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So I recognise what an electrophile and a nucleophile is, but not all mechanisms (that I know of at least) have one species substituting or adding. For example, propanone added to NaBH$_4$ has an initial step of a hydride (nucleophilic addition), and then a H$^+$ being added to an O$^-$ (electrophilic addition).

I'm a bit wary of this example as the second step doesn't involve a carbon. So, another example would be the addition of HBr to ethene. The first step has the addition of H$^{\delta+}$ (electrophile) and then Br$^{-}$ being added as a nucleophile.

This website offers a solution by specifying that the rate determining step takes precedence, but I can't find anything to corroborate this

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  • $\begingroup$ so what exactly is your doubt, is it like identifying which step occurs first or whether it is entirely an SN or EN mechanism reaction ? $\endgroup$
    – PSR_123
    Commented Jun 5, 2022 at 17:54
  • $\begingroup$ The latter; (The site suggest it's to do with the rate determining step) $\endgroup$
    – yolo
    Commented Jun 5, 2022 at 18:09
  • $\begingroup$ in the end it is the rate determining step and stability of intermediates formed which decides whether an SN or EN reaction occurs. Eg : in a tertiary halide E1 is most preferred, followed by SN1 and SN2 and E2 are almost negligible. $\endgroup$
    – PSR_123
    Commented Jun 5, 2022 at 18:14
  • $\begingroup$ @PSR_123. Thanks. This answers my question $\endgroup$
    – yolo
    Commented Jun 5, 2022 at 19:28
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    $\begingroup$ You know the difference between addition and substitution. Nucleophilic/electrophilic reactions are similar to redox reactions with reapect to reagents involve. There is always a reduction reaction when oxidation invoved. Similarly, there is always nucleophilic reaction when electrophilic reaction involved. $\endgroup$
    – Mathew Mahindaratne
    Commented Jun 5, 2022 at 22:58

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Any reaction that is either a nucleophilic or an electrophilic addition/substitution is in fact both a nucleophilic and an electrophilic addition/substitution. Whatever a nucleophile combines with can also be called an electrophile that attacks the nucleophile; textbooks just adopt a single perspective.

We also have examples where the same reagent is both an electrophile and a nucleophile to different parts of the "substrate". Grignard reactions, in which the Grignard reagent has both a nucleophilic carbon atom and an electrophilic magnesium atom, are one such example.

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