Structure and nomenclature in aldehydes, ketones and carboxylic acids

Aldehydes, ketones, and carboxylic acids are important functional groups in organic chemistry that you will encounter frequently at the high school level. These compounds are not only important for understanding organic reactions, but they also have many applications in everyday life. In this lesson, we will cover their structures, how to name them, and examples to solidify your understanding. Let's take a deeper look at each type:

Aldehyde

The general formula for an aldehyde is RCHO, where R is any alkyl or aryl group, and CHO represents the aldehyde group. In an aldehyde group, a carbon atom is double-bonded to an oxygen atom (carbonyl group C=O) and single-bonded to a hydrogen atom.

      Hey,
    RCH
    

The structure of an aldehyde places greater emphasis on the carbonyl group at the end of the carbon chain, which distinguishes it from a ketone.

Nomenclature of aldehyde

Naming aldehydes in IUPAC nomenclature involves replacing -e ending of the corresponding alkane with -al. Here are some examples:

• Methanal (HCHO) is commonly known as formaldehyde.
• Ethanal (CH3CHO) is known as acetaldehyde.
• Propionaldehyde is obtained from propane, named propanal. Its formula is CH3CH2CHO.

For substituted aldehydes, the chain is numbered starting from the carbon of the aldehyde group.

Ketones

The general formula for a ketone is RCOR', where both R and R' are alkyl or aryl groups. Unlike aldehydes, ketones have a carbonyl group (C=O) located between two carbon atoms.

      Hey,
    Rcr'
    

The carbonyl group is always located within the carbon skeleton, which distinguishes ketones from aldehydes, where the carbonyl is located at the extreme end.

Nomenclature of ketones

In the IUPAC system, the naming of ketones involves replacing -e with -one of the corresponding hydrocarbon. The chain is numbered such that the carbonyl group receives the lowest possible number:

• Propanone, which has the molecular formula CH3COCH3, is often called acetone.
• Butanone, CH3COCH2CH3, is another example, also known as methyl ethyl ketone.

If other substituents are present in the ketone, their positions should be indicated using numbers based on the position of the carbonyl carbon.

Carboxylic acid

Carboxylic acids are characterized by the presence of a carboxyl group, COOH, and have the general formula RCOOH. The structure contains a carbonyl and a hydroxyl group attached to the same carbon.

      Hey,
    RC-OH
    

This unique functional group gives carboxylic acids their acidic properties, as can be seen in their ability to donate hydrogen ions (H⁺).

Nomenclature of carboxylic acids

In IUPAC nomenclature, carboxylic acids are named by removing -e from the alkane name and adding -oic acid:

• Methanoic acid, HCOOH, is also known as formic acid.
• Ethanoic acid, CH3COOH, commonly called acetic acid, is widely used in vinegar.

The carboxyl carbon is considered the first in the chain, so no numbering is needed if the chain remains unbranched. For substituted acids, numbering begins at the carboxyl carbon.

Cumulative example

Let's consolidate our knowledge with some visual examples that demonstrate compound structures and then practice naming those compounds.

Example structures

Consider these examples where you are tasked with identifying the type of compound and proposing an IUPAC name:

• Identify CH3CH2CH2CHO: This is an aldehyde, named butanal.
• CH3COCH2CH2CH3 Identify: This is a ketone, called pentan-2-one.
• Identify CH3CH2COOH: This is a carboxylic acid called propanoic acid.

Comparative aspects

It is important to understand the difference between aldehydes, ketones, and carboxylic acids. Both aldehydes and ketones contain a carbonyl group, but their position changes their reactivity and properties. Aldehydes may be more prone to oxidation than ketones. In contrast, carboxylic acids can form hydrogen bonds due to the hydroxyl group, affecting their boiling point and solubility.

The purpose of this lesson is to provide a comprehensive overview, thereby improving your ability to identify, name, and understand these organic compounds. With consistent practice, you will find that these components are easy to recognize and apply in a variety of contexts.

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