S/P electrophiles & crown ethers

The PowerPoint slides from today’s lecture can be downloaded here. A couple of comments on each slide …

Slide 1 – The potential maps show why S and P make more inviting targets for a weak nucleophile (like ROH) than C. However, bond energy is another important factor. S-Cl and P-Br bonds are much weaker than C-Br. This difference helps explain why nucleophilic attack on S and P is more efficient.

Slide 2 – I hope you remember this phrase: like charges repel. Take another look at the crown ether map and notice where “like charges” are located. The six O surrounding the cavity, right? These atoms actually repel each other. Consequently, 18-crown-6 does not adopt the shape shown on this slide until a positively charged ion wanders into the cavity and provides strong attractive forces to offset O-O repulsion.

Slide 3 – Three points to make here:

  1. The color-energy scale for neutral 18-crown-6 (-180 to +180) is completely different from the scale used for the cations (0 to +1500).
  2. Although all of the cations have the same charge, they do not create identical potentials. Potential and ion radius are inversely related. Li+ (small, +1600) vs. Na+ (medium, +1200) vs. K+ ( large, +900). This makes sense. Since Li is small, another ion can approach it more closely (larger potential). On the other hand, since K is large, another ion cannot approach as closely (smaller potential).
  3. When a crown ether forms a complex with K+, it does not change the size of K+. The potential near K+ is smaller in the complex (+460) because there are offsetting contributions from K+ (very positive) and the ether (very negative). Since the potential near K+ is much smaller in the complex, the ability of complex K+ to form ionic bonds is greatly compromised.
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