Nucleophilic and electrophilic attack

[m25]

When describing reactions in ochem, how do I tell apart between nucleophilic attack and electrophilic attack? I mean, nucleophile and electrophile are both present in the reaction most of the time, so how do you know which was is attacing which??

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[tea guzzling traveler]

The terminology might be what's confusing you. I personally would say that eletrophillic attack as such does not really exist. Electrophillic substitution, like with a carbonyl carbon, or with an addition to a benzene ring, that exists. However, the electrophile is always the thing that is being "attacked". The nucleophile is the thing that is "attacking". So therefore, if you had a reaction with a good nucleophile and a good electrophile, all other things being equal (i.e. no steric hindrance etc), then your nucleophile will probably attack your electrophile.

 

[m25]

The terminology might be what's confusing you. I personally would say that eletrophillic attack as such does not really exist. Electrophillic substitution, like with a carbonyl carbon, or with an addition to a benzene ring, that exists. However, the electrophile is always the thing that is being "attacked". The nucleophile is the thing that is "attacking". So therefore, if you had a reaction with a good nucleophile and a good electrophile, all other things being equal (i.e. no steric hindrance etc), then your nucleophile will probably attack your electrophile.

But there are some instances from the AAMC MCAT question that describes the reaction as electrophilic attack...how does that work?

 

[tea guzzling traveler]

My guess would be a strong enough electrophile that it induces an attack from what would normally not be a nucleophile... but I'd have to see the actual problem in context to give a better answer.

 

[Cawolf]

I have seen some reactions phrased as "electrophilic attacks", but like @tea guzzling traveler said, this is not really what happens. These reactions are generally electrophilic additions to pi bonds, but the nucleophilic pi electrons (HOMO) still move to fill the LUMO of the electrophile.

 

[m25]

I have seen some reactions phrased as "electrophilic attacks", but like @tea guzzling traveler said, this is not really what happens. These reactions are generally electrophilic additions to pi bonds, but the nucleophilic pi electrons (HOMO) still move to fill the LUMO of the electrophile.

What is the HOMO and LUMO?

 

[Cawolf]

Did you take orgo? If not, it is probably beyond the scope of this forum to delve into Frontier Molecular Orbital theory.

The simple answer is that the HOMO is the orbital containing the most nucleophilic electrons and the LUMO is the orbital containing the empty, electrophilic orbital that the nucleophilic electrons attack.