# Relative reactivity of alkyl and aryl halides in nucleophilic-substitution reactions

In case of alkyl halides, the order for relative rates of reactions is $$\ce{RI > RBr > RCl}$$.

The reason is clear that $$\ce{I-}$$ is a better lg because of negative charge stabilisation and also $$\ce{C-I}$$ bond is weaker than $$\ce{C-Br}$$ bond which in turn is weaker than $$\ce{C-Cl}$$ because of overlap of orbitals.

But why is it the reverse order in case of aryl halides, $$\ce{ArF > ArCl > ArBr > ArI}$$? Why is it so even though $$\ce{C-F}$$ bond strength is higher and $$\ce{F}$$ is also not able to stabilise negative charge?

• Two different mechanisms with different features have different relevant factors. – Zhe Jul 10 '20 at 16:08

There are three mechanisms to discuss here: $$S_N2$$, $$S_N1$$ for $$RX$$ and nucleophilic aromatic substitution by addition-elimination for $$ArX$$:
$$S_N2$$ and $$S_N1$$:
$$S_N2$$ is concerted (one step), so this one step is rate determining, or the slowest of the mechanism. In $$S_N1$$, formation of the carbocation is rate determining. In both rate determining steps, the leaving group is being kicked off. This means the easier it is for the $$C-X$$ bond to break, the faster the reaction goes. The bond enthalpy decreases as size of halogen increases, giving the order you stated above.
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