University:
Amherst College
Course:
CHEM 221 | Organic Chemistry I
Academic year:

2021
Views:

141

Pages:

4
Author:

kikarukawce

Name ______________________________________ Section 16.7 revisits ideas about geometry, hybridization, and orbitals that we just discussed in class. Two classic examples of aromatic heterocycles discussed in that section nicely illustrate when lone pair electrons are included in a conjugated cycle and when they're not. 1 (a) Draw pyridine, and show the p-orbitals and the lone pair orbital on N. As always, make sure it's clear how these are oriented in space. (b) The most important resonance structures of aromatic rings illustrate the delocalization of the 4n+2 electrons around the conjugated cycle. Draw the important resonance structures of pyridine. 2 (a) Draw pyrrole, and show the p-orbitals and the lone pair orbital on N. (Is the N pyramidal or planar?!) (b) Draw the important resonance structures of pyrrole. Hints: small steps.... lone pair in, bond pair out... keep track of charges... keep going until that lone pair gets back to where it started (this will take two "laps" around the ring). test for font substitutions... ∆ π sy: Δ π σ ν δ τ ψ φ Ψ Φ + Lecture outline Other aromatic and antiaromatic π-systems Remember that any fully conjugated cycle of 4n + 2 electrons gives a molecule special stability. These compounds have aromatic rings — – ... and these are antiaromatic (4n-e cycles) — But it’s important to appreciate that not all compounds fit into these two classes — many have no conjugated cycle, some are forced out of planarity, some have odd numbers of electrons, and some have structural features that prevent "free" delocalization of electrons with the conjugated cycle. Such molecules are neither especially stable nor unstable — they are non-aromatic — just plain ordinary molecules. Classify each of the following as aromatic, antiaromatic, or non-aromatic. N H N 2– It’s also important to appreciate that aromaticity and antiaromaticity are not "all or nothing" phenomena. Rings can have varying degrees of stabilization and destabilization arising from these effects. For example, cyclopentadienyl anion is more aromatic, i.e. has more resonance stabilization energy, than furan. Why? O Chemical and structural consequences of aromaticity and antiaromaticity This section could go on for hundreds of pages (or more), and we'll see many consequences of aromaticity throughout the course. – Structurally, aromaticity encourages planarity (to maximize conjugation in a 4n+2 π-e cycle), and the structure is an average of several resonance forms — in benzene, for example, the CC bond lengths are all equal and intermediate in length between single and double. In contrast, molecules will do anything they can to disrupt antiaromatic conjugation. Some are more successful than others. Cyclobutadiene can get rectangular to diminish the effect of the nasty resonance, but this doesn't help much, and it is still highly destabilized. Cyclooctatetraene, on the other hand, can avoid antiaromatic destabilization by being non-planar. This disrupts the conjugation between adjacent double bonds, and this molecule behaves just like a "normal" alkene. Because aromatic rings are exceptionally stable, they form easily, and react with great difficulty. Aromatic compounds are almost always greatly favored over non-aromatic products, when both are possible. Examples will be provided in class as time permits...

Lecture Outline Other Aromatic and Antiaromatic π-systems

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