Organic Chem Question

[jlee1986]

Hey guys,

I'm having some trouble distinguishing good/poor bases and good/poor nucleophiles. Is it true that the relationship between bases and nucleophiles is inversely related (if something's a good base, then it's a bad nucleophile, and vice versa)? Also, what are some characteristics of nucleophiles and bases that can help me distinguish them from each other? Thanks !

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[joonkimdds]

Hey guys,

I'm having some trouble distinguishing good/poor bases and good/poor nucleophiles. Is it true that the relationship between bases and nucleophiles is inversely related (if something's a good base, then it's a bad nucleophile, and vice versa)? Also, what are some characteristics of nucleophiles and bases that can help me distinguish them from each other? Thanks !

I don't know if I remember this correctly or not but here u go.

strong base = good nucleophile = bad leaving group.
but this rule only applies in aprotic solvent.

protic solvent(H bond to a high electronegative atom) can form H bond to nucleophile and reduce nucleophilicity. This effect is particularly strong for small atoms with concentrated charge thus larger atoms are more nucleophilic in protic solvent. This goes against strong base = good nucleophile rule because larger atoms(go down the periodic chart) are weak base and strong acid.

also, what I heard is that nucleophiles are only used in Sn1 and Sn2 but bases are used in E1 and E2. I am not sure who said this but that's what I heard and it might be wrong


By the way, good nucleophile is bad leaving group. If not, the leaving group becomes nucleophile and the whole cycle begins.

 

[TimeforDAT]

I don't know if I remember this correctly or not but here u go.

strong base = good nucleophile = bad leaving group.
but this rule only applies in aprotic solvent.

protic solvent(H bond to a high electronegative atom) can form H bond to nucleophile and reduce nucleophilicity. This effect is particularly strong for small atoms with concentrated charge thus larger atoms are more nucleophilic in protic solvent. This goes against strong base = good nucleophile rule because larger atoms(go down the periodic chart) are weak base and strong acid.

also, what I heard is that nucleophiles are only used in Sn1 and Sn2 but bases are used in E1 and E2. I am not sure who said this but that's what I heard and it might be wrong


By the way, good nucleophile is bad leaving group. If not, the leaving group becomes nucleophile and the whole cycle begins.

so what exactly is the difference between using a nucleophile and a base like in e1 and e2 as stated above? Do you mean e1 and e2 just require any base of any strength? I remember reading a thread on it a while back, but can't find it

 

[joonkimdds]

so what exactly is the difference between using a nucleophile and a base like in e1 and e2 as stated above? Do you mean e1 and e2 just require any base of any strength? I remember reading a thread on it a while back, but can't find it

so I guess u read the same thing from this forum somewhere, right?
If I remember it correctly, the person said whatever is used in substitution is called nucleophile and whatever is used in elimination to attack is called base.

I was thinking that he is wrong because my textbook shows Nu for elimination reaction as well.

But I just wrote it there because I am not good at OCHEm and any of my comments might be wrong so I just wrote whatever I remember from reading other's comments~

Anyway I don't think it's a big deal~.