Resonance

Concept of Resonance There are certain compounds, which can be represented by more than one Lewis structures, but none of the structure is able to represent the molecule in agreement with its experimentally determined parameters. The phenomenon of resonance is introduced because of the inability of single Lewis structures to show electron delocalization over the molecule. So Heisenberg introduced the phenomenon of resonance to explain the properties of certain molecules. Take the example of ozone molecule to explain resonance.

In this case, each oxygen atom has an octet of electrons Acc. To structure (1) there is one single bond (O-O) and one double bond. The bond lengths for (O-O) and (O=O) are 148pm and 121pm respectively. So according to the above structures, we expect the two bond lengths in the ozone molecule will be different, but it is found that both the bond lengths are equal i.e. 128pm. This bond length is intermediate between single and double bonds. It means

that above Lewis structures do not account for the observed experimental facts. It is proposed that the actual structure lies in between the above two structures and is called resonance hybrid as shown below.

Resonance hybrid structure of ozone with some electrons delocalized between the atoms. Resonance structures have no real existence. These are the only theoretical based. Also, it does not actual molecule exists as a one form for a certain fraction of time or in another form. The resonating structures are the only an easiest way of picturing a molecule to account for its properties. Thus Resonance can be defined as: when a molecule can’t be represented by a single structure but its characteristic properties can be described by two or more than two structures, then the actual structure is said to be a resonance hybrid of these structures. Conditions/ Rules for writing the resonating structure: 1) Contributing structure should have same atomic positions. They should differ only in the electronic arrangement.

2) These structures should have same number of paired and unpaired electrons. 3) The energy level of contributing structures should be the same. 4) The structure should be written in such a way that the negative charge is present on the electronegative atom and positive charge is present on the electropositive atom

Out of these three structures, 3 more electronegative atom.

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one is ruled out because +ve charge is on

5) The like charges should not reside on the adjacent atoms in contributing structures.

Characteristics of Resonance: 1) Resonating structures are hypothetical and not the real one. 2) The actual molecule has the single structure, which is a resonance hybrid of various cannonical forms and therefore cannot be presented by single Lewis structure. 3) The bond lengths in hybrid structure are intermediate of the bond lengths in various cannonical forms e.g. C – C bind length in benzene molecule is 139 pm which is intermediate of C – C (154 pm) and C=C (134pm) 4) Resonance hybrid has lower energy and thus greater stability than any of its resonating form. 5) The difference of energy between actual bond energy of the molecule and the most stable resonating form is called resonance energy. Larger the value of resonance energy more will be the stability of resonance hybrid.

For example, consider a molecule having three different canonical structures A, B & C. If C has lower energy and it is more stable. Then the resonance energy is equal to the difference between the actual bond energy and the energy of most stable resonating structure. i.e.E3―E 6) Resonating structure are used to calculate the bond order of a particular bond in a molecule.

For example, take the resonating forms of CO32- ion. For calculating C-O bond order, consider bonds of C atom with particular O atom in all the resulting structures.

There are 2 in structure (a),1 in structure (b)and 1 in structure (c) So the bond order is 2+1+1/3 =1.33 Facts about Resonance: While studying the concept of resonance we should be very careful about the following misconceptions • • • •

The sign (↔) present among the resonating structures does not represent any kind of equilibrium. The real molecule cannot be depicted by single Lewis structure. The resonating structures (cannonical forms) have no real existence. The actual molecule can not be assumed to exist in one cannonical form at one time and in other cannonical forms at other times.

Resonance Structures and Hybrid: We can also draw resonance structures from some Lewis structures. For that the Lewis structure should have multiple bonds and an adjacent atom with at least one lone pair of electrons. The general method to draw resonance structures is shown below. The arrows explain shifting of the electrons from one resonance structure to another.

Important steps to write resonance structures: • • • •

Transfer one of the lone pairs of electrons on an adjacent atom down - to form another bond. Transfer one of the bonds from either a double or a triple bond up - to form a lone pair. Repeat the same process (above steps) further with the adjacent atoms to draw additional resonance structures. Put double-headed arrows to separate the resonance structures.

Resonating Structures of some Molecules: a) Resonance structures of carbonate ions:

b) Resonating structure of bisulfate ion (conjugate base of sulfuric acid)

c) Resonating structure of Carbon monoxide

d) Resonating structure of phosphate ion

Explanation of some structures: 1) Sulfur dioxide Let’s consider the sulfur dioxide, SO2, molecule. Structural studies explain that both the bonds in SO2 are of equal length and strength. This electronic distribution is unexplained by either single Lewis structure, but is clearly illustrated when both Lewis formulae are taken into consideration.

In resonance hybrid structure, pi electron pair is delocalized or spread over both S-O bonds and not localized in a single S-O bond. 2) Nitrate ion Lewis structure for the nitrate ion, NO3-, is drawn below.

Two different types of bonds are shown by Lewis structure in nitrate ion - single and double. A double bond is stronger than a single bond as it requires more energy to break a double bond than a single bond.

Bond length of a double bond is always shorter than single bond because the distance between the nuclei of two atoms in a bond is less than the single bond. But, experimentally it is found that all the 3 bonds in nitrate ion are of same strength and same length.

It has been observed that these bonds are longer than double bond but shorter than single bond. These are also stronger than single bonds but not as strong as double bonds. This can be explained for the nitrate ion and polyatomic ions like it with the help of valence-bond model.

Resonating Structure of Nitrate ion:

These resonating structures help the chemists to develop better descriptions of the nitrate ion, which cannot be explained by a Lewis structures. The resonance hybrid of nitrate ion (polyatomic):

The actual geometry of the nitrate ion is trigonal planar with bond angles of 120º

3) Formate ion Resonance structures for the formate ion (HCO2-) are

Second resonance structure can be made from the first by shifting one lone pair of electron down to form another bond and transferring an adjacent bond up to form a lone pair. The arrows show the hypothetical shift of electrons. These resonance structures lead to the resonance hybrid as drawn below.

Exceptions: Sometimes general method of drawing resonance structures fails to draw a reasonable resonance structure. For example: Fluorine atoms do not participate in resonance. For a fluorine atom to form two bonds and two lone pairs, it has to lose an electron which is not possible because of its high electronegativity. Thus any resonance structure is not considered a reasonable resonance structure that has a double bond with fluorine atom.

a) But in case of fluoroethene (CH2CHF), it has a double bond and an adjacent atom with a lone pair of electrons (the factors required for resonance structure). Out of two hypothetical resonance structures only one is reasonable:

So fluoroethene does not show resonance and only the first structure is best described for CH2CHF molecule.

b) Taking the case of formic acid, HCO2H, which has a double bond and an adjacent atom with a lone pair of electrons? By appearance it gives an impression of forming a resonance structures. Similar is the situation with oxygen atoms present in formic acid. Although it is possible for oxygen atoms to have three bonds and one lone pair yet it’s not possible for the oxygen atom which is the second most electronegative element to lose the electron to form the resonance structure of formic acid. Thus, we will do away with resonance structures that have three bonds and a lone pair for an oxygen atom.

The first Lewis structure is reasonable while the second one having three bonds and a lone pair on an oxygen atom is not a reasonable resonance structure. So formic acid does not show resonance and only the first structure is best described for formic acid.

Resonance and the Benzene Molecule: Resonance structures can also be drawn without the participation of lone pairs. The examples for this type of resonance structures are benzene, C6H6, and compounds that contain the benzene ring. All the six carbon atoms of benzene are linked to each other in a six-membered ring. Lewis structure of benzene is often represented with three double bonds as shown below. Its simplest and easiest representation is also shown below.

(1)

(2)

(1) With symbols of the elements (2) Simplest one It appears from the Lewis structures that benzene molecule it contains two types of C-C bonds, double and single. But actually all of C-C bonds of benzene are same, and can be explained why, in terms of resonance. It is as if the benzene ring were resonating between the two structures below.

The resonance hybrid of benzene is shown below.

Implications of resonance are (1) the actual electron distribution is different than would be expected based on a single Lewis structure, and (2) the energy of the actual molecule is lower than expected from a single Lewis structure. This energy lowering is due to electron delocalization and is called resonance stabilization. The degree of energy lowering or molecular stabilization is related to the number of resonance structures and the relative stabilities of the resonance structures.

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