Question

Following are structural formulas for phenol and cyclohexanol along with the acid dissociation constants for each. Propose an explanation for the fact that phenol is a considerably stronger acid than cyclohexanol.

Short answer

Answer: Phenol is a stronger acid than cyclohexanol because the conjugate base of phenol (phenoxide ion) is stabilized by resonance. In contrast, the conjugate base of cyclohexanol (cyclohexoxide ion) does not have resonance stabilization. A more stable conjugate base increases the likelihood of the acid losing its hydrogen ion, making phenol a stronger acid.

Step-by-step solution

Analyze the structure of phenol and cyclohexanol.

To begin, draw the structural formulas of both molecules. Phenol has a hydroxyl group (-OH) attached to a benzene ring, while cyclohexanol has a hydroxyl group attached to a six-membered ring. Both molecules have an -OH functional group, which is responsible for the acidic property.

Identify the acidic hydrogen.

Now, identify the acidic hydrogen in both the molecules. The acidic hydrogen in phenol and cyclohexanol is the hydrogen atom attached to the oxygen atom in the -OH functional group.

Examine the acid dissociation constants.

Compare the Ka values provided for both phenol and cyclohexanol. The larger the value of Ka, the stronger the acid, because it indicates that a greater degree of dissociation into the conjugate base and hydrogen ion occurs. Based on the given Ka values, see that phenol has a considerably higher Ka value than cyclohexanol, making it a stronger acid.

Consider the stability of the conjugate base formed.

After the acidic hydrogen dissociates, both phenol and cyclohexanol will form conjugate bases. Phenol forms a phenoxide ion (C6H5O-) while cyclohexanol forms a cyclohexoxide ion (C6H11O-). The greater the stability of the conjugate base, the more likely the acid is to lose its hydrogen ion and thus, the stronger the acid.

Investigate the influence of resonance.

In the case of phenol, the negative charge on the oxygen atom in the phenoxide ion can be delocalized over the benzene ring through resonance, which stabilizes the conjugate base. In contrast, the negative charge on the oxygen atom in the cyclohexoxide ion cannot delocalize, as there are no available p-orbitals for the electrons to move into.

Propose the explanation.

The fact that phenol's conjugate base can participate in resonance, while cyclohexanol's conjugate base cannot, provides an explanation for the observed difference in acidity. When an acid dissociates, it forms a more stable conjugate base if that conjugate base is stabilized by resonance (as in the case of phenol). Thus, phenol is a considerably stronger acid than cyclohexanol due to the resonance stabilization of its conjugate base (phenoxide ion).

Key concepts

Acid Dissociation Constant (Ka)

Understanding the acid dissociation constant, often abbreviated as Ka, is crucial in evaluating the strength of an acid. It measures an acid's tendency to donate a proton (H+) to the surrounding environment, a process we term as 'dissociation'. The dissociation of an acid in water is a reversible reaction, where the acid, HA, gives away a proton, resulting in the formation of water and a conjugate base, A-.

The acid dissociation constant equation is represented as follows: \[ Ka = \frac{[H^+][A^-]}{[HA]} \]
Higher Ka values are indicative of a stronger acid, as they suggest a greater concentration of H+ ions, signifying more complete dissociation. In the context of phenol and cyclohexanol, the Ka value for phenol is higher, signifying that it donates protons more readily, confirming its status as the stronger acid.

Conjugate Base Stability

Upon losing a proton, acids form what we call a 'conjugate base'. The stability of this conjugate base plays a paramount role in the acid's strength; greater stability means a stronger acid. This stability is often influenced by several factors, such as the inductive effect, hybridization of the atom bearing the negative charge, and resonance.

In comparing phenol and cyclohexanol, we observe that the phenoxide ion (the conjugate base of phenol) is more stable than the cyclohexoxide ion (the conjugate base of cyclohexanol). This is primarily due to resonance stabilization, where the negative charge on the oxygen in phenoxide can be delocalized over the aromatic ring. The inability of the cyclohexoxide ion to distribute the negative charge similarly means it's less stable, making cyclohexanol a weaker acid by comparison.

Resonance in Organic Chemistry

Resonance is a concept in organic chemistry that describes the delocalization of electrons within certain molecules, providing heightened stability. It involves the distribution of a charge or bonding among multiple atoms through overlapping p-orbitals, effectively creating multiple structures, often termed 'resonance structures', which contribute to a molecule's overall hybrid structure.

Regarding phenol, the phenoxide ion benefits from resonance stabilization. The negative charge on the oxygen atom post-dissociation is not localized but instead is 'spread out' across the benzene ring. This dispersal of charge diminishes its intensity at any given point, thereby increasing stability and, correspondingly, the acidity of phenol. Cyclohexanol lacks this property as its structure does not allow for resonance, highlighting the significance of resonance in determining acid strength in organic molecules.