Question

All of the following are true with respect to carbonyls EXCEPT: (A) \(\quad\) the carbonyl carbon is electrophilic. (B) \(\quad\) the carbonyl oxygen is electron-withdrawing. (C) a resonance structure of the functional group places a positive charge on the carbonyl carbon. (D) the \(\pi\) electrons are mobile and are pulled toward the carbonyl carbon.

Short answer

D

Step-by-step solution

- Identify Electrophilicity of Carbonyl Carbon

The carbonyl carbon is electrophilic because it is bonded to a more electronegative oxygen atom which withdraws electron density from the carbon, making the carbon electron-deficient and electrophilic.

- Confirm Electron-Withdrawing Nature of Carbonyl Oxygen

The carbonyl oxygen is electron-withdrawing due to its high electronegativity, which pulls electron density towards itself, increasing the partial positive charge on the carbonyl carbon.

- Examine Resonance Structure

A resonance structure of the carbonyl group can place a positive charge on the carbonyl carbon and a negative charge on the oxygen. This occurs when the \(\pi\) electrons move toward the oxygen.

- Analyze Mobility and Direction of π Electrons

The \(\pi\) electrons in the carbonyl double bond are not pulled toward the carbon; they are pulled toward the more electronegative oxygen atom due to its higher electronegativity.

- Identify the Correct Answer

Based on steps 1 to 4, option (D) is incorrect because the \(\pi\) electrons are pulled toward the oxygen, not the carbon.

Key concepts

Electrophilicity of carbonyl carbon

The carbonyl carbon is highly electrophilic, which means it is an electron-seeking entity. This happens because the carbon is double-bonded to an oxygen atom, which is much more electronegative. The oxygen atom pulls the electron density away from the carbonyl carbon. This causes the carbon to become electron-deficient, making it an attractive site for nucleophiles (electron-rich species) to attack. This property is exceptionally crucial in many organic reactions such as nucleophilic addition, which frequently appears on the MCAT.
Understanding this concept will help you grasp why carbonyl compounds are so reactive in organic chemistry.

Electron-withdrawing nature of carbonyl oxygen

The carbonyl oxygen is known for its strong electron-withdrawing capability. Oxygen is highly electronegative, meaning it has a strong tendency to attract electrons. In a carbonyl group, the oxygen pulls electron density away from the carbonyl carbon.
This withdrawal of electron density increases the partial positive charge on the carbonyl carbon, making it even more electrophilic.
Hence, in organic reactions, the carbonyl carbon is often the site of attack by nucleophiles.
By comprehension of this concept, you will understand why oxygen's electronegativity plays a critical role in the reactivity of carbonyl compounds.

Resonance structures of carbonyls

Resonance structures depict the delocalization of electrons within a molecule. For a carbonyl group, one resonance structure places a positive charge on the carbonyl carbon and a negative charge on the oxygen.
This happens when the πelectrons move towards the oxygen atom. Resonance structures are not actual structures but a way of showing the electron distribution within a molecule.
They provide a more comprehensive understanding of the molecule's reactivity and structure.
Indicatively, the fact that a resonance structure can place a positive charge on the carbonyl carbon further enhances its electrophilic nature.

Mobility of π electrons in carbonyls

The πelectrons in a carbonyl group are not stationary; they are mobile and contribute to the molecule's reactivity. However, these electrons are more likely to be found nearer the oxygen atom rather than the carbon.
This is because oxygen is more electronegative and effectively

pulls

the πelectrons towards itself.
This understanding is key to knowing why the carbonyl carbon does not attract πelectrons, contrary to some misconceptions.
This concept is important when considering the electronic structure of carbonyl compounds and predicting how they will react in different chemical environments.