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Introduction of Resonance

Structural formulas of Kekulé are essential tools for understanding organic chemistry. Even though, by a single formula, the structures of some ions and compounds cannot be represented. For instance, nitric acid (HNO3) and sulfur dioxide (SO2) may each be defined by two equivalent formulas (equations 1 & 2). For better understanding two ambiguous bonds to oxygen are given different colors in the formulas that are given below.

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Double bond to oxygen would be shorter and stronger than the single bond If only one formula for sulfur dioxide was accurate and correct. Because the experimental evidence point outs that this molecule is bent (bond angle 120º) and has equivalent length sulfur: oxygen bonds (1.432 Å) single formula is insufficient and the actual structure may be similar to an average of the two formulas. Resonance is the averaging of the electron distribution over two or more hypothetical contributing structures (canonical forms) to produce a hybrid electronic structure. Likewise, the structure of nitric acid is best defined as aresonance hybrid of two structures, for resonance double headed arrow being the unique symbol.

The examples which have been shown above represent one extreme in the application of resonance. Here, the two energetically and structurally equal electronic structures for a stable compound can be written but no single structure presents an accurate or even an adequate representation of the true molecule. In the examples like these, the electron delocalization demonstrated by resonance improves the stability of the molecules and compounds or ions composed of this type of molecules often show exceptional stability.

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The electronic structures of most covalent compounds do not suffer the inadequacy as noted above. So, for complete satisfactory the formulas of Kekulé can be drawn for water (H2O), methane (CH4) and acetylene C2H2). Nevertheless the principles of the resonance are important in rationalizing the chemical behaviour of many this type of compounds. For a case that carbonyl group of formaldehyde (the carbon-oxygen double bond) reacts readily to give addition products. By a small contribution of a dipolar resonance contributor Course of these reactions can be explained, as displayed in equation no.3. There in the diagram, first contributor which is on the left is specifically the best representation of this molecular unit, there is no charge separation and by covalent electron sharing both the carbon and oxygen atoms have achieved valence shell neon-like configurations. If double bond is broken heterolytically, the resultant formal charge pairs shown in other two structures. Preferred charge distribution will have the negative charge on the more electronegative atom (oxygen) and the positive charge on the less electronegative atom (carbon). So the middle formula stands for a more stable and reasonable structure than the one on the right. Resonance's Application to this case necessitates a weighted averaging of these canonical structures. Double bonded structure is considered as the main contributor and the middle structure is regarded as a minor contributor and the right hand structure a non-contributor. Because middle, charge-separated contributor has an electron deficient carbon atom; this describes the tendency of the electron donors (nucleophiles) to bond at this site.

The resonance's fundamental principles may now be summarized.
For a specified compound, a set the structures of Lewis / Kekulé are written, keeping the relative positions of all component atoms similar. These are canonical forms to be considered, and all must have similar number of unpaired and paired electrons.
Following factors are important in evaluating the contribution each of canonical structures which described before makes to the actual molecule.

  1. In a structure the number of covalent bonds. (Greater the bonding, the more important and stable the contributing structure).
  2. Formal charge separation. (Other factors aside, charge separation decreases stability and importance of the contributing structure.)
  3. Charge bearing atoms and charge density's Electronegativity. (The High charge density is destabilizing. negative charge on the high electronegative atoms, and unlike the negative charge Positive charge is the best contained on atoms of low electronegativity.)

The stability of any canonical contributor is always smaller than the stability of a resonance hybrid. So, if one canonical form has a greater stability than all others, hybrid will closely resemble it energetically and electronically. This is the example for carbonyl group. At the left hand C=O structure has much greater total bonding than either charge-separated structure, so it describes this functional group rather well. Alternatively, if two or more canonical forms have the same low energy structures, the resonance hybrid will have the unique properties and exceptional stabilization. This is the case for sulfur dioxide that is defined in eq.1 and nitric acid that is defined in eq.2.

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We shall consider carbon monoxide (that is eq.4) and azide anion (that is eq.5) to demonstrate these principles. The most stable canonical form is on the left, in every case. For carbon monoxide, additional bonding is most important than charge separation. Also, double bonded structure has an electron deficient carbon atom (valence shell sextet). A similar destabilizing factor is described in the two azide canonical forms on the top row of the bracket (the three bonds vs. four bonds in the left most structure). Bottom row pair of structures has four bonds but are destabilized by the high charge density on single nitrogen atom.

All the cases demonstrate a significant restriction that must be remembered when using resonance. Within the common structural framework there is No atoms change their positions, only the electrons are moved.

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