Harpoon reaction

A harpoon reaction is a type of

.

The main feature of these redox reactions is that, unlike most reactions, they have steric factors greater than unity; that is, they take place faster than predicted by collision theory. This is explained by the fact that the colliding particles have greater cross sections than the pure geometrical ones calculated from their radii, because when the particles are close enough, an electron "jumps" (therefore the name) from one of the particles to the other one, forming an anion and a cation which subsequently attract each other. Harpoon reactions usually take place in the gas phase, but they are also possible in condensed media.^[4][5]

The predicted

can be improved by using a better estimation of the steric factor. A rough approximation is that the largest separation R_x at which charge transfer can take place on energetic grounds, can be estimated from the solution of the following equation that determines the largest distance at which the Coulombic attraction between the two oppositely charged ions is sufficient to provide the energy ΔE₀

${\displaystyle {\frac {-q_{e}^{2}}{R_{x}}}+\Delta E_{0}=0}$^[6]

With ${\displaystyle \Delta E_{0}=E_{i}-E_{e}a}$, where Ei is the

of the halogen.

Examples of harpoon reactions

• Generically: Rg + X₂ + hν → RgX + X,
  rare gas
  and X is a halogen
• Ba...FCH₃ + hν → BaF^(*) + CH₃[8]
• K + CH₃I → KI + CH₃^[9]

References