Question

An aromatic aldehyde and an aromatic ketone each have the molecular formula \(\mathrm{C}_{8} \mathrm{H}_{8} \mathrm{O}\). Draw the structure for each isomer.

Short answer

Aromatic aldehyde: Benzaldehyde, aromatic ketone: Acetophenone.

Step-by-step solution

Understanding the Formula

The given molecular formula is \( \mathrm{C}_8 \mathrm{H}_8 \mathrm{O} \). This formula indicates compounds that are constituted by 8 carbons, 8 hydrogens, and 1 oxygen atom. Since we are dealing with an aromatic aldehyde and a ketone, the presence of a benzene ring (aromatic ring) is implicit in both isomers.

Drawing the Aromatic Aldehyde

For the aromatic aldehyde isomer, the aldehyde group \( (-CHO) \) must be attached to the aromatic ring. The structure consists of a benzene ring with an aldehyde group (\( -\mathrm{CHO} \)) directly attached: .This structure represents benzaldehyde, an aromatic aldehyde.

Drawing the Aromatic Ketone

In the aromatic ketone isomer, the ketone group \( (>\mathrm{C}=\mathrm{O}) \) needs to be bonded to the benzene ring. The structure of phenylacetone includes a carbonyl group \( (>\mathrm{C}=\mathrm{O}) \) attached to the benzene with an intervening methylene (\(\mathrm{CH}_2\)) group: .This is the structure of acetophenone, which is an aromatic ketone.

Key concepts

Aldehyde

Aldehydes are organic compounds characterized by the presence of a carbonyl group connected to at least one hydrogen atom. The carbonyl group (\( >\text{C}=\text{O}\)) is a significant functional group where the carbon atom is double-bonded to an oxygen atom. For aldehydes, this carbon is also bonded to a hydrogen atom and possibly a hydrocarbon group.

• The simplest aldehyde is formaldehyde (\(\text{HCHO}\)).
• The distinguishing feature of aldehydes from other carbonyl containing compounds is that the carbonyl carbon is always at the 'end' of the molecule.
• In aromatic aldehydes, like benzaldehyde, the carbonyl group is attached to a benzene ring.

Aromatic aldehydes often exhibit sweet, pleasant odors and are used in fragrances and flavorings. Their reactivity is also utilized in various chemical reactions, especially in the creation and study of other organic compounds.

Ketone

Ketones are a class of organic compounds that include a carbonyl group bonded to two other carbon atoms. Unlike aldehydes, the carbonyl carbon in ketones is situated within the carbon chain rather than at the end.

• An example of a simple ketone is acetone (\(\text{CH}_3\text{COCH}_3\)).
• In aromatic ketones, the carbonyl group is bonded to one or more aromatic rings. In the case of acetophenone, the carbonyl group is directly attached to a benzene ring way of an intervening methylene group (\(\text{CH}_2\)).
• Ketones are important in both industrial applications and biological processes.

They typically have distinct smells and are found in many fragrances and essential oils. The structure and properties of ketones grant them extensive use in the synthesis of nylon and other polymers.

Molecular Formula

A molecular formula represents the number and type of atoms present in a molecule. It provides the simplest way to report the actual number of each type of atom in a compound without conveying how they are connected. For example, the molecular formula for the exercise we are examining, \(\text{C}_8\text{H}_8\text{O}\), indicates that the compound is made up of 8 carbon atoms, 8 hydrogen atoms, and a single oxygen atom.

• The molecular formula does not directly provide information about the structure or arrangement of atoms.
• Different compounds can share the same molecular formula, while having distinct structures; these are known as isomers.
• In the case of the given exercise, the placement of carbonyl groups (\(\text{-CO}\)) will distinguish the aromatic aldehyde from the ketone.

Understanding molecular formulas is essential for recognizing isomers and predicting physical and chemical properties. This basic knowledge supports further exploration into the vast world of organic chemistry.