Chapter 2: Q47 P (page 140)

The following compounds can all react as acids.
For each compound, show its conjugate base. Show any resonance forms if applicable.
Rank the conjugate bases in the order you would predict, from most stable to least stable.
Rank the original compounds in order from strongest acid to weakest acid.

Short Answer

Expert verified

(a)

(b)

(c)

Step by step solution

Conjugate acid-base pair

The residual part of acid after losing a proton (H⁺) will have a tendency to accept a proton (H⁺). Therefore, it will behave as a base. These pairs of substances which differ from one another by a proton(H⁺) are known as conjugate acid-base pairs. Consider a general example of an acid:

Resonance structures

A single structural formula sometimes cannot explain all the properties of a compound that is given. In such cases, the compound may be represented by two or more structural formulae which differ from each other only in the arrangement of electrons. None of this structural formula alone can explain all the observed properties of the compound. The compound is then said to show resonance. The various structures are known as resonating structures. The true structure of the molecule is not represented by any of the resonating structures but is considered to be a resonance hybrid of the various resonating structures. Resonance delocalization is considered as a dominating effect in stabilizing the conjugate base.

Step 3: Ka and pKa

K_ais known as the acid dissociation constant and its value indicates the relative strength of the acid. pK_a is defined as the negative logarithm (base ) of K_a . Mathematically, pK_a =-logK_a . value of strong acids is usually zero or even negative while pK_a the value of weaker acids are greater than 4.

pK_a values of different compounds can be compared to determine which acid is better. Lower value ofpK_a implies higher acidity.