Question

which one is more nucleophilic: hydroxide ion (OH) , or ammonia (NH3)? Explain why

Answer 1

Nucleophilicity trends (compared with basicity)

1. Across a row in the periodic table nucleophilicity (lone pair donation) C^- > N^- > O^- > F^- since increasing electronegativity decreases the lone pair availability. This is the same order as for basicity.
2. If one is comparing the same central atom, higher electron density will increase the nucleophilicity,
3. 
   
4. e.g. an anion will be a better Nu (lone pair donor) than a neutral atom such as HO^- > H₂O. This is the same order as for basicity.
5. Within a group in the periodic table, increasing polarisation of the nucleophile as you go down a group enhances the ability to form the new C-X bond and increases the nucleophilicity, so I^- > Br^- > Cl^- > F^-. The electron density of larger atoms is more readily distorted i.e. polarised, since the electrons are further from the nucleus.
6. 
   
7. Note that is the opposite order to basicity (acidity increases down a group) where polarisability is much less important for bond formation to the very small proton.

Here is a table of relative nucleophilicities as measured in methanol (CH₃OH):

Very Good	I-, HS-, RS-
Good	Br-, HO-, RO-, NC-, N3-
Fair	NH3, Cl-, F-, RCO2-
Weak	H2O, ROH
Very Weak	RCO2H