Question

Arrange these compounds in order of increasing acidity: 2,4-dichlorophenol, phenol, cyclohexanol.

Short answer

Question: Arrange the compounds 2,4-dichlorophenol, phenol, and cyclohexanol in order of increasing acidity. Answer: The order of increasing acidity for the given compounds is: Cyclohexanol < Phenol < 2,4-dichlorophenol.

Step-by-step solution

Identify the acidic proton in each compound

The first step in comparing the acidity of the three compounds is to identify the acidic proton in each molecule, which is typically the most labile proton bonded to the most electronegative atom. In 2,4-dichlorophenol and phenol, the acidic protons are the hydroxyl protons (OH) bonded to the benzene ring. In cyclohexanol, the acidic proton is also the hydroxyl proton (OH) bonded to the cyclohexane ring.

Determine the stability of the conjugate bases

After a compound donates its acidic proton, it forms a conjugate base with a negative charge. Comparing the stability of the conjugate bases will help us predict the relative acidity of these compounds. A more stable conjugate base implies that the original compound is more acidic. For 2,4-dichlorophenol, the conjugate base will have a negative charge on the oxygen atom, with the two chlorine atoms on the benzene ring. For phenol, the conjugate base will also have a negative charge on the oxygen atom. For cyclohexanol, the conjugate base will have a negative charge on the oxygen atom as well.

Compare the delocalization and inductive effects on the conjugate bases

Now, we need to compare the electronic effects that stabilize the conjugate bases. Two primary effects to consider are resonance delocalization and the inductive effect of electron-withdrawing groups. In 2,4-dichlorophenol, the negative charge on the oxygen atom can be delocalized via resonance through the benzene ring. Additionally, the chlorine atoms withdraw electron density through an inductive effect, further stabilizing the conjugate base. In phenol, the negative charge can also be delocalized via resonance through the benzene ring, but there are no electron-withdrawing groups to stabilize the conjugate base. In cyclohexanol, the negative charge cannot be delocalized as the aliphatic ring does not have a π system, making its conjugate base the least stabilized of the three.

Arrange compounds in order of increasing acidity

Based on the stability of their conjugate bases, we can arrange the given compounds in order of increasing acidity: 1. Cyclohexanol (least acidic) - due to the lack of resonance stabilization and electron-withdrawing groups. 2. Phenol - due to resonance stabilization but no electron-withdrawing groups. 3. 2,4-dichlorophenol (most acidic) - due to resonance stabilization and the presence of electron-withdrawing chlorine atoms. So, the order of increasing acidity is: Cyclohexanol < Phenol < 2,4-dichlorophenol.

Key concepts

Conjugate Base Stability

Conjugate base stability is a crucial factor in determining the acidity of a compound. When an acid donates a proton, it forms its conjugate base, which carries a negative charge. The stability of this negatively charged species directly impacts how readily the compound will give up its proton. A more stable conjugate base means that the acid is more willing to lose the proton, making it more acidic.

For example, consider the conjugate bases of 2,4-dichlorophenol, phenol, and cyclohexanol. When these compounds lose their \( \text{OH} \) proton, their conjugate bases each have a negative charge on the oxygen atom. However, the stability of these negative charges differs due to their unique structures. The conjugate base of 2,4-dichlorophenol is more stable because it benefits from additional stabilizing factors like resonance delocalization and inductive effects, which we will explore further in this article. Cyclohexanol's conjugate base, however, lacks such features, resulting in its lower acidity.

Resonance Delocalization

Resonance delocalization plays a significant role in stabilizing the conjugate bases of phenolic compounds. This electronic stabilization occurs when a negative charge is spread over multiple atoms via a structure known as resonance. This effect lowers the energy of the conjugate base and increases the tendency of the original compound to release its proton.

Take 2,4-dichlorophenol and phenol as examples. Both compounds have a benzene ring that allows the negative charge, after the \( \text{OH} \) proton is lost, to be delocalized across the aromatic ring. This distribution of charge reduces the energy of the conjugate base, making the original molecules more acidic. In contrast, cyclohexanol lacks such a \( \pi \)-system, resulting in the inability of its negative charge to resonate, leading to a less stable conjugate base.

Inductive Effect

The inductive effect refers to the electron-withdrawing or electron-donating properties of atoms or groups attached to a compound. This effect can also impact the stability of conjugate bases and, in turn, influence acidity. An electron-withdrawing group, often electronegative, helps stabilize a negative charge, making the overall molecule more acidic.

In the case of 2,4-dichlorophenol, the chlorine atoms exert a strong inductive effect. They pull electron density towards themselves, away from the oxygen atom carrying the negative charge, which stabilizes the conjugate base. No such groups are present in phenol and cyclohexanol, making them less capable of stabilizing the additional negative charge. Thus, the presence of chlorines in 2,4-dichlorophenol contributes significantly to its increased acidity compared to the other compounds.