Chapter 6, Problem 6.42P

Interpretation Introduction

Interpretation:

The curved arrow notation for the proton transfer reaction between the hydride anion and ethanol is to be drawn. Using the $\text{pKa}$ values, the most favored direction of the reaction is to be determined. The difference in their $\text{pKa}$ values is to be calculated.

Concept introduction:

The curved arrow starts with unshared electrons pair, which has a higher electron density and points in the direction of a site of electron deficiency. The curved arrow begins with a nucleophile and points toward an electrophile. A hydride anion acts as a nucleophile, and an ethanol molecule acts as an electrophile. So, the curved arrow starts from ${\text{H}}^{\text{- }}$ and points toward ${\text{CH}}_{\text{3}}{\text{CH}}_{\text{2}}\text{OH}$. At equilibrium, the strongest acid that can occur in solution is the protonated solvent, and the strongest base that can occur in solution is the deprotonated solvent. The proton transfer reaction favors the side opposite the stronger acid (i.e., opposite the lower $\text{pKa}$ value). Each difference of $\text{1}$ in $\text{pKa}$ values between two acids corresponds to a power of $\text{10}$ by which that side of the reaction is favored.