Question

Aldehydes and ketones both contain the carbonyl group, yet the properties of these two types of compounds are different enough that they are classified separately. Without looking back at the text, draw the structures of the ketone and the aldehyde that have three carbon atoms. Name each of these compounds.

Short answer

The 3-carbon aldehyde is propanal, with the structure: H H H | | | H-C-C-C=O The 3-carbon ketone is propanone (acetone), with the structure: H H H | | | H-C-C-C || O

Step-by-step solution

Draw the aldehyde structure with three carbon atoms

We need to create a 3-carbon chain (prop-) and add a carbonyl group on one end. Remember that aldehydes have the carbonyl group bonded to at least one hydrogen atom. The structure will be: H H H | | | H-C-C-C=O

Name the aldehyde

The aldehyde has three carbon atoms, so it is a prop- (for three) aldehyde. Its name is thus "propanal."

Draw the ketone structure with three carbon atoms

Similar to the aldehyde, we need to create a 3-carbon chain (prop-) and add a carbonyl group, but this time within the carbon chain. Remember that ketones have the carbonyl group bonded to two carbon atoms. The structure will be: H H H | | | H-C-C-C || O

Name the ketone

The ketone has three carbon atoms and the carbonyl group on the second carbon atom, so it is a prop- (for three) ketone. Its name is thus "propanone" or "acetone." Now we have drawn and named both the aldehyde and ketone with three carbon atoms. The aldehyde is propanal, and the ketone is propanone (acetone).

Key concepts

Carbonyl Group

The carbonyl group is a functional group that is crucial in organic chemistry. It consists of a carbon atom double-bonded to an oxygen atom, which is represented as \(C=O\). This group is found in several classes of organic compounds, including aldehydes and ketones.

What makes the carbonyl group incredibly significant is its reactivity. The electronegativity difference between carbon and oxygen creates a polar bond, with oxygen being more electronegative. This polarity facilitates reactions with various nucleophiles, making carbonyl compounds central to numerous chemical reactions.

- In aldehydes, the carbonyl group is positioned at the end of the carbon chain, bonded to at least one hydrogen atom.
- In ketones, this group is bonded to two carbon atoms, never at the end of a carbon chain.

The position and configuration of the carbonyl group significantly impact the properties and reactivity of the compound they are part of.

Chemical Structures

Understanding chemical structures is vital when dealing with organic compounds like aldehydes and ketones. These structures offer a visual representation of the arrangement of atoms within molecules and provide insights into the chemical properties of the compound.

For aldehydes, the general formula is \(RCHO\), where \(R\) represents an alkyl group, and \(CHO\) denotes the aldehydic carbonyl group. The carbonyl feature is always terminal, meaning it appears at the end of the carbon chain. For example, in propanal, the structure is:

• Three carbon atoms in a straight chain
• The carbonyl group at the terminal position
• Represented as \(CH_3CH_2CHO\)

Ketones, on the other hand, have a general formula of \(RCOR'\), indicating the carbonyl group is flanked by two carbon groups. With propanone, or acetone, the structure is:

• Three carbon atoms with a carbonyl group in the middle
• Each end of the carbonyl is attached to a carbon
• Represented as \(CH_3COCH_3\)

These structural differences between aldehydes and ketones lead to variations in their chemical behavior.

Organic Chemistry Nomenclature

Naming organic compounds correctly is essential in chemistry to ensure precise communication of molecular structures. Organic chemistry nomenclature helps in identifying and describing the molecular composition and structural attributes of compounds.

For aldehydes, naming starts with identifying the longest carbon chain that includes the carbonyl group, followed by the suffix '-al' indicating an aldehyde. For example:

• 'Propanal' is derived from a three-carbon chain (prop-) and the aldehyde suffix '-al.'

Ketone naming involves the longest carbon chain containing the carbonyl group combined with the suffix '-one.' For instance:

• 'Propanone' highlights a three-carbon chain (prop-) with the ketone suffix '-one.'

Substituents or branches stemming from the main carbon chain are indicated by prefixes. Numerical locants show the position of the carbonyl group in ketones but are not needed in aldehydes, as it is always at the terminal end. Proper nomenclature ensures clarity and precision in chemical identification.