[fahraynk]

A book is telling me $$CH_3CH_2OH$$ is a weak nucleophile and $$CH_3CH_2SH$$ is a strong nucleophile.
Is there any way for me to know this out without memorizing it? Is it because sulfur is bigger? If so... what would be the atomic number I could cut off the "this molecule is bigger" rule.

[sjb]

Strong and weak nucleophicility, like ionic / covalent bonding, is not a yes / no concept. Especially when you're trying to get ideas of synthetic approaches, a mixture of compounds will often be formed. Having said that, because oxygen is generally more electronegative than sulfur, it is unlikely to want to share electrons (which is a basic idea behind nucleophilic attack). So ethaneselenol is likely to be an even stronger nucleophile, for instance.

[rolnor]

I find this usefull;
https://en.wikipedia.org/wiki/Nucleophile

[fahraynk]

Thanks both of you.
Wikipedia was good, "In general, sulfur is very nucleophilic because of its large size, which makes it readily polarizable, and its lone pairs of electrons are readily accessible."

So I guess below oxygen in that column everything would be pretty much a good nucleophile due to size/polarizability even if they are not negatively charged.
Thanks, I am less stressed out now.

[Paul2lyfe]

The thiol is more nucleophilic than alcohol due to a variety of reasons electronegativity, polarizability, ionization energy, atomic size etc.

Nucleophilicty has to do with ability to donate a pair of electrons. To donate means it must be able to make it available for bonding.

The bond between O and H is stronger than S and H due to oxygen being very electronegative and hence will not make a pair of electrons readily available for bond formation like SH.