Question

2,4-Pentanedione is a considerably stronger acid than is acetone (Chapter 19). Write a structural formula for the conjugate base of each acid and account for the greater stability of the conjugate base from 2,4-pentanedione.

Short answer

Answer: 2,4-pentanedione is a stronger acid compared to acetone because its conjugate base has two carbonyl groups, allowing for greater resonance effect and thereby making it more stable. More stable conjugate base indicates a stronger acid.

Step-by-step solution

Identify the acidic protons

In both acetone and 2,4-pentanedione, the acidic protons are attached to the carbonyl (C=O) group. In acetone, there is only one carbonyl group and thus only one acidic proton. In 2,4-pentanedione, however, there are two carbonyl groups and therefore two acidic protons.

Write the structural formula for the conjugate base of each acid

The conjugate base is formed by removing the acidic proton from the molecule. To do this, we can represent the dissociation of each acid as follows: Acetone: \(\text{CH}_3\text{COCH}_3 \rightleftharpoons \text{CH}_3\text{COCH}_2^-\) + \( \text{H}^+\) 2,4-Pentanedione: \(\text{CH}_3\text{C(O)CH}_2\text{C(O)CH}_3 \rightleftharpoons \text{CH}_3\text{C(O)CH}_2\text{C(O)CH}_2^-\) + \(\text{H}^+\) Now, we can write the structural formula for the conjugate base of each acid: Acetone conjugate base: \(\text{CH}_3\text{COCH}_2^-\) 2,4-Pentanedione conjugate base: \(\text{CH}_3\text{C(O)CH}_2\text{C(O)CH}_2^-\)

Explain the greater stability of the conjugate base from 2,4-pentanedione

In order to compare the stability of the two conjugate bases, we need to analyze their resonance structures. Resonance means that the conjugate base can delocalize its negative charge across multiple atoms, which helps stabilize the molecule. In the conjugate base of acetone, the negative charge is localized on the alpha carbon (next to the carbonyl group) and cannot be delocalized elsewhere: \(\text{CH}_3\text{COCH}_2^-\) In the conjugate base of 2,4-pentanedione, there are two carbonyl groups, and the negative charge can be delocalized between both oxygen atoms: \(\text{CH}_3\text{C(O)CH}_2\text{C(O)CH}_2^-\) The presence of two carbonyl groups in 2,4-pentanedione enhances the resonance effect, which makes the conjugate base more stable. More stable conjugate base means stronger acid. Thus, 2,4-pentanedione is a considerably stronger acid than acetone.