Question

\(a\) -hydrogens of a ketone are acidic due to: resonance stabilization. the electron-withdrawing properties of the alkyl groups. the electronegative carbonyl oxygen. A. I only B. I and III only C. II and III only D. I, II, and III

Short answer

B. I and III only

Step-by-step solution

Understand the given options

First, identify the properties listed in the problem: I. Resonance stabilization. II. Electron-withdrawing properties of the alkyl groups. III. Electronegativity of the carbonyl oxygen.

Analyze Resonance Stabilization

Ketones have a carbonyl group which can participate in resonance. The α-hydrogens, once deprotonated, create an enolate ion that can be stabilized by resonance with the carbonyl group. Hence, option I is correct.

Evaluate Electron-Withdrawing Properties of Alkyl Groups

Alkyl groups are generally electron-donating and do not exhibit electron-withdrawing properties. Therefore, option II is incorrect.

Check Electronegativity of Carbonyl Oxygen

The carbonyl oxygen is highly electronegative and withdraws electron density from the α-hydrogens, making them acidic. Therefore, option III is correct.

Conclusion

Since options I and III are correct, the correct answer is B. I and III only.

Key concepts

Resonance Stabilization

Resonance stabilization plays a crucial role in the acidity of α-hydrogens in ketones. When an α-hydrogen is removed (deprotonated), an enolate ion is formed. This enolate ion is stabilized through resonance with the carbonyl group.
The resonance effect allows the negative charge to be delocalized over the oxygen atom and the carbon atom, reducing its overall energy.
Lower energy states are more stable, making the proton removal easier, and thus, increasing the acidity of the α-hydrogens.
Here’s a simple way to visualize it:

• Start with your ketone, which has at least one α-hydrogen.
• Remove the α-hydrogen to form an enolate ion.
• The enolate ion will have a negative charge which can resonate between the carbon atom that lost the hydrogen and the oxygen atom of the carbonyl group.

Understanding resonance stabilization is essential to grasp why the α-hydrogens in ketones exhibit higher acidity.

Carbonyl Group

The carbonyl group (C=O) is a defining feature of ketones and plays a significant role in many of their chemical properties.
The carbonyl carbon is electrophilic due to the polarization caused by the electronegative oxygen atom. This means it attracts electron density away from the α-hydrogens, making them more acidic.
Consider these points about the carbonyl group:

• The polar nature of the carbonyl group means it has partial positive and negative charges.
• The partial positive charge on the carbon makes it susceptible to nucleophilic attack.
• The double bond between carbon and oxygen allows for resonance stabilization when the α-hydrogen is removed.

By understanding the nature of the carbonyl group, one can appreciate its influence on the acidity of α-hydrogens in ketones.

Electronegativity of Carbonyl Oxygen

The electronegativity of the carbonyl oxygen is another key factor influencing the acidity of α-hydrogens. Oxygen is highly electronegative, meaning it strongly attracts electron density.
This creates a polarized carbonyl group where the oxygen has a partial negative charge, and the carbon has a partial positive charge.
The implications of oxygen’s high electronegativity are:

• It withdraws electron density from the surrounding atoms, including the α-hydrogens.
• By pulling electron density away, it increases the ease with which the α-hydrogen can be removed (deprotonated).
• This makes the α-hydrogen more acidic compared to hydrogens in other types of molecules.

Therefore, the high electronegativity of the carbonyl oxygen is a potent factor in making the α-hydrogens in ketones acidic.
Understanding this concept is crucial when studying ketone reactivity and their behavior in different chemical environments.