1.3 The Resonance Explanation of the Structure of Benzene

  Aromatic Compounds

    1.0 The Structure of Benzene
    2.0 Electrophilic Aromatic Substitution Reactions
    3.0 Nitration
    4.0 Sulphonation
    5.0 Halogenation
    6.0 Friedel-Crafts Alkylation
    7.0 Friedel-Crafts Acylation
    8.0 Orientation and Reactivity in Electrophilic Aromatic Substitution
    9.0 Ortho / Para Ratio
    10.0 Reactions of Alkyl Benzenes

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1.3 The Resonance Explanation of the Structure of Benzene

A basic postulate of resonance theory is that whenever two or more Lewis structures can be written for a molecule that differ only in the positions of their electrons, none of the structures will be in complete accord with the compound’s chemical and physical properties. If we recognize this, we can now understand the true nature of the two Kekulé structures (I and II) for benzene. The two Kekulé structures differ only in the positions of their electrons. Structure I and II, then, do not represent two separate molecules in equilibrium as Kekulé had proposed. Instead, they are the closest we can get to a structure for benzene within the limitations of its molecular formula, the classic rules of valence, and the fact that the six hydrogen atoms are chemically equivalent. The problem with the Kekulé structures is that they are Lewis structures, and Lewis structures portray electrons in localized distributions. (With benzene, as we shall see, the electrons are delocalized). Resonance theory, fortunately, does not stop with telling us when to expect this kind of trouble; it also gives us a way out. Resonance theory tells us to use structures I and II as resonance contributors to a picture of the real molecules of benzene. As such, I and II should be connected with a double headed arrow and not with two separate ones (because we must reserve the symbol of two separate arrows for chemical equilibria). Resonance contributors, we emphasize again, are not in equilibrium. They are not structures of real molecules. They are the closest we can get if we find found by simple rules of valence, but they are very useful in helping us to visualize the actual molecule as a hybrid.

Look at the structures carefully. All of the single bonds in structure I are double bonds in structure II. If we blend I and II, that is, if we fashion a hybrid of them, then the carbon-carbon bonds in benzene are neither single bonds nor double bonds. Rather, they have a bond order between that of a single bond and that of a double bond. Bond order of benzene ring is found to be 1.5. This is exactly what we find experimentally. Spectroscopic measurements show that the molecule of benzene is planar and that of all of its carbon-carbon bonds are of equal length. Moreover, the carbon-carbon bond lengths in benzene figure are 1.39Å, a value in between that for a carbon-carbon single bond between sp2 hybridized atoms (1.47Å) and that for a carbon-carbon double bonds (1.33Å)