[Catherine S.P]

Im asked to compare the 3 nucleophiles, PhO-, PhS- and PhOH and list them in order of reactivity....

With regards to PhO- and PhS-, is PhS- more reactive because the Sulphur is a bigger atom so the overlap between the pi system and the Sulphur is greater whereas Oxygen being a smaller atom cannot overlap as much? But I would have thought that if the atom was smaller it would be more reactive because the nucleus of the oxygen would be closer to the phenyl pi system!!
And also Im unsure whether these two phenyl nucleophiles would be more or less reactive than PhOH, my guess is that having a hydrogen attatched means its less reactive as it hasnt got that negetive charge anymore because the hydrogen cancels it out. Is this right?!

[Honclbrif]

I think you've got the overlap of S vs. O with a phenyl ring reasoning backwards. C and O tend to have comparable radii, and therefore overlap between the two is pretty favorable, but S tends to be much larger, and does not overlap as efficiently. Look up the stability of thioesters vs. regular esters.

As for the rest of the question: what qualities tend to make a species more nucleophilic and how are they displayed by the options you have?

[zoork34]

See this website for a good explanation of S- vs. O- nucleophilicity.


http://chemwiki.ucdavis.edu/Organic_Chemistry/Organic_Chemistry_With_a_Biological_Emphasis/Chapter__8%3A_Nucleophilic_substitution_reactions_I/Section_3%3A_More_about_nucleophiles

[Vidya]

You can compare the nucleophilicity on electronegativity also.More electronegative atom has less tendency to donate electrons.