Question on Sn2 vs. E2 reactions

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I understand that Sn2 favors methyl and primary groups and wants a strong nucleophile and
E2 reactions favors secondary and tertiary groups and wants a strong base.

But what is the difference between a strong nucleophile and a strong base?
I tried to find it out but I could not find a satisfying answer anywhere. I even saw someone asking the same question in this forum but I could not really understand.

I know that halogens Cl-,Br-, I- are strong nucleophiles but a weak base (since they are conjugate base of strong acid(?))
and a tert-ButO- is a strong base but a weak nucleophile.

but other than that how can you determine if a molecule is a strong nucleophile or strong base?

Thanks guys

look up polarizability

Originally Posted by mario0923

I understand that Sn2 favors methyl and primary groups and wants a strong nucleophile and
E2 reactions favors secondary and tertiary groups and wants a strong base.

But what is the difference between a strong nucleophile and a strong base?
I tried to find it out but I could not find a satisfying answer anywhere. I even saw someone asking the same question in this forum but I could not really understand.

I know that halogens Cl-,Br-, I- are strong nucleophiles but a weak base (since they are conjugate base of strong acid(?))
and a tert-ButO- is a strong base but a weak nucleophile.

but other than that how can you determine if a molecule is a strong nucleophile or strong base?

Thanks guys

Unfortunately you can NOT.

you have to look at the electrophile you are given, (primary, secondary, etc) , protic or aprotic solvent, and if it is a bulky base in order to determine what reaction you are doing. You can have 3 scenarios for sn1,sn2,e1,e2

1) strong nucleophile strong base
2) strong nucleophile weak base
3) weak nucleophile strong base



and Yes the conjugate base of a strong acid is so weak that they don't have any basic properties. And T-butyl is bulky so it can't do backside attack.

Thanks!!
I guess I was on the right track. But I thought there would be a "definition" of how to identify a strong nucleophile and a strong base.