Arrange the following according to their increasing acidity:

A. 2,4,6-Trinitrophenol

B. Propan-1-ol

C. 3-Nitrophenol

D. 3,5-Dinitrophenol

E.4-Nitrophenol

Choose the correct answer from the options given below:

B, E, C, D, A

The correct answer is option 2. B, E, C, D, A.

Let us explore the acidity of the given compounds in detail, focusing on how substituents affect the acidity of phenol and related compounds.

Factors Affecting Acidity:

Inductive Effect: Electron-withdrawing groups (EWGs) like nitro groups (-NO₂) stabilize the negative charge on the phenoxide ion, thus increasing acidity. Electron-donating groups (EDGs) like alkyl groups decrease acidity by destabilizing the phenoxide ion.

Resonance Effect: EWGs stabilize the phenoxide ion through resonance, making the compound more acidic. EDGs have the opposite effect by destabilizing the phenoxide ion.

Detailed Analysis of Each Compound:

B. Propan-1-ol (B)

Propan-1-ol is a simple alcohol with no aromatic ring. Its acidity is primarily due to the hydroxyl group, and it does not benefit from additional stabilization mechanisms. It is less acidic compared to phenolic compounds. The absence of an aromatic ring and electron-withdrawing groups means it has the lowest acidity among the listed compounds.

E. 4-Nitrophenol 

The nitro group at the 4-position is an electron-withdrawing group that stabilizes the phenoxide ion via resonance. This increases the acidity of phenol but not as much as compounds with multiple electron-withdrawing groups. The nitro group at the 4-position increases the acidity of phenol by stabilizing the phenoxide ion, but the effect is less pronounced than in compounds with multiple EWGs.

C. 3-Nitrophenol 

The nitro group at the 3-position exerts an electron-withdrawing effect through resonance, which increases the acidity of phenol. However, the effect is somewhat less than in 4-nitrophenol because the nitro group at the 3-position is less effective in stabilizing the phenoxide ion compared to the 4-position. The nitro group at the 3-position contributes to increased acidity, but not as significantly as the 4-position nitro group.

D. 3,5-Dinitrophenol

The presence of two nitro groups (at the 3 and 5 positions) significantly increases the acidity. These nitro groups provide strong electron-withdrawing effects via both inductive and resonance effects, leading to enhanced stabilization of the phenoxide ion. The combined electron-withdrawing effects of the two nitro groups make 3,5-dinitrophenol more acidic than compounds with only one nitro group.

A. 2,4,6-Trinitrophenol 

The presence of three nitro groups significantly enhances the acidity of phenol. These groups exert strong electron-withdrawing effects both through inductive and resonance effects, leading to maximum stabilization of the phenoxide ion. The three nitro groups provide the greatest degree of electron-withdrawing effects, resulting in the highest acidity among the listed compounds.

Increasing Order of Acidity: Based on the analysis:

B. Propan-1-ol : Lowest acidity due to the lack of aromatic stabilization and electron-withdrawing groups.

E. 4-Nitrophenol : Higher acidity compared to phenol, but less than compounds with multiple nitro groups.

C. 3-Nitrophenol : Slightly more acidic than 4-nitrophenol, but less than compounds with two or three nitro groups.

D. 3,5-Dinitrophenol : More acidic than the single nitro groups due to two nitro groups.

A. 2,4,6-Trinitrophenol : Highest acidity due to three nitro groups providing maximum electron-withdrawing effects.

So, the correct order of increasing acidity is: Option 2: B, E, C, D, A.