Question

Which of the following correctly lists hexanol, phenol, and cyclohexanol by increasing acidity of the hydroxyl hydrogen? (A) Phenol \(<\) hexanol \(<\) cyclohexanol (B) Cyclohexanol \(<\) hexanol \(<\) phenol (C) Cyclohexanol \(<\) phenol \(<\) hexanol (D) \(\quad\) Phenol \(<\) cyclohexanol \(<\) hexanol

Short answer

(B) Cyclohexanol < Hexanol < Phenol.

Step-by-step solution

Understanding Acidity

Acidity of a compound is determined by the ability of a substance to donate a hydrogen ion (proton). More acidic compounds have hydroxyl hydrogens that dissociate more easily.

Analyzing Phenol

Phenol has a hydroxyl group attached to a benzene ring. The resonance stabilization of the phenoxide ion makes phenol more acidic compared to aliphatic alcohols like hexanol and cyclohexanol.

Analyzing Cyclohexanol

Cyclohexanol has a hydroxyl group attached to a cyclohexane ring. This structure does not have the resonance stabilization present in phenol, making it less acidic.

Analyzing Hexanol

Hexanol is an aliphatic alcohol with a hydroxyl group attached to a straight chain of six carbon atoms. This structure lacks the resonance stabilization of phenol and the cyclic structure of cyclohexanol, making it the least acidic.

Determining Correct Order

Based on the analysis, the correct order by increasing acidity is: hexanol (least acidic), cyclohexanol, phenol (most acidic).

Choosing the Correct Option

The correct option that lists hexanol, phenol, and cyclohexanol by increasing acidity of the hydroxyl hydrogen is (B) Cyclohexanol < Hexanol < Phenol.

Key concepts

organic chemistry

Organic chemistry is a branch of chemistry that deals with the study of carbon-containing compounds. This field covers a wide range of substances, from small molecules like methane to large complex molecules such as DNA. At its core, organic chemistry focuses on understanding the properties, structures, and reactions of organic molecules.
Organic compounds are vital to life and include not only biological molecules but also synthetic materials like plastics and pharmaceuticals.
In our specific exercise, organic chemistry principles help us understand the acidity of alcohols, crucial for various chemical reactions and applications.

acidity

Acidity refers to the tendency of a substance to donate a hydrogen ion (H⁺), also known as a proton. The measure of a compound's acidity is often represented by its pKa value.
The lower the pKa, the stronger the acid, meaning it more readily donates a proton.
For alcohols, acidity is influenced by the structure of the molecule and the ability to stabilize the resulting anion after proton donation.
For example, phenol is more acidic than hexanol and cyclohexanol because its conjugate base is more stabilized through resonance.
Understanding these concepts allows us to correctly rank the compounds by their acidity.

alcohols

Alcohols are organic compounds containing a hydroxyl group (-OH) attached to a carbon atom. The general formula for an alcohol is R-OH, where 'R' represents an alkyl or aryl group.
Examples of alcohols include hexanol, cyclohexanol, and phenol. Hexanol is an aliphatic alcohol with a straight chain structure, whereas cyclohexanol features a cyclic structure, and phenol has a hydroxyl group attached to an aromatic benzene ring.
The structure of an alcohol influences its physical and chemical properties, including its acidity.
Differentiating between these types of alcohols is crucial in many chemical reactions and industrial processes.

resonance stabilization

Resonance stabilization occurs when a molecule can be represented by two or more structures (resonance forms) that contribute to the overall hybrid structure.
This concept is essential in understanding why phenol is more acidic compared to hexanol and cyclohexanol.
In phenol, the negative charge on the oxygen after losing a proton can be delocalized over the aromatic ring through resonance forms, making the conjugate base (phenoxide ion) more stable.
In the case of hexanol and cyclohexanol, such resonance stabilization is not possible, leading to less acidic compounds.
Therefore, resonance effects significantly impact the acidity of alcohols and other organic compounds.