Examine the structure of benzene

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Benzene (a) is unusually stable for an alkene. Normal alkenes readily react with bromine to give dibromoalkane addition products [1]. However, benzene reacts only in the presence of a Lewis acid catalyst and the product is a monosubstituted benzene not an addition product [2]. Why does benzene not behave like other alkenes?

 

•	Display the structure of benzene by typing the reference code (refcode) BENZEN02 in the Entry box above the structure list on the left hand side of the WebCSD page.

 

•	Inspect the structure using the 3D viewer. You will notice that benzene is a planar symmetrical hexagon (internal bond angles close to 120 degrees) with six trigonal (sp2) carbon atoms, each with one hydrogen atom in the plane of the ring.

 

To manipulate structures in the 3D viewer:

 

1. Structures can be rotated by moving the cursor in the display area while keeping the left-hand mouse button pressed down.

 

2. To zoom in and out move the cursor up and down in the display area while keeping both the shift key and the left-hand mouse button pressed down.

 

3. To translate structures move the cursor in the display area while keeping both the left-hand mouse button and the keyboard Ctrl key pressed down.

 

4. At any stage the display area can be returned to the default view by hitting the r button on the keyboard.

 

•	Next, measure each of the carbon-carbon bond lengths in the structure. You will see that all bond lengths are around 1.38Å. How does this compare to typical carbon-carbon double and single bond lengths? Typical C=C double bonds are 1.33Å and C-C single bonds are 1.46Å.

 

To measure geometric parameters in the 3D viewer:

 

1. Select the type of measurment you wish to make by right-clicking within the 3D viewer and selecting Measure, followed by either Distances, Angles or Torsions

 

2. Geometrical measurements (intramolecular or intermolecular) can now be made by clicking on e.g., two atoms for a distance, three atoms for an angle or four atoms for a torsion angle.

 

3. To remove all geometrical measurements from the display right-click within the 3D viewer and select Measure, followed by either Clear distances, Clear angles or Clear torsions.