Chapter 2, Problem 47P

a. Rank the following alcohols from strongest to weakest acid:

b. Explain the relative acidities.

Interpretation Introduction

Interpretation:

The given alcohols have to be ranked from strongest to weakest acid.

Concept introduction:

If a base receives one proton, then the formed species is a conjugate acid whereas an acid lose one proton, then the formed species is a conjugated base.

If an acid lose one proton, then the formed species is a conjugated base. Weak base forms stronger conjugated acid.

Acidity of species depends on the electronegativity of atom attached to the acidic proton. Order of electronegativity of hybridization is $sp>s{p}^2>s{p}^3$ so $s{p}^3$ hybridized molecule is weak acid.

Answer to Problem 47P

The given alcohols are ranked from strongest to weakest acid as follows,

$CC{l}_{3}C{H}_{2}OH>CHC{l}_{2}C{H}_{2}OH>C{H}_{2}ClC{H}_{2}OH$

Explanation of Solution

In hydrocarbons, if hydrogen atoms are replaced by electronegative atoms, it causes inductive electron withdrawal. It stabilizes its conjugate base thus increases the strength of the acid. The conjugated base of a weak acid is very strong. As the electronegativity of substituent increases, the greater will be the inductive electron withdrawal of the substituent making it a strong acid.

Therefore, the acidity order is:

$CC{l}_{3}C{H}_{2}OH>CHC{l}_{2}C{H}_{2}OH>C{H}_{2}ClC{H}_{2}OH$

The compound with three chlorine atoms near to the $OH$ group is more acidic. This is because as the number of electron-withdrawing chlorine atom increases from the same distance of $OH$ group, the acid gets stronger. The least acidic compound is the alcohol with one chlorine atom near to the $OH$ group.

Interpretation Introduction

Interpretation:

The relative acidities of the given alcohol compounds have to be explained briefly.

Concept introduction:

If a base receives one proton, then the formed species is a conjugate acid whereas an acid lose one proton, then the formed species is a conjugated base.

If an acid lose one proton, then the formed species is a conjugated base. Weak base forms stronger conjugated acid.

Acidity of species depends on the electronegativity of atom attached to the acidic proton. Order of electronegativity of hybridization is $sp>s{p}^2>s{p}^3$ so $s{p}^3$ hybridized molecule is weak acid.

Explanation of Solution

In hydrocarbons, if hydrogen atoms are replaced by electronegative atoms, it causes inductive electron withdrawal. It stabilizes its conjugate base thus increases the strength of the acid. The electron density near $OH$ group is reduced when the electron withdrawing substituent is present which makes the conjugate base more stable and increases the acidity of acid. As the number of electron-withdrawing chlorine atom increases from the same distance of $OH$ group, the acid gets stronger.