Both nBuLi and tBuLi deliver a carbanion to the aryl bromide in the Li–Br exchange, but the two carbanions differ sharply in shape.
- n-Butyl carbanion (n-Bu⁻): a straight four-carbon chain. The nucleophilic carbon at the end is unhindered; the chain extends away from the reactive center. This makes n-Bu⁻ a good nucleophile toward electrophilic carbons.
- tert-Butyl carbanion (t-Bu⁻): a central carbon bearing three methyl groups. The nucleophilic carbon is surrounded on three sides by methyl substituents, so any electrophile approaching it must thread through a tight steric pocket.
The oxy-carbene carbon (the Fischer-type carbene carbon) is electrophilic but sits within a metal–carbonyl framework that already occupies significant space. A small nucleophile such as n-Bu⁻ can approach this carbon along a viable trajectory. A bulky nucleophile such as t-Bu⁻ cannot: the three methyl groups clash with the surrounding ligands before the nucleophilic carbon gets close enough to bond.
This steric difference, not any difference in basicity, is the primary reason the carbene discriminates between the two butyl groups.