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The Arrhenius Law: Direction Matters

Complicated molecular structures increase the likelihood that the rate constant depends on the trajectories with which the reactants approach each other. This kind of electrophilic addition reaction is well-known to all students of organic chemistry.

Consider the addition of a hydrogen halide such as HCl to the double bond of an alkene, converting it to a chloroalkane:

Experiments have shown that the reaction only takes place when the HCl molecule approaches the alkene with its hydrogen-end, and in a direction that is approximately perpendicular to the double bond, as shown inbelow:

       

 

The reason for this is apparent: HCl is highly polar owing to the high electronegativity of chlorine, so that the hydrogen end of the molecule is slightly positive. The double bond of ethene consists of two clouds of negative charge corresponding to the σ (sigma) and π (pi) molecular orbitals. The latter, which extends above and below the plane of the C2H4 molecule, interacts with and attracts the HCl molecule.

If, instead, the HCl approaches with its chlorine end leading as in , electrostatic repulsion between the like charges causes the two molecules to repel each other before any reaction can take place. The same thing happens in : the electronegativity difference between carbon and hydrogen is too small to make the C–H bond sufficiently polar to attract the incoming chlorine atom.

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