Question

Which of the following statements is not correct? (a) Cyclopentadiene has an acidic hydrogen (b) Squaric acid is highly acidic (c) Aniline is more basic than ammonia (d) Aniline is less basic than ammonia

Short answer

(c) Aniline is more basic than ammonia is not correct.

Step-by-step solution

Understand Acidic and Basic Nature

First, we need to determine the chemical nature of each compound mentioned in the statements. Cyclopentadiene is known to have an acidic hydrogen due to the resonance stabilization of the resulting anion. Squaric acid is a known highly acidic compound because it can donate protons and stabilize the negative charge through resonance. Aniline (an aromatic amine) contains an amine group bonded to a benzene ring, which is less basic than aliphatic amines or even ammonia because the lone pair of electrons on nitrogen can participate in resonance with the benzene ring.

Analyze Basicity

Compare the basicity of aniline and ammonia. Ammonia (NH_3) is a simple non-aromatic compound with a lone pair on the nitrogen making it quite basic. Aniline, however, due to the lone pair on the nitrogen being delocalized into the benzene ring, is less able to accept protons compared to ammonia. Hence, aniline is less basic than ammonia.

Evaluate Each Statement

(a) Cyclopentadiene has an acidic hydrogen - This statement is correct based on its chemical properties and resonance stabilization. (b) Squaric acid is highly acidic - This statement is correct as squaric acid can donate protons effectively. (c) Aniline is more basic than ammonia - Since aniline is less basic due to its resonance involvement with the benzene ring, this statement is incorrect. (d) Aniline is less basic than ammonia - As explained earlier, this is a correct statement.

Identify the Incorrect Statement

By evaluating each of the statements, the incorrect one is statement (c), which states that aniline is more basic than ammonia, contrary to the comparative basicity explained earlier.

Key concepts

Cyclopentadiene acidity

Cyclopentadiene, despite being a hydrocarbon, exhibits interesting acidic properties. Its acidity comes from the presence of an acidic hydrogen atom in its chemical structure. When this hydrogen is removed, it leaves behind a negatively charged ion, known as a cyclopentadienyl anion.
The key factor in cyclopentadiene's acidity is the resonance stabilization of the anion formed. The cyclopentadienyl anion is aromatic, with a ring structure that allows the negative charge to be delocalized over a larger area, enhancing its stability.
This resonance stability increases the tendency of cyclopentadiene to lose a hydrogen ion, making it acidic, even though it is a hydrocarbon, which are typically non-acidic.

Squaric acid properties

Squaric acid's highly acidic nature is attributed to its unique molecular structure, which allows for effective resonance stabilization upon donation of protons. Squaric acid has a square planar geometry and can donate protons from its hydroxyl groups.
When these protons are released, squaric acid forms a stable anion. This stability is due to resonance, where the negative charge is spread over several atoms, reducing its reactivity.
Moreover, its cyclic, stable structure supports the negative charge, making squaric acid more acidic than many other common acids. Its ability to stabilize extra electrons via resonance makes it a classic example in acidity discussions.

Aniline basicity

Aniline, an aromatic amine, is less basic compared to other amines like ammonia, owing to its unique structure. The nitrogen in aniline is directly bonded to a benzene ring, and the lone pair of electrons on the nitrogen can partake in resonance with the benzene's pi electron system.
This resonance effect delocalizes the lone pair, making it less available to react with protons, which in turn decreases its basicity. This differs sharply from ammonia, where the lone pair on nitrogen is free and more accessible for bonding.
Aniline's delocalized lone pair means it is not as strong a base as aliphatic amines, where the nitrogen atom does not participate in resonance.

Ammonia basicity

Ammonia ( H_3 ) is characterized by its strong basic nature, due to the presence of a lone pair of electrons on its nitrogen atom.
This lone pair is ready to accept protons, which makes ammonia a good base. Unlike aniline, ammonia does not have any benzene ring attachment that would offer resonance stabilization, thus its lone pair is localized and highly reactive.
This reactivity increases ammonia's basicity significantly, making it a stronger base than aniline, where the involvement of the lone pair in resonance within the benzene ring reduces its ability to attract protons.