Question

The correct order of acidity among the following compounds is (a) Cyclohexane-1,3-dione (b) Cyclohexanone (c) Cyclohexene (d) Cyclohexane carboxylic acid (A) \(a>d>b>c\) (B) \(a>b>d>c\) (C) \(\mathrm{d}>\mathrm{a}>\mathrm{b}>\mathrm{c}\) (D) \(a>d>c>b\)

Short answer

The short answer is: The correct order of acidity among the given compounds is (C) \(d > a > b > c\).

Step-by-step solution

Write down the chemical structures of the given compounds

Draw the chemical structures of the given compounds: (a) Cyclohexane-1,3-dione (b) Cyclohexanone (c) Cyclohexene (d) Cyclohexane carboxylic acid

Identify the acidic protons in each compound

Determine the acidic protons in each compound, which are the protons that can be easily lost to form a conjugate base: (a) Cyclohexane-1,3-dione: Two carbonyl groups are present, so the protons in between the carbonyl groups are acidic (b) Cyclohexanone: The alpha position (carbon next to the carbonyl group) has acidic protons (c) Cyclohexene: The protons in the allylic position (carbon adjacent to the double bond) are slightly acidic (d) Cyclohexane carboxylic acid: The carboxylic acid group has an acidic proton

Analyze the stability of the conjugate bases

Analyze the stability of the conjugate bases formed after losing the acidic proton. Consider resonance effect, inductive effect, and hybridization: (a) Cyclohexane-1,3-dione: The conjugate base has resonance stabilization between the two carbonyl groups (b) Cyclohexanone: The conjugate base has resonance stabilization with the carbonyl group (c) Cyclohexene: The conjugate base has resonance stabilization with the double bond (d) Cyclohexane carboxylic acid: The conjugate base has resonance stabilization with the carbonyl group and a nearby oxygen atom

Compare the stability of the conjugate bases and determine the acidity

Compare the stability of the conjugate bases based on resonance, inductive effects, and hybridization: (a) Cyclohexane-1,3-dione: High stability due to two carbonyl groups stabilizing the conjugate base (b) Cyclohexanone: Lesser stability compared to (a), as only one carbonyl group is present for stabilization (c) Cyclohexene: Even lesser stability due to less effective double bond stabilization (d) Cyclohexane carboxylic acid: Maximum stability due to resonance stabilization with both carbonyl group and additional oxygen atom Considering the stability of the conjugate bases, we can determine the order of acidity: (d) > (a) > (b) > (c) Answer: (C) d > a > b > c

Key concepts

Acid-Base Stability

Understanding acid-base stability is essential when comparing the acidity of different molecules. In organic chemistry, the strength of an acid is directly related to the stability of its conjugate base. An acid is more likely to donate a proton if the resulting conjugate base is stable. Factors that contribute to the stability include resonance, inductive effects, and electron-withdrawing groups.

For example, the conjugate base of a carboxylic acid is stabilized by resonance between the two oxygen atoms, making carboxylic acids generally stronger acids than alcohols or ketones. Similarly, cyclohexane carboxylic acid, in our exercise, has a highly stable conjugate base due to resonance and the electronegative oxygen atom, making it the strongest acid among the given compounds.

Conjugate Bases

A conjugate base is formed when an acid loses a proton. The stability of a conjugate base greatly influences the original compound's acidity. Stable conjugate bases often result from delocalization of the negative charge over more atoms, which effectively spreads out and reduces the charge density.

In our example, when comparing the conjugate base of cyclohexane-1,3-dione to that of cyclohexanone, the former is more stabilized due to delocalization over two carbonyl groups. Consequently, the acid that leads to a more stable conjugate base is the stronger acid, hence cyclohexane-1,3-dione is a stronger acid than cyclohexanone.

Resonance Effect in Acidity

The resonance effect is a crucial factor in determining acidity. It refers to the delocalization of electrons within a molecule, which can stabilize the negative charge of a conjugate base after an acid releases a proton. The presence of a resonance effect typically makes a molecule a stronger acid.

For instance, the conjugate base of cyclohexene is less stable when compared to the ones from cyclohexane-1,3-dione or cyclohexane carboxylic acid because its negative charge can only be shared between two positions. On the other hand, in the carboxylate ion formed from cyclohexane carboxylic acid, the charge is spread over two oxygen atoms through resonance, resulting in a much more stable ion.

Inductive Effect in Acidity

The inductive effect plays an important role in acidity. It is the effect of electron withdrawal by substituent groups through sigma bonds. Electron-withdrawing groups can stabilize the conjugate base by delocalizing the negative charge away from the site where the proton was lost.

In the comparison of the given compounds, the carboxylate ion formed from cyclohexane carboxylic acid benefits from the inductive effect due to the electronegativity of the adjacent oxygen atoms which draw electron density away from the negative charge. This stabilizing interaction makes cyclohexane carboxylic acid one of the most acidic compounds among those listed.