Aluminum ions are seen moving throughout an aqueous solution. Because of their high charge, there is a very strong charge dipole interaction between the water molecules and the aluminum plus three ions. Six water molecules surround each aluminum and are held in place partly by the charge-dipole interaction and partly by covalent bonding. The hydrogens of the attached water molecules are more susceptible to a proton transfer reaction because of the removal of electron density by the aluminum ion. Hence, proton transfers occur to surrounding water molecules, until hydrated aluminum hydroxide remains. This is insoluble and sinks to the bottom of the container.
This animation portrays the formation of the 5-coordinate, trigonal bipyramidal adduct SnCl5- from tin tetrachloride and chloride ion. When additional chloride ion is added the 6-coordinate, octahedral adduct SnCl62- is formed.
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 mechanism is called SN1 because the rate-determining step is first order and the reaction involves the displacement or substitution of one nucleophile by another.
This animation shows the mechanism for the reaction of the hydroxide ion with methyl iodide. The reaction only occurs when the hydroxide ion attacks 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.