Organic Chemistry/Introduction to reactions/Polar and radical reactions

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Homolytic vs heterolytic cleavage

Two bonded atoms can come apart from each other in one of two ways. Either

• each atom gets away with half of the shared electrons, or
• one of the atoms leaves with more of the shared electrons than the other.

In homolytic cleavage, each atom leaves with one-half of the shared electrons (one electron for a single bond, or two for double bonds).

A—B → A* + B*

A* and B* represent uncharged radicals. The "*" represents an unbonded, unpaired valence electron.

In heterolytic cleavage, one atom leaves with all of the previously shared electrons and the other atom gets none of them.

A—B → A⁻ + B⁺

Homo (from the Greek for same) indicates that each atom leaves with the same number of electrons from the bond. Hetero (from the Greek for different) refers to the fact that one atom gets all of the bonding electrons, while the other gets none.

Polar reactions

Polar reactions occur when two bonded atoms come apart, one taking more of the shared electrons than the other. They involve heterolytic cleavage. The result is two charged species—one cation and one anion.

Radical reactions

Radical reactions don't deal with charged particles but with radicals. Radicals are uncharged atoms or molecules with an incomplete octet of valence electrons.

When a molecule comes apart by homolytic cleavage the result is two radicals. Although uncharged, radicals are usually very reactive because the unfilled octet is unstable and the radical can lower its energy by forming a bond in a way that allows it to fill its valence shell while avoiding any electrostatic charge..