Question

Select wrong statements: (a) Phenols turn blue litmus to red. (b) Reactivity of methanol with sodium metal is more than that of isopropyl alcohol (c) Methanol gives iodoform test. (d) Phenol reacts with \(\mathrm{Na}_{2} \mathrm{CO}_{3}\) and liberates \(\mathrm{CO}_{2}\) gas

Short answer

Statements (c) and (d) are wrong.

Step-by-step solution

Understand Phenol's Reaction with Litmus

Phenols are acidic and tend to turn blue litmus paper to red due to their ability to donate protons (H+) and form phenoxide ions. Thus, statement (a) is correct.

Compare Reactivity with Sodium

Methanol, being a primary alcohol, is more reactive towards sodium than isopropyl alcohol, a secondary alcohol. This is because the steric hindrance around the hydroxyl group is less in methanol, making it more reactive. Therefore, statement (b) is correct.

Evaluate the Iodoform Test for Methanol

The iodoform test is positive for compounds with a methyl ketone structure or a secondary alcohol with a methyl group adjacent to the alcohol-bearing carbon. Methanol does not have such a structure and does not give a positive iodoform test. Thus, statement (c) is incorrect.

Analyze Phenol with Sodium Carbonate

Phenols are not acidic enough to react with \[\mathrm{Na}_{2} \mathrm{CO}_{3} \\] (sodium carbonate) to liberate \[\mathrm{CO}_{2} \\] gas, as they are weaker acids than carbonic acid. Therefore, statement (d) is incorrect.

Key concepts

Phenol

Phenol is a unique compound in organic chemistry characterized by a hydroxyl group directly bonded to an aromatic benzene ring. This structure gives phenol distinctive properties that differ from typical alcohols. Phenols are slightly acidic due to the ability of the hydroxyl group to donate a proton, resulting in the formation of a stable phenoxide ion. This acidic nature explains why phenols can turn blue litmus paper to red.
Unlike stronger acids like hydrochloric acid, phenol does not fully ionize in water, but the resonance stability of the phenoxide ion enhances its acidity compared to regular alcohols. One of the practical implications of this is seen in laboratory settings, where phenols can undergo various reactions, including forming esters and ethers, which are not as readily accessible to other types of alcohols.

Reactivity of Alcohols

Alcohols are a class of compounds known for containing an -OH group attached to a carbon atom. This hydroxyl group is responsible for the general reactivity of alcohols. Among different alcohols, methanol, a primary alcohol, is less hindered compared to secondary or tertiary alcohols like isopropyl alcohol.

• **Methanol and Sodium Reaction:** Methanol is more reactive with sodium metal because it can more easily donate a proton to form sodium methoxide. The smaller molecular structure of methanol allows the reaction to proceed faster.
• **Effect of Steric Hindrance:** In bulkier alcohols, such as isopropyl alcohol, the presence of additional alkyl groups surrounding the hydroxyl group makes it less accessible, reducing its reactivity compared to methanol.

Alcohols, therefore, have varying reactivity based on their structure, with primary alcohols typically showing higher reactivity due to less steric bulk.

Iodoform Test

The iodoform test is a qualitative chemical test used to detect the presence of methyl ketones and specific secondary alcohols. This test is characterized by the formation of yellow precipitate iodoform ( Chi _3 ). However, not all alcohols give a positive iodoform test.
For the iodoform test to be positive, the compound must possess a methyl group adjacent to the carbonyl group in ketones or the hydroxyl-bearing carbon in secondary alcohols. Methanol does not fulfill these structural requirements as it lacks the necessary methyl ketone structure and thus gives a negative iodoform test, demonstrating that it's essential to understand the molecular structure for predicting test outcomes.
This classic test is widely used to distinguish between various types of alcohols in organic chemistry.

Acid-Base Reactions

Acid-base reactions are fundamental in chemistry, involving the transfer of protons ( H ^+ ) between reactants. Phenols, although acidic, are much weaker acids than carbonic acid, a common inorganic acid. This explains why phenols do not react with sodium carbonate ( Na _2 CO _3 ) to release carbon dioxide gas.
Phenols' inability to displace carbonate ions in sodium carbonate highlights their lower acidity. In practical terms, this knowledge helps chemists predict reaction pathways and the feasibility of forming specific products. Understanding acid-base behavior also aids in the selection of proper reagents and conditions necessary for various synthesis processes.
Comprehending acid-base properties assists in grasping how phenolic compounds interact in different chemical environments—critical for students engaging in advanced organic chemistry studies.