Question

Resonance results in a weaker base and a weaker nucleophile?

Answer 1

A GOOD NUCLEOPHILE OR BASE MUST HAVE ABILITY TO ABSTRACT AN ACIDIC PROTON OR ATTACK ON ELECTRON DEFFICIENT CENTRES RESPECTIVELY.

FOR THIS THESE SPECIES MUST HAVE ELECTRON PAIR AVAILABLE FOR DONATION TO PROTON OR ELECTROPHILIC CENTRE.

SO HIGHER THE AVAILABLITY OR UNSTABLITY OF UNSHARED ELECTRON PAIR ON BASE OR NUCLEOPHILE HIGHER WILL BE THEIR BASICITY OR NUCLEOPHILICITY..

FOR EXAMPLE METHYL ANION IS A STRONGER BASE AND NUCLEOPHILE THAN HYDROXIDE ANION .BECAUSE THE NEGATIVE CHARGE ON CARBON OF METHYL GROUP IS NOT STABLISED BY CARBON DUE TO ITS LOW ELECTRONEGATIVITY.

BUT NEGATIVE CHARGE ON OXYGEN ATOM OF HYDROXIDE ANION IS SLIGHTLY STABLISED BY ELECTRONEGATIVITY OF OXYGEN ATOM.

THINK WHICH IS STRONG BASE AMIDE ANION OR HYDROXIDE ANION?

RESONANCE ALSO EFFECTS THE BASICITY AND NUCLEOPHILICITY AS FOLLOWS.

DUE TO RESONANCE THE LONE PAIR OF ELECTRONS ARE NOT READILY AVAILABLE FOR DONATION SO THEIR BASICITY OR NUCLEOPHILICITY DECREASES..

FOR EXAMPLE PYRROLE IS A WEAK BASE BECAUSE LONE PAIR OF ELECTRONS ON NITROGEN ATOM OF PYRROLE RING ARE INVOLVED IN DELOCLALISATION TO AROMATIC SEXTET.

UNSTABLE WITTIG YLIDES ARE STRONG NUCLEOPHILES THAN STABLE WITTIG YLIDES.HERE STABLE WITTIG YLIDES ARE STABLISED BY RESONANCE BETWEEN NEGATIVE CHARGE ON CARBON AND ADJACENT PI BOND .

DIMETHYL SULPHONIUM METHYLIDE IS UNSTABLE AND STRONG NUCLEOPHLE THAN DIMETHYL OXO SULPHONIUM METHYLIDE WHICH IS STABLISED BY RESONANCE BETWEEN NEGATIVE CHARGE ON CARBON AND PI BOND BETWEEN SULPHUR AND OXYGEN.

PROTON IS ALSO AN ELECTROPHILE.