Examine the structure of benzene

•

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 clicking on the identifier BENZEN02 from the Structure Navigator on the right hand side of the main Mercury window.

•	Inspect the structure. 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 Mercury

 

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 the right-hand mouse button pressed down.

 

3. To translate structures hold down the middle mouse button while moving the cursor in the display area (three-button mouse only). Alternatively, 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 Reset button at the bottom of the window.

 

•	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 distances in Mercury

 

1. Set Picking Mode in the tool bar (near the top of the main Mercury window) to the required parameter type, viz. Measure Distance, Measure Angle or Measure Torsion

 

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 click on the Clear Measurements button in the tool bar near the top of the main Mercury window.