Question

An aldehyde and a ketone each have the molecular formula \(\mathrm{C}_{3} \mathrm{H}_{6} \mathrm{O}\). Draw the structure for each isomer.

Short answer

The aldehyde is propanal (\(\text{CH}_3\text{CH}_2\text{CHO}\)) and the ketone is propanone (\(\text{CH}_3\text{COCH}_3\)).

Step-by-step solution

Identify the Aldehyde

Aldehydes have the general form \( ext{R-CHO}\), where \( ext{R}\) is a hydrocarbon group. For the given formula \( ext{C}_3 ext{H}_6 ext{O}\), consider the simplest aldehyde structure which is propanal: \( ext{CH}_3 ext{CH}_2 ext{CHO}\). This fits the formula and structural requirements.

Identify the Ketone

Ketones have the general structure \(\text{RC(=O)R'}\), where \( ext{R}\) and \( ext{R'}\) are hydrocarbon groups. For \(\text{C}_3\text{H}_6\text{O}\), the appropriate ketone is propanone (commonly known as acetone): \(\text{CH}_3\text{COCH}_3\). The carbonyl group is at the central carbon, satisfying both ketone structural requirements and the molecular formula.

Key concepts

Aldehyde

Aldehydes are a common class of organic compounds found in many natural and synthetic substances. These compounds are characterized by their carbonyl group, which consists of a carbon atom double-bonded to an oxygen atom (represented as C=O). In aldehydes, this carbonyl group is always located at the end of a carbon chain, giving it the general formula

• R-CHO,

where R represents any hydrocarbon group. Unlike other carbonyl-containing compounds, aldehydes have their C=O bond attached directly to a hydrogen atom, which makes them easier to oxidize.

One familiar example of an aldehyde is formaldehyde, which is used as a preservative. Meanwhile, acetaldehyde is found in plants, ripe fruits, and is also a byproduct of alcohol metabolism in the human body. The aldehyde in the exercise is propanal, or

• (CH₃CH₂CHO).

This structure fulfills the

• C₃H₆O

formula by having three carbon atoms, six hydrogen atoms, and one oxygen atom bonded as required. This positioning and structure confer unique reactivity and properties to aldehydes, making them pivotal in many chemical reactions.

Ketone

Ketones are another set of organic compounds similar to aldehydes but with a distinctive structural feature. Instead of having the carbonyl group at the end of the carbon chain, ketones have it positioned between two carbon atoms. This leads to the general structural formula

• RC(=O)R',

where R and R' stand for any alkyl or aryl group.

The most well-known ketone, and incidentally the ketone in our exercise, is propanone, commonly known as acetone. Its structure is

• (CH₃COCH₃),

where the oxygen is double-bonded to the central carbon atom, making it fulfill its molecular formula

• C₃H₆O.

Ketones differ from aldehydes in their reactivity; they do not oxidize easily and generally require more vigorous conditions for such reactions to occur. Because of this stability, ketones are widely used in industrial applications as solvents and in chemical synthesis. Acetone, for example, is a common solvent in products like nail polish remover.

Isomers

Isomers are molecules that share the same chemical formula but have different structural arrangements. This concept is crucial in chemistry because even a simple change in arrangement can lead to vastly different properties and behaviors in a molecule.

There are different types of isomerism:

• Structural isomerism involves differences in the connectivity of atoms in the molecule.
• Stereoisomerism involves the same connectivity but different spatial orientations of the atoms.

In the given exercise, we see structural isomers with

• propanal and propanone

as examples. Although both have the same formula,

• C₃H₆O,

their different placements of the carbonyl group define their distinct properties. Such differences underscore the diversity and adaptability of chemical compounds, enabling a vast array of functionalities and applications.