Heterolytic and Homolytic Cleavage

[andyrk]

When heterolytic cleavage occurs, an anion and a cation are formed. Why don't they get attracted towards each other to form an ionic bond? And does this cleavage occur in an aqueous solution or just by itself?

Similarly, when homolytic cleavage occurs, none of the atoms produced have a complete octet as before. So why did the cleavage occur?

 

[TeethWhitener]

When heterolytic cleavage occurs, an anion and a cation are formed. Why don't they get attracted towards each other to form an ionic bond? And does this cleavage occur in an aqueous solution or just by itself?

They do attract. Generally, though, something keeps them from recombining. In solution, solvent molecules can form a complex around the newly formed ions called a solvent shell. Even without solution, heterolytic cleavage can still happen, but it's often disfavored energetically because, as you have pointed out, you have to get the fragments far enough away from each other that they don't react immediately.

Similarly, when homolytic cleavage occurs, none of the atoms produced have a complete octet as before.

Homolytic cleavage of a bond in a closed-shell species (a "normal" chemical compound that obeys the octet rule or otherwise has a complete valence shell without unpaired electrons) generates radicals, which are species with unpaired electrons, or (sticking with the octet rule) unfilled octets. Just because a compound no longer obeys the octet rule doesn't mean it doesn't exist. But the octet rule does allow you to predict that radicals are generally unstable and reactive--although there are exceptions here as well.

So why did the cleavage occur?

A bond doesn't usually cleave spontaneously. A chemical bond in equilibrium represents a minimum in potential energy, so that to break the bond requires adding energy to the system. So to break a bond, you pump energy into it to push the two fragments apart. For each extra unit of length you stretch the bond, the amount of extra energy required gets a little less until you asymptotically approach what's known as the bond dissociation energy. This is the amount of energy required to separate the fragments by an infinite distance (which is a pretty good definition for a broken bond).