistry example to make sense of this idea. Hybrid cars run using both a traditional gasoline engine and an electric motor. It would not be accurate to just call it an electric car, because it has two different power systems. The two systems together means the car can run longer on a tank of gas and also gets those really nice spots in the parking garages!
Returning to chemistry, molecules that have multiple resonance structures cannot be fully described by just one Lewis Structure. Each Lewis Structure tells us something different about the molecule.
Take a look at Aldehyde and Ketone 1. The puzzle starts with the commonly shown resonance structure of 2-butanone.
Perhaps a more informative resonance structure though is that in which the carbon-oxygen pi bond has been moved toward the more electronegative oxygen atom.
In this structure carbon is assigned a formal positive charge and oxygen a formal negative charge but the overall charge of the molecule is still neutral. Stated differently, a mechanistically helpful resonance structure shows that the carbon is electrophilic and the oxygen is nucleophilic. These are not full charges though since this structure is just a contributor to the real structure. Instead we call them partial charges.
So why not just draw the, arguably, more informative second resonance structure? Typically, when writing out reactions, organic chemists will use the lowest energy resonance structure. Remember not all resonance structures contribute equally to they hybrid. The level of contribution relates to the relative energy of each structure. Also, the hybrid will most closely resemble the major contributor. To make a more complete image though, sometimes, as in the case of 2-butanone, we will add in delta 𝛿 + and 𝛿 - to indicate the location of partial charges.
However, when writing out a mechanism it is perfectly fine to use a less contributing resonance structure. In fact it is encouraged. As Robert B. Grossman says in The Art of Writing Reasonable Organic Reaction Mechanisms, “when in doubt, draw in all the lone pairs, and draw resonance structures until the cows come home.” (1)
Finally, it is VERY important to point out that the molecule does not interconvert between the two resonance structures. Just like with a hybrid car the owner does not switch out the engine with a motor and vice versa, the molecule is not in equilibrium with its resonance structures. The electrons in the structure are spread out, or delocalized, throughout the molecule. Yes, they are in multiple places at the same time.
Part 2 - Resonance stabilization
Molecules with a formal charge are typically higher in energy than their neutral counterparts. If the charge can be delocalized across the molecule, then the energy difference is lower. In other words, the ability to delocalize a charge stabilizes a molecule.
The implications of this statement are most salient using Acid Base 7 as an example.
A typical alcohol has a pka of 16, but phenol is more acidic with a pka of 10. The reason for this increased acidity is that the conjugate base, phenoxide, is stabilized by resonance delocalization. The negative charge can be distributed through the ring to carbons at positions 2, 4, and 6.
See the guided examples for more examples like Acid Base 7!
Hopefully this quick review has helped refresh your memory and you feel more ready to learn more complex mechanisms.
- Grossman, R. B. The Art of Writing Reasonable Organic Reaction Mechanisms, Second Edition; Springer: New York, 2003; pp 30.