[kor]

Why I^- is a good nucleophile but it is a weak base??
Isn't a good nucleophile is always a good base ?

[cliverlong]

I think the "environment" of the I^- may be relevant here

Hydroiodic acid, http://en.wikipedia.org/wiki/Hydrogen_iodide, has a very large (10¹⁰) value for Ka. So HI readily dissociates, so HI(aq) is a strong acid. Now I^- is the complementary base (attracts H⁺). By the simple rule: Strong acid  weak (complementary) base, I^- is a weak base.

The "strength" of a nucleophile is determined solely by the density of its negative charge. Since, I^- is actually an ion rather then a slightly (delta) negative part of a polar molecule, then I- (like the other halide ions) will be an effective nucleophile. Now, in practical terms, how is the I^-  "delivered" to the delta positive centre in an organic molecule? We can't use an aqueous solution of a sodium or potassium salt since the ionic aqueous solution won't mix with an organic compound - so I guess we have to use polar covalent HI gas.

That's my opinion.

Clive

[kor]

I think the "environment" of the I^- may be relevant here

Hydroiodic acid, http://en.wikipedia.org/wiki/Hydrogen_iodide, has a very large (10¹⁰) value for Ka. So HI readily dissociates, so HI(aq) is a strong acid. Now I^- is the complementary base (attracts H⁺). By the simple rule: Strong acid  weak (complementary) base, I^- is a weak base.

The "strength" of a nucleophile is determined solely by the density of its negative charge. Since, I^- is actually an ion rather then a slightly (delta) negative part of a polar molecule, then I- (like the other halide ions) will be an effective nucleophile. Now, in practical terms, how is the I^-  "delivered" to the delta positive centre in an organic molecule? We can't use an aqueous solution of a sodium or potassium salt since the ionic aqueous solution won't mix with an organic compound - so I guess we have to use polar covalent HI gas.

That's my opinion.

Clive

thank you 
A good nucleophile is a good base unless the ions/atoms is too big ,RIGHT??

[renge ishyo]

I was taught that I^- was a poor nucleophile and a weak base (it is a good leaving group).

[kor]

I was taught that I^- was a poor nucleophile and a weak base (it is a good leaving group).

]
However, I was taught that I^- was a good nucleophile and a weak base
So, I use google
http://walba.colorado.edu/substitution%20and%20elim%27n%20%2702.htm