Lewis structure of a resonance hybrid

Resonance form (Section 2.4) An individual Lewis structure of a resonance hybrid. 

The second convention in Section 1-5 is the use of curved arrows to show the movetnem of electron pairs. In (his section the only application is in showing how the electron pairs shift in going from one Lewis structure of a resonance hybrid to another. Pictorial descriptions of electron movement using these arrows will be very u.seful tools to help you learn and understand organic chemistry. 

EXAMPLE 10-9 Representing the Lewis Structure of a Resonance Hybrid... 

Does your transition state drawing look more like a sing Lewis structure or a resonance hybrid If the latter, whi resonance contributors must you combine to generate a of the features of this hybrid ... 

Contribution of individual Lewis structures to a resonance hybrid... 

We can write the structure of formic acid as a Lewis structure as a resonance hybrid of three contributing resonance structures as shown below. 

In completing the octet of the C atom on the right, we discover that we can write two completely equivalent Lewis structures, depending on which of the two O atoms furnishes the lone pair of electrons to form a carbon-to-oxygen double bond. The true Lewis structure is a resonance hybrid of the following two contributing structures. 

Because all three bonds are identical, a better model of the nitrate ion is a blend of all three Lewis structures with each bond intermediate in properties between a single and a double bond. This blending of structures, which is called resonance, is depicted in (9) by double-headed arrows. The blended structure is a resonance hybrid of the contributing Lewis structures. A molecule does not flicker between different structures a resonance hybrid is a blend of structures, just as a mule is a blend of a horse and a donkey, not a creature that flickers between the two. 

STRATEGY Write a Lewis structure for the molecule by using the method outlined in Toolbox 2.1. Decide whether there is another equivalent structure that results from the interchange of a single bond and a double or triple bond. Write the actual structure as a resonance hybrid of these Lewis structures. 

Sometimes more than one satisfactory Lewis structure can be written and there is no reason to select one over another. In such cases a single structural formula is inadequate for a correct representation, and we say that the true structure is a resonance hybrid of the several structures. Common examples of species requiring resonance structures are ozone, 03, carbonate ion, CO " and benzene, C6H6. These... 

Let s consider the formate anion, shown in Figure 3.15. The first Lewis structure (a) does not accurately represent the structure of this covalent ion, because a second Lewis structure can be drawn (b) that is equivalent to the first. The actual structure is a resonance hybrid of these two structures. Experiments confirm that the two CO bonds are identical, with a bond length between that of a single and a double bond, and that the charge on each oxygen is the same, approximately —y. 

Formate anion is an example of an ion for which two equivalent Lewis structures (0 and ) can be drawn.The actual structure is a resonance hybrid of these two structures. Remember to use the double-headed arrow only between resonance structures. Never use equilibrium arrows ( .) between resonance structures. 

Often, one Lewis structure is not sufficient to describe the electron distribution in a molecule. In many cases, the use of a resonance hybrid is necessary. All resonance forms are valid Lewis structures. In resonance forms, only the electrons move and not the atomic nuclei. Each resonance form does not have a separate existence but is a part of a hybrid whole. The use of a double-headed arrow, between the forms reinforces the notion of a hybrid representation of a single structure. The forms are not in equilibrium with each other (equilibrium is shown by two opposing arrows ). 

In a multiple covalent bond, all electrons in the bond are confined to the region between the bonded atoms, and together constitute one group of electrons. Let us test this idea by predicting the molecular geometry of sulfur dioxide. S is the central atom, and the total number of valence electrons is 3 X 6 = 18. The Lewis structure is the resonance hybrid of the three contributing structures shown below. 

Resonance—Often, more than one plausible Lewis structure can be written for a species this situation is called resonance. In these cases the true structure is a resonance hybrid of two or more contributing structures. 

By using delocalized bonding schemes, we can avoid writing two or more contributing structures to a resonance hybrid, as is so often required in the Lewis theory. Consider the ozone molecule, O3, that we used to introduce the concept of resonance in Section 10-5. In place of the resonance hybrid based on these contributing structures. 

Benzene, C6H(l, is another molecule best described as a resonance hybrid. It consists of a planar hexagonal ring of six carbon atoms, each one having a hydrogen atom attached to it. One Lewis structure that contributes to the resonance hybrid is shown in (11) it is called a Kekulc structure. The structure is normally written as a line structure (see Section C), a simple hexagon with alternating single and double lines (12). 

Different Lewis structures do not in general make the same contribution to a resonance structure. It is possible to decide which structures are likely to make the major contribution by comparing the number of valence electrons distributed around each atom in a structure with the number of valence electrons on each of the free atoms. The smaller these differences for a structure, the greater is its contribution to a resonance hybrid. 

Select from each of the following pairs of Lewis structures the one that is likely to make the dominant contribution to a resonance hybrid. Explain your selection. 

Long before their theories were supported by computations, organic chemists found a way to use resonance structures to explain the product distribution in electrophilic substitution. Thus, the Lewis structure for phenol is regarded as a resonance hybrid of the following structures ... 

Nitrous oxide is a good example of a molecule that shows resonance. The structure on the left in Figure 7.6 has two double bonds, with two lone pairs on each of the distal atoms. The structure on the right has a triple bond. Neither structure fully describes nitrous oxide, nor does either structure actually exist. The real nitrous oxide molecule is a resonance hybrid of the two Lewis structures. 

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