Benzene delocalized electrons

It may also be represented as a hexagon with a circle in the middle. The circle is a symbol of the n cloud encircling the benzene ring. The delocalized electrons associated with the benzene ring impart very special properties to aromatic hydrocarbons. They have chemical properties of single-bond compounds such as paraffin hydrocarbons and doublebond compounds such as olefins, as well as many properties of their own. 

Aromatic hydrocarbons that contain ring systems with delocalized electrons, for example, benzene. 

Each carbon-carbon double bond is constructed from four electrons. In benzene, the electrons that create the apparent double bonds fall into two classes. Two of the electrons are localized between two carbon atoms, just as we have come to expect. The other two electrons that contribute to the apparent double bonds are, in contrast, delocalized over the entire molecule. Since there are three apparent double bonds, we have a total of six electrons that are delocalized over the six carbon atoms. Think of these as free-range electrons. Basically, each of the carbon-carbon bonds in benzene is a 1.5 bond (technically, we say that the bond order in benzene carbon-carbon bonds is 1.5). Hence, the two models for benzene employed above, though universally used in chemistry, leave something to be desired. Benzene is better thought of as a hybrid of the two. Chemists have struggled with ways to depict the reality of benzene better than the stractures A and B. The struggle has not been notably successful. 

Figure 2-2. Spatial representation (ball-and-stick model) of benzene, with C-atoms in grey and H-atoms in white. The dotted lines between the C-atoms represent the delocalized electrons. The image on the right shows the surface area of the highest occupied molecular orbital (HOMO). Note how the 71-electrons are above and below the benzene ring.

The actual geometry of a conjugated system is determined by a fine balance of the distortive r-component and the symmetrizing force of the o-frame.12 4 This o— -balance and opposition is summarized schematically in Figure 4. In allylic species and in benzene, the cr-frames win and the species remain symmetric, with equal C—C distances and with delocalized -electrons. In contrast, in cyclobutadiene the a-resistance is insufficient and the structure relaxes to a rectangular geometry. 

These structures are called canonical forms, and the actual resulting blended structure is called a resonance hybrid. They are usually represented as an aromatic sextet of delocalized electrons, represented by a circle within the benzene ring ... 

Aromatic hydrocarbons are cyclic, planar compounds containing delocalized electrons. The delocalization of the electrons in the pi orbitals makes these very stable structures. The most common aromatic hydrocarbon is C6H6, benzene. The structure of benzene is shown in Figure 19.13 ... 

The applied field sets up a ring current in these delocalized electrons that produces a local field rather like the field produced by the electrons around a nucleus. Inside the benzene ring, the induced field opposes the applied field but, outside the ring, it reinforces the applied field. The carbon atoms are in the ring itself and experience neither effect, but the hydrogens are outside the ring, feel a stronger applied field, and appear less shielded. 

Benzene has six n electrons delocalized in six p orbitals that overlap above and below the plane of the ring. These loosely held n electrons make the benzene ring electron rich, and so it reacts with electrophiles. 

On the other hand, it is believed by some that Kekule had intuitively anticipated by some 75 years our present concept of delocalized electrons, and drew two pictures (VIlI and IX)—as we shall do, too- as a crude representation of something that neither picture alone satisfactorily represents. Rightly or wrongly, the term Kekul6 structure has come to mean a (hypothetical) molecule with alternating single and double bonds—just as the term D war benzene has come to mean a structure (II) that James Dewar devised in 1867 as an example of what benzene was not. 

Electron orbitals in benzene overlap to form continuous orbitals that allow the delocalized electrons to spread uniformly over the entire ring. 

---