Question

My understanding is that nucleophiles are lewis bases. Apparently nucleophile and base strength are not the same though. I thought Nu strength/Base strength is determined by how well it can donate an e- pair which is thus affected by the electron charge density on the e- donating atom.

However I've come across weak bases but strong nucleophiles Eg: Br-. It's a weak base cause it is large and has low electron density. However Br- is a strong nucleophile and this was attributed to its polarisability.
However I thought the issue of polarisability was more of deciding the hardness of the lewis acids/bases and am thus very confused as to what really determines the strength of a nucleophile/base and how polarisability comes into play.

Answer 1

Nucleophiles and bases have different strength determinants, with nucleophilicity being determined by the ability to donate electrons and attack an electrophile, and basicity being the ability to accept a proton. Br- is a weak base but a strong nucleophile due to its high polarizability, which allows it to distort its electron cloud in response to electric fields. Steric hindrance and solvent effects also play a role in determining nucleophilicity.

Step-by-step explanation:

The strength of a nucleophile and a base are determined by different factors. While nucleophilicity is primarily determined by the ability to donate a pair of electrons and attack an electrophilic center, basicity is the ability to accept a proton. In the case of Br-, it is a weak base because of its size and low electron density, but it is a strong nucleophile due to its high polarizability. Polarizability refers to the ability of an atom or ion to undergo distortion of electron cloud in response to an external electric field.

The differences in nucleophilicity and basicity are also influenced by steric factors and the nature of the solvent. Steric hindrance caused by bulky groups can hinder the approach of a nucleophile, making it a poor nucleophile despite being a strong base. Solvents can affect nucleophilicity by solvating the nucleophile and altering its reactivity.