Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Bruice8th EditionOrganic ChemistryISBN: 9780135213711Not the one you use?Change textbook

Bruice 8th Edition

Ch. 8 - Delocalized Electrons: Their Effect on Stability, pKa, and the Products of a Reaction • Aromaticity and Electronic Effects: An Introduction to the Reactions of Benzene

Problem 94e,f

Chapter 9, Problem 94e,f

e. Which has a greater Ka: cyclohexanol or phenol?
f. Which is a stronger acid: cyclohexanol or phenol?

Verified step by step guidance

Step 1: Understand the concept of Ka and acid strength. Ka, the acid dissociation constant, measures the extent to which an acid dissociates in water. A higher Ka value indicates a stronger acid because it dissociates more readily to release H⁺ ions.

Step 2: Analyze the chemical structures of cyclohexanol and phenol. Cyclohexanol is an alcohol with a hydroxyl group (-OH) attached to a cyclohexane ring, while phenol has a hydroxyl group attached to a benzene ring.

Step 3: Consider the resonance stabilization of the conjugate base. Phenol's conjugate base (phenoxide ion) is stabilized by resonance because the negative charge on the oxygen can delocalize into the aromatic ring. Cyclohexanol's conjugate base lacks this resonance stabilization, making it less stable.

Step 4: Compare the Ka values. From the image, cyclohexanol has a Ka of 1 × 10⁻¹⁶, which is extremely small, indicating it is a very weak acid. Phenol, although its Ka is not explicitly provided in the image, is known to have a significantly higher Ka due to the resonance stabilization of its conjugate base.

Step 5: Conclude the comparison. Phenol has a greater Ka and is a stronger acid than cyclohexanol because its conjugate base is stabilized by resonance, allowing it to dissociate more readily in water.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acidity and pKa

Acidity in organic chemistry is often measured by the acid dissociation constant (Ka), which quantifies the strength of an acid in solution. A higher Ka value indicates a stronger acid, meaning it dissociates more completely in water. The pKa, which is the negative logarithm of Ka, is commonly used to compare acid strengths; lower pKa values correspond to stronger acids.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, allowing for the delocalization of electrons. In the case of phenol, the negative charge on the phenoxide ion (the conjugate base) can be delocalized over the aromatic ring, enhancing its stability compared to cyclohexanol, which lacks such resonance stabilization.

Comparative Acid Strength

To determine which compound is a stronger acid, one must compare their ability to donate protons (H+). Phenol is generally a stronger acid than cyclohexanol due to the resonance stabilization of its conjugate base, phenoxide. This stabilization lowers the energy of the conjugate base, making it more favorable for phenol to lose a proton compared to cyclohexanol.

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Related Practice

Textbook Question

Which dienophile in each pair is more reactive in a Diels–Alder reaction?

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Textbook Question

Draw the resonance contributors for the phenolate ion.

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Textbook Question

Protonated cyclohexylamine has a Ka = 1 * 10-11. Using the same sequence of steps as in Problem 94, determine which is a stronger base: cyclohexylamine

or aniline.

e. Which has a greater Ka: cyclohexylammmonium ion or anilinium ion?

f. Which is a stronger acid: cyclohexylamine or aniline?

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Textbook Question

Draw the resonance contributors for phenol.

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Textbook Question

Which dienophile in each pair is more reactive in a Diels–Alder reaction?

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Textbook Question

The acid dissociation constant (K_a) for loss of a proton from cyclohexanol is 1 × 10^–16.