I recently came across a question in which an option is 'Breaking a chemical bond is the first step in any chemical reaction', which lead me to think of reactions which do not involve bond breaking. The only reaction I can think of is dimer formation, but a Google search reveals no more. Surely this cannot be the only example?


The reaction of a Lewis acid with a Lewis base results only in bond formation. Your question gets at the essence of chemistry, though. A chemical reaction is the rearrangement of electrons between atoms. So generally, most reactions will involve breaking bonds. Even for monomers (e.g. alkenes) to polymerize, $\pi$-bonds within the monomers have to be broken to form $\sigma$-bonds in the polymer. You seldom have one without the other.

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    $\begingroup$ Electron transfer is such an example; $\ce{D + A \to D^+ = A^-}$. $\endgroup$
    – porphyrin
    Jul 28 '18 at 6:53

Precipitation reactions? If you will grant me that an aqueous ion is free. (But maybe you don't and consider the solvated ion bonded to nearby water.)


Per a source, to quote:

Like ozone, the predominant oxidation reaction mechanism for chlorine dioxide proceeds through a process known as free radical electrophilic (i.e. electron-attracting) abstraction rather than by oxidative substitution or addition (as in chlorinating agents such as chlorine or hypochlorite).

So, the reaction:

$\ce{.ClO2 + e- -> ClO2-}$


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