Question

In nucleophilic aliphatic substitution the nucleophiles are generally:
A. Acids
B. Bases
C. Salt
D. Neutral molecules

Answer 1

Hint: A nucleophile may be defined as any substance that has the tendency to donate an electron pair to form a covalent bond.

Complete Step by Step Solution:
In nucleophilic aliphatic substitution, an atom or group of atoms is replaced by the nucleophile bonded to a carbon atom which is neither a member of the aromatic ring nor a member of carbonyl carbon. For example, the carbon-halogen bond in alkyl halides is known to be polar because of the greater electronegativity of the halogen as compared to carbon.

A nucleophilic attack is possible due to the presence of a partial positive charge on the carbon atom. Thus, when a stronger nucleophile approaches the positively charged carbon of the alkyl halide, the halogen is readily displaced as a halide ion.

Nucleophiles are also known as Lewis bases. A Lewis base may be defined as a species that can donate one or more electron pairs, i.e., a base is an electron pair donor. Thus, any substance having an unshared pair of electrons can behave as a Lewis base.

All the anions can act as Lewis bases since they have a tendency to donate electron pairs. For example, oxides \[{\rm{(}}{{\rm{O}}^{2 - }}{\rm{)}}\], hydroxides \[{\rm{(O}}{{\rm{H}}^ - }{\rm{)}}\], halides \[{\rm{(}}{{\rm{X}}^ - }{\rm{)}}\], amides \[{\rm{(N}}{{\rm{H}}_2}^ - {\rm{)}}\] etc. Moreover, substances having unshared or lone pairs of electrons can act as Lewis bases. For example, water \[{\rm{(}}{{\rm{H}}_{\rm{2}}}{\rm{O)}}\], ammonia \[{\rm{(N}}{{\rm{H}}_3}{\rm{)}}\]etc.

Hence, it is evident that the bases act as nucleophiles in nucleophilic aliphatic substitution.
Therefore, option B is correct.

Note: The greater the charge density on the anion, the stronger the base. Nucleophiles undergo both nucleophilic addition and nucleophilic substitution reactions. Nucleophiles are either negatively charged or neutrally charged.