Consider the relative thermodynamic stabilities of the following carbanionic conjugate bases: (I) , — GOC and Organic Chemistry Basics Chemistry Question
Question
Consider the relative thermodynamic stabilities of the following carbanionic conjugate bases: (I) $CCl_3^-$, (II) $C_6H_5CH_2^-$, (III) $(CH_3)_2CH^-$, (IV) $(CH_3)_3C^-$. What is the correct decreasing order of their stability, which dictates the acidic strength of their parent hydrocarbons?
💡 Solution & Explanation
The trichloromethyl anion ($CCl_3^-$) is exceptionally stabilized by strong $-I$ effects and $p\pi-d\pi$ back-bonding into the vacant d-orbitals of the chlorine atoms (I is most stable). The benzyl anion ($C_6H_5CH_2^-$) is stabilized by extensive resonance delocalization (II is second). The isopropyl and tert-butyl anions are destabilized by strong $+I$ effects from two and three methyl groups respectively, making IV the least stable. Order: I > II > III > IV.