Wired Chemist

Blue atoms are seen approaching a diatomic molecule from a variety of angles. Only when a blue atom approaches from the side of the molecule away from the larger atom, does reaction occur and a new molecule is formed.

This animation utilizes the same atoms as Animation 12.1 and shows that even when the angle of approach is correct, the reaction will not occur until the blue atom approaches with sufficient kinetic energy.

This animation shows the mechanism for the reaction of the hydroxide ion with methyl iodide. The reaction only occurs when the hydroxide ion attacks from the carbon from the side opposite the iodine. As the oxygen-carbon bond begins to form, the hydrogens begin to bend away from the oxygen. At the transition state both the oxygen and iodine are partially bonded to carbon and the hydrogens are in a plane perpendicular to the oxygen-carbon-iodine bond. Finally, the carbon-oxygen bond is fully formed and the iodine is fully displaced as an iodide ion. The reaction profile at the bottom shows how the energy of the system changes as the reactions progresses.

The reaction of the hydroxide ion with 2-methyl-2-iodopropane follows a two-step mechanism. In the first, slow step the carbon-iodine bond dissociates leaving a planar carbocation. Because this carbocation is positively charged the hydroxide rapidly attacks it in the second step to form 2-methyl-2-hydroxypropane. The first step controls the rate of the reaction.

The transfer of an oxygen atom from nitrogen dioxide to carbon monoxide occurs in a single step process. Notice that in the transition state both the carbon-oxygen and nitrogen-oxygen bonds are partially formed.