19. The Simple Compounds
                                   of Carbon
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      Reactions of Hydrocarbons

The answer is that a carbon atom is well-shielded from any attacking group by the four atoms tetrahedrally placed around it. There are no "open sides" exposed to attack (see movie, right).

In addition, carbon and hydrogen are of approximately equal electronegativity. The C-H bond is nonpolar, so there is no excess or deficiency of charge on either type of atom.

Most organic reactions take place by electrophilic or nucleophilic attack (see boxes on next page). A slight positive charge on an atom encourages attack by a nucleophile (e.g., OH-, Cl-, NH3) and a slight negative charge invites electrophilic attack (e.g. by H+).

In hydrocarbons, carbon has neither charge, hence it is little affected by these attacking groups.

This property, plus the tetrahedral shielding around the carbon atom, means thatalthough saturated hydrocarbons may be thermodynamically able to react, they have no convenient mechanism for doing so, and will react only slowly.



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