Structure and Bonding:  Reaction Rates and Molecular Crowding

Let's examine a reaction where the fluoride ion, F-, attacks a molecule of chloromethane.  For any reaction to occur, what has to happen between the chloromethane and fluoride ion?

CH3Cl    +       F-                 CH3F         +       Cl-

  

How would you describe the attack?

When the reaction is finished, how have the three hydrogen atoms changed their position on the molecule?

The image below shows the geometry of the transition state of the reaction given above.  How would you describe the transition state?

The incoming fluoride ion must attack from the backside of the chloromethane molecule.  The collision of the two must be in this preferred direction to be an effective collision or cause reaction.  This causes an inversion of the three hydrogen atoms.  The molecule behaves like an umbrella that turns inside out in the wind!  At the transition state, the geometry is trigonal bipyramidal, with the C-F bond forming and the C-Cl breaking.  Notice the hydrogen in the equatorial positions are planar at this point.  This is a very high energy unstable species. 

If the incoming fluoride ion attacked from the front side of the chloromethane molecule, would inversion of the hydrogen occur?

The relative rates of the reaction for the following molecules are given below.  Why does the rate of reaction change as the methyl groups are substituted on the original CH3Cl?

30 1 0.002  0.001

This reaction is a classical example of an SN2 reaction in organic chemistry.  The reaction kinetics are second order as the two species must collide.  So the rate law is given by the equation:    

Rate = k (CH3Cl)(F-)

For the series of compounds given above, the backside attack is hindered by increased substitution, hence the rate decreases.  When the reaction does occur the molecule goes through an inversion in configuration.  This inversion would not occur if the attack was from the front side.  The importance of the inversion will be discussed further in organic chemistry.

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