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Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: xshadow on June 25, 2018, 09:58:11 AM

Title: better nucleophile
Post by: xshadow on June 25, 2018, 09:58:11 AM
 Hi  :)
Which is the best nucleofile in CH3OH between:

a) OH-/H2O
b)H2S /H2O
c)OH- /NH2-
d)I-/Br-

And why?


I have some doubt because there is a protic solvent

a) Usually a charged specie like OH- is a better nucleophile than H2O but here i have a protic solvent: OH- is much more solvatated than water so I really don't know which is better

b) H2S  (S is bigger than O,so its polarizability is better = better nucleophile)

c)  NH2- is a strong base,much more than OH----so it should be the better one....but hey,we are in a protic solvent and NH2- has  stronger H-bond with solvent...is this enough to overcome the usually trend (NH2 better )

d) I- si better...less solvatated



Is it correct?
Some suggestions for my doubt(s) ??

Thanks!!


Title: Re: better nucleophile
Post by: Babcock_Hall on June 25, 2018, 12:48:14 PM
How did iodide and bromide compare in polarizability?
Title: Re: better nucleophile
Post by: rolnor on June 25, 2018, 01:15:46 PM
Is it possible to have NH2- in CH3OH? I think not.
Title: Re: better nucleophile
Post by: xshadow on June 25, 2018, 01:40:42 PM
Quote from: Babcock_Hall on June 25, 2018, 12:48:14 PM
How did iodide and bromide compare in polarizability?

I- is bigger...so its polarizability is higher
I know this for I/Br
Title: Re: better nucleophile
Post by: xshadow on June 26, 2018, 04:51:15 AM
Quote from: rolnor on June 25, 2018, 01:15:46 PM
Is it possible to have NH2- in CH3OH? I think not.

My  textbook (paula bruice)asks tihis..m


Other doubt:
H2S is always a better nucleophile than  H2O or it ' s only true with protic solvent

(Because my textbook says that RO-  is better than RS-  in an aprotic solvent)
Title: Re: better nucleophile
Post by: Babcock_Hall on June 26, 2018, 01:04:35 PM
That might well be true for some pairs of nucleophiles; certainly hydrogen bonding to the nucleophile cannot be ignored.  In dipolar, aprotic solvents, the familiar order of nucleophilicity, iodide > bromide > chloride > fluoride, is reversed.  The only way to know for certain is to look at the rate constants for particular pairs of nucleophiles in the specified solvent.  Thiophenoxide ion is a stronger nucleophile than phenoxide, as judged by their values of n the nucleophilicity value in methanol (Lowry and Richardson, pp. 333-334, Mechanism and Theory in Organic Chemistry, 2nd ed).  However, if R is alkyl, the story might be a little different, and I don't have any information handy on polar, aprotic solvents for these two classes of nucleophiles.