Chapter 8, Problem 37P

Interpretation Introduction

Interpretation: The direction of equilibrium in the indicated reaction should be identified.

  ${\text{H}}^{-}+{\text{H}}_2\text{O}\rightleftharpoons {\text{H}}_2+{\text{OH}}^{-}$

Concept introduction: In accordance with Bronsted definition an acid can act as a proton donor and a base can act as a proton acceptor. Thus in a typical acid-base reaction, the fundamental principle is a lone pair of base reaches out for an acidic proton. Similar curved arrows are used to show the movement of electrons. After deprotonation, the species left with a negative charge is referred as the conjugate base of acid while the other with a positive charge is termed conjugate acid of given base. For example;

  

The strength of various conjugate acid-base pairs varies inversely to one another; the strong acid has a weak conjugate base and the strong base has weak conjugate acid and vice-versa.

The order of acidic strength of various alcohols is as follows:

  ${\text{CH}}_{\text{3}}\text{OH}<1°<2°<3°$

For tertiary alcohols, the steric bulk is maximum that leads to inhibition in solvation of $\text{tert-butoxide}$ ion. This in turn raises the ${\text{pK}}_{\text{a}}$ of corresponding tertiary alcohol and thus reduces their acidity. On the other hand, methanol has lowest ${\text{pK}}_{\text{a}}$ of theses alcohols. It can readily solvated by solvent and thus is most acidic.