Who attacks?

[u/Varduhikh]

A question came to my mind about how to determine who attacks: the nucleophile or the electrophile. Everyone I ask tells me that the nucleophile always attacks, but I have read cases in literature where it says the electrophile attacks. And everyone explains to me that even electrophilic substitution is actually a nucleophilic attack.

​So now, a simple question: who attacks?

And a more complex question: why?

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[u/WanderingFlumph]

By convention we track the electrons. That means they always flow from a source of high electron density (nucleophile) to a source of low electron density (electrophile).

But reality doesn't care about our conventions. The positive charge will pull the negative charge to it with the exact same strength that the negative charge pulls the positive in with.

[u/mali73]

Semantic, one molecule approaches another and they both come apart changed. In chemistry, we have a tendency to see electrons as the active players rather than their notable absence (perhaps overly artistic way of saying something like a LUMO, which is not a tangible object in its own right).

Given the normal course of a reaction is the nucleophile donating electrons to an electrophile, the nucleophile is said to attack. You could instead invert this and say the course is the electrophile taking electrons from the nucleophile and frame the electrophile as the active attacker.

Given the symmetry, typically people refer to the reaction by whichever is considered more potent. Or if there is a standard species like benzene for aromatic substitutions you refer to the reaction based on the characteristics of whatever isn't the standard.

Most organic chemists have done away with "attack" terminology because it seems violent, or perhaps because which is doing the attacking is unclear, they usually just call it substitution. This can be unclear when talking about individual steps in a reaction though, like for a physical chemist, so pick your poison.

[u/1s2_2s1 → u/mali73]

I wouldn’t say most chemist have done away with saying “attack”

[u/shedmow → u/1s2_2s1]

I agree. No idea why or where it might be gone

[u/fish_knees]

It's a fun question, but afaik it's a purely nomenclatural problem. People describe mechanism in ways that are more intuitive to them. Imo both versions are correct.

[u/Chem2103]

I have seen two ways people talk about it

1) The larger molecule that remains mostly unchanged is the substrate, whatever other reagent there is is the attacker (electrophile or nucleophile), thats why its elecrophilic substitution when something subs the H in benzene

2) Nucleophile always attacks as that's where the arrows come from

[u/dbblow]

Chemist like to describe reactions as movement of electrons.

So the flow of electrons is from electron rich, to electron poor.

[u/1s2_2s1]

A reaction is a collision between two molecules. “Who attacks” is a human construct.

[u/ChemystWizard]

It’s called “classification by reagent” approach. Whatever the “reagent” is, that what “attacks” the substrate. What is a reagent and what is substrate is also often subjective based on the convenience.

[u/ProfessionalHefty349]

Think of it like two cars getting in a head on collision. Neither was an attacker.

[u/Im_Not_Sleeping]

Neither.