One of the following hydrocarbons is much more acidic than the others. Indicate which one, and explain why it is unusually acidic.

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Step 1: Analyze the structures of the hydrocarbons provided in the image. Each structure contains a five-membered ring with varying substituents. Pay attention to the presence of double bonds and the type of substituents attached to the ring.

Step 2: Recall that acidity in hydrocarbons is influenced by the stability of the conjugate base formed after deprotonation. A more stable conjugate base corresponds to a more acidic compound.

Step 3: Examine the substituents on the ring. In structure C, the substituent is an electron-withdrawing carbonyl group (C=O). Electron-withdrawing groups stabilize the negative charge on the conjugate base through resonance or inductive effects.

Step 4: Compare the other structures (A, B, and D). These structures lack electron-withdrawing groups like the carbonyl group in C. Their conjugate bases are less stabilized, making them less acidic.

Step 5: Conclude that structure C is unusually acidic due to the presence of the carbonyl group, which stabilizes the conjugate base through resonance and inductive effects, making it much more acidic than the other hydrocarbons.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Acidity of Hydrocarbons

The acidity of hydrocarbons is determined by their ability to donate protons (H+ ions). In organic chemistry, the presence of electronegative atoms or groups can stabilize the negative charge of the conjugate base formed after deprotonation, thus increasing acidity. Aromatic compounds often exhibit unique acidity due to resonance stabilization.

Resonance Stabilization

Resonance stabilization occurs when a molecule can be represented by multiple valid Lewis structures, allowing for the delocalization of electrons. This delocalization can stabilize the negative charge on a conjugate base, making the corresponding acid more acidic. In aromatic compounds, resonance can significantly influence acidity by distributing charge over the aromatic system.

Aromaticity

Aromaticity refers to the enhanced stability of cyclic compounds with conjugated pi electron systems that follow Huckel's rule (4n + 2 pi electrons). Aromatic compounds are often more stable than their non-aromatic counterparts, which can affect their reactivity and acidity. The unique electronic structure of aromatic compounds can lead to unusual acidity patterns among different hydrocarbons.