2: Conjugation, Resonance, and Aromaticity
After Chapter 2, you should be able to
- identify conjugated and non-conjugated bonding in molecules
- use the information presented in this chapter, along with material from earlier chapters, to solve problems, particularly road-map problems and those requiring an understanding of spectroscopy.
- explain the concept of aromaticity and the stability of aromatic compounds.
- define, and use in context, the key terms introduced.
In Chapter 2, we describe conjugation within molecules and how it relates to electronic communication. Lewis' method for drawing organic molecules does not always accurately predict some chemical and/or physical phenomenon. Resonance theory is a way to represent molecules as a combination of multiple Lewis structures - the hybrid of which best describes the chemical and physical properties of the molecule. Molecules that are considered aromatic 1) are conjugated, 2) can be represented using a combination of Lewis structures, and 3) have chemical properties that make them uniquely stable.
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- 2.3: Resonance
- Resonance structures are a set of two or more Lewis Structures that collectively describe the electronic bonding a single polyatomic species including fractional bonds and fractional charges. Resonance structure are capable of describing delocalized electrons that cannot be expressed by a single Lewis formula with an integer number of covalent bonds.
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- 2.4: Rules for Resonance Forms
- The above resonance structures show that the electrons are delocalized within the molecule and through this process the molecule gains extra stability. Ozone with both of its opposite formal charges creates a neutral molecule and through resonance it is a stable molecule. The extra electron that created the negative charge on one terminal oxygen can be delocalized by resonance through the other terminal oxygen.
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- 2.5: Drawing Resonance Forms
- Resonance structures are used when one Lewis structure for a single molecule cannot fully describe the bonding that takes place between neighboring atoms relative to the empirical data for the actual bond lengths between those atoms. The net sum of valid resonance structures is defined as a resonance hybrid, which represents the overall delocalization of electrons within the molecule. A molecule that has several resonance structures is more stable than one with fewer.
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- 2.7: Aromaticity and the Hückel 4n + 2 Rule
- In 1931, German chemist and physicist Erich Hückel proposed a theory to help determine if a planar ring molecule would have aromatic properties. His rule states that if a cyclic, planar molecule has 4n+2 π electrons, it is considered aromatic. This rule would come to be known as Hückel's Rule.