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

Step-by-step solution

Analyze Cyclopentadiene

Cyclopentadiene is a compound with the formula C\(_5\)H\(_6\). It has a cyclic structure and one of its hydrogens can be removed to form a stable anion known as the cyclopentadienyl anion. The stability is due to aromaticity, which means cyclopentadiene has an acidic hydrogen. Thus, statement (a) is correct.

Evaluate Squaric Acid

Squaric acid, C\(_4\)H\(_4\)O\(_4\), is a cyclic compound with acidic properties due to its structure, allowing it to donate protons easily, making it highly acidic. Therefore, statement (b) is correct.

Compare Aniline and Ammonia

Aniline is an aromatic amine and its basicity is decreased due to the resonance interaction with the aromatic ring, which withdraws electron density from the nitrogen. Ammonia, on the other hand, does not have this resonance effect and is a stronger base. This means aniline is less basic than ammonia. Hence, statement (c) is incorrect and statement (d) is correct.

Key concepts

Cyclopentadiene

Cyclopentadiene is a fascinating compound with a formula of C\(_5\)H\(_6\). Although it is a hydrocarbon, it possesses a unique acidic property. This may seem unusual given that hydrocarbons are generally neutral. However, cyclopentadiene can lose a hydrogen ion (H\(^+\)), leaving behind the cyclopentadienyl anion. This anion is remarkably stable, thanks to a concept called aromaticity. Aromaticity in cyclopentadienyl arises because the anion is cyclic, planar, and has a fully conjugated \pi electron system that follows Huckel's rule of 4n+2 \pi electrons, with n being an integer. In this case, there are six \pi electrons, fitting n=1, which makes the anion especially stable. This stability makes cyclopentadiene more likely to lose a hydrogen ion, acting as an acid. Therefore, the statement that cyclopentadiene has an acidic hydrogen is indeed correct.

• Cyclopentadiene's hydrogen can be abstracted to form a stable anion.
• The aromatic stability of the anion is due to Huckel's rule.
• This makes cyclopentadiene behave as an acid, which is rare for hydrocarbons.

Squaric Acid

Squaric acid, chemically known as C\(_4\)H\(_4\)O\(_4\), is a compound with highly acidic properties. Its acidity comes from its ability to easily donate protons. This capability is supported by the unique structure of squaric acid, which features a cyclic framework composed of four carbon atoms and oxygen that is poised to release protons.The high acidity of squaric acid can be attributed to resonance stabilization. When squaric acid donates a proton, the negative charge that results can be delocalized over multiple atoms within the molecule. This delocalization spreads the charge, thereby stabilizing the structure of the deprotonated form.Thus, it is correct to say that squaric acid is highly acidic due to its stability upon losing a proton. It's an interesting compound not only in terms of its chemical behavior but also due to its versatile uses.

• Squaric acid can readily donate protons due to its structure.
• Resonance allows the negative charge to be distributed, enhancing stability.
• Its intrinsic stability contributes to its high acidity.

Aniline versus Ammonia

Aniline and ammonia are both nitrogen-containing compounds, but they differ notably in their basicity. Aniline is an aromatic amine, which means the nitrogen is bonded to an aromatic benzene ring. On the other hand, ammonia is a simple amine with no aromatic influence. The basicity of an amine like aniline is decreased due to resonance effects. In aniline, the lone pair of electrons on the nitrogen can participate in resonance with the aromatic ring, delocalizing the electron density. This interaction withdraws electron density from the nitrogen, making it less able to donate its lone pair. As such, aniline is less basic compared to ammonia, where the nitrogen's lone pair is localized and better available for donation. Therefore, the notion that aniline is more basic than ammonia is incorrect. Ammonia, having no resonance interaction, retains its electron pair fully for donation, making it a stronger base.

• Aniline's basicity is reduced due to resonance with the aromatic ring.
• Ammonia has localized electrons, increasing its ability to act as a base.
• Ammonia is stronger in basicity compared to aniline despite both containing nitrogen.