please rank the following from least to most nucleophilic:
CH3S-, CH3O-, CH3OH, CH3COO-, CH3COOH
I keep getting it wrong over and over again. Please help, I’d really appreciate it!
Negatively charged species are more nucleophilic than neutrally charged ones, so CH3OH (methanol) and CH3COOH (acetic acid) will be last. As for the others, consider electronegativity, polarizability, and size.
The acetate anion is a weak nucleophile, because its negative charge is resonance-stabilized; each oxygen only has a -1/2 charge, so it isn’t as strong a nucleophile as either methoxide or methanethiolate. It’s also bigger; in general, small, linear species make better nucleophiles than bulky ones.
There are two main differences between methanethiolate and methoxide. One is that sulfur is less electronegative than oxygen, which means that its electrons are more willing to participate in a reaction (i.e. sulfur is a better nucleophile). The other is that sulfur is a bigger atom, so it’s more polarizable than oxygen. This mean that methanethiolate is a stronger nucleophile than methoxide.
Finally, acetic acid will be a worse nucleophile than methanol, both because of size and resonance.
The final order, from worst to best nucleophile, is:
CH3COOH < CH3OH < CH3COO- < CH3O- < CH3S-
ch3s- > ch3o- > ch3coo- > ch3oh > ch3cooh