Addition Elimination vs. Nucleophilic Substitution

[pwad]

Can someone clarify what the difference is between an addition-elimination reaction versus a nucleophilic substitution reaction?

For example (on the BS section of TPR #4):
p-aminophenol + acetic anhydride --> acetaminophen
is an addition-elimination reaction

I thought it was a nucleophilic substitution, since the NH2 acts as a nucelophile, attacking the carbonyl carbon of the acetic anhydride, and then eventually the electrons shift, kicking off the CH3COOH (acting as what I thought was a leaving group)

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[Kalamazoo]

In addition-elimination, the nucleophile attacks the carbonyl compound and one pair of electrons from the double bond of C-O are shifted to the Oxygen (giving it a negative charge), then come back down, and kick off the leaving group.

Sn2 reactions are one step. The nucleophile attacks and the leaving group is kicked off. There is no redistribution of electrons as in addition-elimination.

Make sense?

 

[Arctangent]

It is kind of similar, but in nucleophilic substitution, the carbon is usually sigma bonded to 4 different atoms/chains, so that if a nucleophile were to attack, you cannot have 5 bonds to the carbon for any significant amount of time, so the leaving group leaves before the nucleophile attacks (SN1) or as the nucleophile attacks (SN2).

In addition-elimination, you have in this case two single bonds and one double bond to oxygen. In my mind, I like to think of the bonds to oxygen as able to "buffer" any sort of nucleophilic addition to the central carbon, as the pi bond can quickly shift to being a lone pair. So in this case, when the resonance structure shifts and the oxygen withdraws the pi bond electrons, the carbon will have only three sigma bonds, leaving room for another constituent, which will be the nucleophile. While this tetrahedral molecule is somewhat stable (moreso than a pentavalent carbon), the carbonyl will regenerate, by kicking off the leaving group, but basically it can do so after nucleophilic addition as opposed to before/during.

 

[pwad]

okay so pretty much the defining factor is the electron shift (for eliminations)

and then for substitution reactions, the nucleophile prompts the leaving group to leave (SN2) or the leaving group leaves on its own (SN1), but neither are through electron shifts, correct?

thanks guys!