Question: Consider following nucleophiles (A) and (B) Select correct statements about (A) and (B)....

Question

Consider following nucleophiles (A) and (B)

Select correct statements about (A) and (B).

Answer 1

Solution

The question asks us to compare the basicity and nucleophilicity of two species:

(A) $(CH₃)₃C-O⁻$ (tert-butoxide ion) (B) $HO⁻$ (hydroxide ion)

Let's analyze their basicity and nucleophilicity separately.

1. Basicity Comparison

Basicity is the ability of a species to abstract a proton ( $H⁺$ ). A stronger base has a weaker conjugate acid (higher pKa of conjugate acid).

Conjugate acid of (A) (tert-butoxide ion): $(CH₃)₃C-OH$ (tert-butanol)
Conjugate acid of (B) (hydroxide ion): $H₂O$ (water)

Let's compare the pKa values of their conjugate acids:

pKa of $H₂O ≈ 15.7$
pKa of $(CH₃)₃C-OH ≈ 18$

Since the pKa of tert-butanol (18) is higher than that of water (15.7), tert-butanol is a weaker acid than water. Consequently, its conjugate base, $(CH₃)₃C-O⁻$ (A), is a stronger base than $HO⁻$ (B). This is also supported by the inductive effect: the three electron-donating methyl groups in tert-butoxide increase the electron density on the oxygen atom, making it more available to abstract a proton, thus increasing its basicity compared to hydroxide.

Conclusion for Basicity: (A) is a stronger base than (B).

2. Nucleophilicity Comparison

Nucleophilicity is the ability of a species to donate an electron pair to an electron-deficient center (electrophile), typically a carbon atom. It is a kinetic property.

Both (A) and (B) are negatively charged oxygen species. The key factor differentiating their nucleophilicity here is steric hindrance.

(A) $(CH₃)₃C-O⁻$ (tert-butoxide ion): The oxygen atom is bonded to a carbon which is further bonded to three bulky methyl groups. These methyl groups create significant steric hindrance around the oxygen atom. This bulkiness makes it difficult for the oxygen to approach and attack an electrophilic carbon center, especially in an SN2 reaction where the nucleophile must attack from the backside.
(B) $HO⁻$ (hydroxide ion): The oxygen atom is only bonded to a small hydrogen atom. There is minimal steric hindrance.

Due to the substantial steric hindrance, (A) is a very poor nucleophile, even though it is a strong base. It is often referred to as a "non-nucleophilic base" because it preferentially acts as a base (abstracts protons, leading to elimination reactions like E2) rather than a nucleophile (attacks carbon, leading to substitution reactions like SN2). In contrast, $HO⁻$ is a relatively good nucleophile due to its small size and high charge density.

Conclusion for Nucleophilicity: (A) is a weaker nucleophile than (B).

Summary of Comparison:

(A) is a stronger base than (B).
(A) is a weaker nucleophile than (B).

Therefore, A is a weaker nucleophile and stronger base compared to B, which is a strong nucleophile and weaker base.

Explanation of the solution:

Tert-butoxide (A) is a stronger base than hydroxide (B) because its conjugate acid (tert-butanol) is weaker than water (pKa of tert-butanol ≈ 18, pKa of water ≈ 15.7). However, due to the bulky methyl groups, tert-butoxide experiences significant steric hindrance, making it a much weaker nucleophile compared to the small and unhindered hydroxide ion. Therefore, A is a weaker nucleophile and stronger base compared to B, which is a strong nucleophile and weaker base.