Chemistry · Ch 9 — Organic Chemistry – Some Basic Principles and Techniques
Resonance Structure
Resonance Structure
The Problem with a Single Lewis Structure
Many organic molecules cannot be adequately described by a single Lewis structure. The classic example is benzene. Its cyclic structure, drawn with alternating single and double bonds, fails to explain its observed properties.
If benzene truly had alternating C–C single bonds and C=C double bonds, it should exhibit two distinct bond lengths. Experimentally, however, all six carbon–carbon bonds in benzene are identical, with a bond length of 139 pm. This value lies neatly between the length of a typical C–C single bond (154 pm) and a C=C double bond (134 pm). A single, static Lewis structure cannot account for this uniformity.
Benzene can be represented equally well by two energetically identical structures, I and II. Neither structure alone is correct.
The two structures of benzene differ only in the placement of the π electrons; the positions of the nuclei are the same in both.
The Concept of Resonance
According to resonance theory, the actual structure of benzene is not adequately represented by either structure I or structure II. Instead, the real molecule is a resonance hybrid of these two hypothetical structures. These individual hypothetical structures are called resonance structures, canonical structures, or contributing structures.
Resonance structures are purely hypothetical. No single resonance structure represents the real molecule. They contribute to the actual structure in proportion to their stability. The more stable a contributing structure, the greater its contribution to the hybrid.
Another Example: Nitromethane
Nitromethane () provides another clear example. It can be represented by two Lewis structures, I and II. In these structures, the two N–O bonds appear different: one is a single bond and the other is a double bond.
However, experimental evidence shows that the two N–O bonds in nitromethane are of the same length. This bond length is intermediate between a typical N–O single bond and a N=O double bond. Therefore, the actual structure of nitromethane is a resonance hybrid of its two canonical forms, I and II.
Resonance Energy
The energy of the actual molecule (the resonance hybrid) is always lower than the energy of any of its individual canonical structures. This energy difference is a real, measurable quantity.
The more important contributing structures a molecule has, the greater its resonance energy. Resonance is particularly significant when the contributing structures are equivalent in energy, as in the case of benzene.
Rules for Writing Resonance Structures
When writing resonance structures, the following rules must be applied:
- Same positions of nuclei. The atoms themselves do not move. Only the placement of electrons (specifically π electrons and lone pairs) changes between resonance structures.
- Same number of unpaired electrons. The total number of unpaired electrons must be the same in all resonance structures. This is a consequence of the fact that the total number of electrons does not change.
Stability of Resonance Structures
Not all resonance structures contribute equally to the hybrid. The following factors determine the relative stability of a given resonance structure. A more stable structure makes a greater contribution.
The actual resonance hybrid is more stable than any of its contributing structures. The stability of the hybrid is determined by the most stable contributing structures.
A resonance structure is more stable if it has:
- More covalent bonds. A structure with more bonds is generally more stable.
- All atoms with a complete octet of electrons. (Hydrogen is an exception, requiring only a duplet.) Structures where every atom has a full octet are highly stable.
- Less separation of opposite charges. Structures with minimal charge separation are more stable than those with significant charge separation. …