Question

Draw structural formulas for (a) 2 -pentanone,(b) hexanal, and (c) pentanoic acid.

Short answer

2-pentanone: CH3-C(=O)-CH2-CH2-CH3; hexanal: CH3-(CH2)4-CHO; pentanoic acid: CH3-(CH2)3-COOH.

Step-by-step solution

Identify Functional Groups

2-pentanone is a ketone, hexanal is an aldehyde, and pentanoic acid is a carboxylic acid. Recognizing these functional groups helps in constructing the structural formula for each compound.

Draw 2-Pentanone

2-pentanone has five carbon atoms with a ketone functional group at the second carbon. Draw a five-carbon chain and place an oxygen double-bonded to the second carbon atom (C=O). The structural formula is CH3-C(=O)-CH2-CH2-CH3.

Draw Hexanal

Hexanal contains six carbon atoms with an aldehyde functional group at the end. Draw a six-carbon chain and place an oxygen double-bonded to the terminal carbon along with a single hydrogen (C=O-H). The structural formula is CH3-(CH2)4-CHO.

Draw Pentanoic Acid

Pentanoic acid has a carboxylic acid group at the end of a five-carbon chain. Draw a five-carbon chain and attach -COOH (carboxylic group) at the last carbon. The structural formula is CH3-(CH2)3-COOH.

Key concepts

Structural Formulas

In organic chemistry, structural formulas represent the arrangement of atoms within a molecule. They provide a visual map of the chemical structure, displaying how the atoms are bonded together. This information is crucial for understanding the properties and behaviors of organic compounds.
For instance, understanding the structural formula of 2-pentanone involves mapping out its five-carbon chain and placing functional groups correctly. This formula is CH3-C(=O)-CH2-CH2-CH3, clearly depicting each atom's placement and the double-bonded oxygen, which is part of the ketone functional group.

• Structural formulas help predict molecular geometry.
• They show functional groups present in a compound.
• Provide insight into chemical reactivity and interactions.

Understanding structural formulas is a stepping stone to mastering organic chemistry as they bridge theoretical chemistry with practical application.

Functional Groups

Functional groups are specific groupings of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. Recognizing these groups is essential in identifying and classifying organic compounds.
Functional groups determine how the molecule behaves and reacts with other substances. For instance:

• Keto: Identified by the presence of a carbonyl group (C=O) within the carbon chain.
• Aldehyde: Contains a carbonyl group at the end of the carbon chain, coupled with a hydrogen (C=O-H).
• Carboxylic Acid: Features a carboxyl group (COOH), which is acidic and reacts with bases.

Understanding these groups provides students with insight into the chemistry of the compounds and how they can be manipulated for various reactions.

Organic Compounds

Organic compounds are molecules primarily made of carbon and hydrogen, often containing other elements like oxygen, nitrogen, and sulfur. They are the basis of life on earth and are characterized by their carbon-based backbones. The versatility of carbon allows it to form stable bonds with many elements, paving the way for a diverse range of compounds.
In organic chemistry, these compounds are classified based on their functional groups and structure. For example:

• Alkanes: Saturated hydrocarbons with single bonds.
• Ketones: Organic compounds with a carbonyl group bonded to two carbon atoms.
• Aldehydes and carboxylic acids highlight the diversity by the placement of the carbonyl group either within or terminal to the carbon chain.

The vast diversity of organic compounds stems from the variety of possible combinations and arrangements of carbon with other elements.

Ketones

Ketones are organic compounds characterized by the presence of a carbonyl group (C=O) bonded to two carbon atoms within the carbon chain. They are a prevalent functional group in organic chemistry and are often used in the synthesis of other compounds.
The simplest ketone is acetone, but the exercise specifically mentions 2-pentanone, which has the structure CH3-C(=O)-CH2-CH2-CH3 with the carbonyl group on the second carbon. This exhibits the general formula of ketones, R-CO-R', where R and R' are carbon-containing groups.

• Ketones are important in the manufacture of plastics and other polymers.
• They often have distinct odors and are found in various biological processes.
• They serve as solvents in many industrial and consumer products.

Understanding ketones is key to recognizing their role in bio-synthetic pathways and industrial applications.

Aldehydes

Aldehydes consist of a carbonyl group (C=O) connected to at least one hydrogen atom, and are located at the terminal end of a carbon chain. They are essential in a range of chemical reactions and are prevalent in various biological processes and industries.
Hexanal, mentioned in the exercise, exemplifies an aldehyde with the general formula RCHO, where R is a carbon-containing group. Its structural formula CH3-(CH2)4-CHO demonstrates the presence of the carbonyl group at the end of a six-carbon chain.

• Aldehydes are key intermediates in the synthesis of alcohols, acids, and resins.
• They naturally occur in many fragrances and flavors.
• Common aldehydes include formaldehyde and acetaldehyde.

The understanding of aldehydes facilitates comprehension of various synthesis processes in both natural and artificial contexts.

Carboxylic Acids

Carboxylic acids are organic compounds that contain a carboxyl functional group (–COOH). They are weak acids but play significant roles in organic synthesis and metabolic pathways. The carboxyl group, made up of a carbonyl group attached to a hydroxyl group, is highly reactive.
Pentanoic acid from the exercise features a five-carbon chain attached to this group, with its formula CH3-(CH2)3-COOH. Carboxylic acids can be found naturally and synthetically and have a wide range of applications in pharmaceuticals, food, and industry.

• They react with alcohols to form esters, which are used in perfumes and flavorings.
• Carboxylic acids are precursors in the synthesis of more complex chemicals.
• They serve as intermediates in the biochemical synthesis of fatty acids and amino acids.

Recognizing carboxylic acids is crucial to understanding their function and utility in everyday life and industrial processes.