Answer to Question #86911 in Organic Chemistry for Divya Jangir

Question

Explain nucleophilicity is different from basicity

Answer 1

Nucleophilicity refers to the ability of a nucleophile to displace a leaving group in a substitution reaction. Nucleophiles are also bases, and they can abstract protons in elimination reactions. However, although nucleophilicity and basicity are related to the availability of the same electron pair, the reactions of a series of nucleophiles do not necessarily parallel those of the same species as bases.

In comparing nucleophiles whose attacking atom is in the same row of the periodic table, nucleophilicity is approximately in order of basicity, although basicity is thermodynamically controlled and nucleophilicity is kinetically controlled. Kinetically controlled reactions of lone pair nucleophile with an electrophilic carbon (usually) atom resulting in the formation a new C-X bond. Equilibrium (thermodynamic) reaction of the lone pair base with a proton, forming a new H-X bond.

An anion that is reacting as a nucleophile will result in a substitution, but if it reacts as a base, then elimination will result, be it on a carbocation (SN1 vs E1) or with the loss of a leaving group (SN2 vs E2).

References:

1.    Smith, M.B. March’s Advanced Organic Chemistry: Reactions, Mechanisms, and Structure / M.B. Smith, J. March. - 6th edit. - Hoboken, New Jersey : John Wiley & Sons, Inc., 2007. - P. 490-495.

2.    University of Calgary [Electronic resource] : Department of chemistry. – Mode of access: http://www.chem.ucalgary.ca/courses/350/Carey5th/Ch08/ch8-6.html. – Date of access: 25.03.2019.