phthalimide

[SaintJude]

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Why is phthalimide a good nucleophile when deprotonated? It has resonance structures (which should decrease its nucleophicity) and it has electronegative atoms (which should also decrease nucleophicity)

Please help. I'm so tired of my test prep making a statement and not explaining it!

 

[bananaz]

Deprotonating phthalimide places a full negative charge on a less electronegative atom (nitrogen). Since it's not sterically hindered, that alone is enough to make it nucleophilic, even in the presence of the electron-withdrawing factors you mentioned.

 

[SaintJude]

Thanks!

 

[MedPR]

You might also recall that weak bases are typically good nucleophile. Here, resonance will reduce basicity, thus good nucleophile.

At least that's how I think about it. For MCAT I don't think too many exceptions will be made to the "good nucleophile = weak base" rule.

 

[SaintJude]

You might also recall that weak bases are typically good nucleophile. Here, resonance will reduce basicity, thus good nucleophile.

At least that's how I think about it. For MCAT I don't think too many exceptions will be made to the "good nucleophile = weak base" rule.

Actually, Phthalimide is a very strong base.

 

[MedPR]

Actually, Phthalimide is a very strong base.

Really? Where did you read that?

 

[chiddler]

Actually, Phthalimide is a very strong base.

you are mistaken my compadre

http://en.wikipedia.org/wiki/Phthalimide

Acidity (pKa) 8.3
Basicity (pKb) 5.7

it's weak both

 

[SaintJude]

Oh, sorry. Kaplan says potassium phthalimide is "not only a good nucleophile, but a strong base" and so I erroneously extended that to phthalimide. Why would a K+ make a difference? Is it because it's a salt of KOH and phthalimide?

P.s. in case you're wondering, what this relates to : Gabriel synthesis of amines.

http://en.wikipedia.org/wiki/Potassium_phthalimide

 

[chiddler]

it's not because it's K specifically, but it's because the K is replacing an amine hydrogen. When in water, the K dissociates so in effect you just deprotonated the amine group. and amines are not very acidic. So K dissociates, amine holds a negative charge, and it will grab a hydrogen very strongly.

same reason why t-butyl lithium is such a strong base. hydrocarbons, like amine groups, are not acidic. So when they lose a hydrogen, they will want one back very strongly.

 

[MedPR]

Oh, sorry. Kaplan says potassium phthalimide is "not only a good nucleophile, but a strong base" and so I erroneously extended that to phthalimide. Why would a K+ make a difference? Is it because it's a salt of KOH and phthalimide?

P.s. in case you're wondering, what this relates to : Gabriel synthesis of amines.

http://en.wikipedia.org/wiki/Potassium_phthalimide

it's not because it's K specifically, but it's because the K is replacing an amine hydrogen. When in water, the K dissociates so in effect you just deprotonated the amine group. and amines are not very acidic. So K dissociates, amine holds a negative charge, and it will grab a hydrogen very strongly.

same reason why t-butyl lithium is such a strong base. hydrocarbons, like amine groups, are not acidic. So when they lose a hydrogen, they will want one back very strongly.

Maybe chiddler is saying the same thing, but I can't tell for sure..?

K+ is just the counterion, since when phthalimide gets deprotonated, it will have a full negative charge the same way that OH- does. "Potassium phthalimide" is basically the same as saying "insert name of deprotonated phthalimide here." So,


Either way, my theory about weak base is wrong here, since the deprotonated phthalimide is a strong base AND good nucleophile. Go figure.

 

[milski]

Maybe chiddler is saying the same thing, but I can't tell for sure..?

K+ is just the counterion, since when phthalimide gets deprotonated, it will have a full negative charge the same way that OH- does. "Potassium phthalimide" is basically the same as saying "insert name of deprotonated phthalimide here." So,


Either way, my theory about weak base is wrong here, since the deprotonated phthalimide is a strong base AND good nucleophile. Go figure.

Deprotonated phthalimide and phthalimide are not the same thing. Phthalimide would not be a strong base, the ion will be. Your OH- is example is great - OH- is a strong base, H2O not at all.

 

[MedPR]

Deprotonated phthalimide and phthalimide are not the same thing. Phthalimide would not be a strong base, the ion will be. Your OH- is example is great - OH- is a strong base, H2O not at all.

Yes, but isn't "potassium phthalimide" the same thing as deprotonated pthalimide? The same way that H+ is the same as H3O+. Obviously not really the same thing, but when someone says "H+" you know they mean H3O+, and when someone asks you about potassium tertbutoxide in solution, you know it's the same nucleophile as "tertbutoxide in solution"

 

[milski]

Yes, but isn't "potassium phthalimide" the same thing as deprotonated pthalimide? The same way that H+ is the same as H3O+. Obviously not really the same thing, but when someone says "H+" you know they mean H3O+, and when someone asks you about potassium tertbutoxide in solution, you know it's the same nucleophile as "tertbutoxide in solution"

Yes, we agree on this one. I think the thread started about the deprotonated compound which is an anion and will be a good nucleophile. The compound itself being weak base would not matter in that case.