The term indicates a sequence of reactions such as shown in equations (1)–(3), leading from A to B :

\[\text{A} + \text{e}^{-} \rightarrow \text{A}^{\cdot- }\]	(1)
\[\text{A}^{\cdot- } \rightarrow \text{B}^{\cdot- }\]	(2)
\[\text{B}^{\cdot- } + \text{A} \rightarrow \text{B} + \text{A}^{\cdot- }\]	(3)

An analogous sequence involving radical cations (A⁺·, B⁺·) is also observed. The most notable example of electron-transfer catalysis is the S RN 1 (or T + D N + A N) reaction of aromatic halides. The term has its origin in a suggested analogy to acid-base catalysis, with the electron instead of the proton. However, there is a difference between the two catalytic mechanisms, since the electron is not a true catalyst, but rather behaves as the initiator of a chain reaction. 'Electron-transfer induced chain reaction' is a more appropriate term for the situation described by equations (1)–(3).