Structure of Ketones and Aldehydes

Aldehydes and ketones are organic compounds characterized by the presence of the carbonyl group, a carbon atom double-bonded to an oxygen atom (C=O). This functional group is crucial to their chemical reactivity and physical properties.

The Carbonyl Group (C=O) and Resonance

The carbonyl group is the defining structural feature of both aldehydes and ketones. It consists of a carbon atom double-bonded to an oxygen atom. This bond is highly polar due to the greater electronegativity of oxygen compared to carbon. The oxygen atom carries a partial negative charge (δ-), while the carbon atom carries a partial positive charge (δ+). This polarity makes the carbonyl group reactive towards nucleophiles and electrophiles. This polarity can be further understood by considering resonance.

We can represent the carbonyl group with two resonance structures:

The major resonance contributor shows the C=O double bond, with the oxygen bearing the partial negative charge and the carbon the partial positive charge. However, the minor resonance contributor depicts a single bond between carbon and oxygen, with a full positive charge on carbon and a full negative charge on oxygen. While the minor contributor is less stable, it contributes to the overall electronic distribution of the carbonyl group. Critically, it shows that the carbonyl carbon has significant partial positive character, making it *electrophilic* – that is, it is attracted to and reacts with electron-rich species (nucleophiles).

The resonance hybrid, a combination of these two contributors, emphasizes the partial positive charge on the carbonyl carbon. This is why carbonyl compounds are so often attacked by nucleophiles. The carbon is electron-deficient and seeks to accept a pair of electrons.

Aldehydes

Aldehydes have the general structure R-CHO, where R represents an alkyl or aryl group, and CHO is the aldehyde group. The carbonyl carbon in an aldehyde is always bonded to at least one hydrogen atom. The simplest aldehyde is formaldehyde (HCHO), where R = H. Other examples include acetaldehyde (CH₃CHO) and benzaldehyde (C₆H₅CHO).

• Formaldehyde (HCHO): The simplest aldehyde.
• Acetaldehyde (CH₃CHO): An aldehyde with a methyl group attached to the carbonyl carbon.
• Benzaldehyde (C₆H₅CHO): An aldehyde where the R group is a phenyl ring.

Ketones

Ketones have the general structure R-CO-R', where R and R' represent alkyl or aryl groups. The carbonyl carbon in a ketone is bonded to two carbon atoms. Examples include acetone (CH₃COCH₃), also known as propanone, and butanone (CH₃CH₂COCH₃).

• Acetone (CH₃COCH₃): The simplest ketone.
• Butanone (CH₃CH₂COCH₃): A ketone with an ethyl group and a methyl group attached to the carbonyl carbon.

Key Structural Features

Understanding the structure of aldehydes and ketones, particularly the nature of the carbonyl group, is essential for understanding their chemical behavior in the reactions discussed in subsequent chapters.