Question

Draw the condensed structural formula for alkanes or a skeletal formula for cycloalkanes for each of the following: a. propane b. hexane c. heptane d. cyclopentane

Short answer

Propane: CH₃-CH₂-CH₃, Hexane: CH₃-CH₂-CH₂-CH₂-CH₂-CH₃, Heptane: CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃, Cyclopentane: pentagon shape.

Step-by-step solution

Understanding the structures

Identify if each compound is an alkane or a cycloalkane. Linear alkanes will have a condensed structural formula, while cycloalkanes will have a skeletal formula.

Drawing propane

Propane is a linear alkane with three carbon atoms. The condensed structure is shown by writing the atoms in sequence. the condensed structural formula for propane is: CH₃-CH₂-CH₃

Drawing hexane

Hexane is a linear alkane with six carbon atoms. The atoms are written in sequence: the condensed structural formula for hexane is: CH₃-CH₂-CH₂-CH₂-CH₂-CH₃

Drawing heptane

Heptane is a linear alkane with seven carbon atoms. The atoms are written in sequence: the condensed structural formula for heptane is: CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃

Drawing cyclopentane

Cyclopentane is a cycloalkane with five carbon atoms forming a ring structure. The skeletal formula represents each carbon by a corner in a pentagon: the skeletal formula for cyclopentane is:

Key concepts

Condensed Structural Formula

A condensed structural formula is a compact way of writing the structure of a molecule. It shows the sequence of atoms in the molecule without drawing all the bonds explicitly. For example, let’s take propane, which consists of three carbon atoms and eight hydrogen atoms.
Instead of drawing all the individual bonds, we simply write it as CH₃-CH₂-CH₃.
Here, the CH₃ and CH₂ represent the number of hydrogen atoms attached to each carbon. This format helps to quickly understand the structure while saving space.
Linear alkanes generally follow this type of notation as it makes it easier to comprehend the sequence of carbon and hydrogen atoms in a molecule.

Linear Alkanes

Linear alkanes are hydrocarbons that consist of only carbon and hydrogen atoms arranged in a straight or branched chain. They do not form rings. In linear alkanes, each carbon atom forms four single covalent bonds with either carbon or hydrogen atoms.
For example:
- Propane is a linear alkane with the condensed structural formula CH₃-CH₂-CH₃.
- Hexane, with six carbon atoms, is represented as CH₃-CH₂-CH₂-CH₂-CH₂-CH₃.
- Heptane, having seven carbon atoms, is written as CH₃-CH₂-CH₂-CH₂-CH₂-CH₂-CH₃.
Understanding these formulas is crucial for visualizing the carbon chain length and the hydrogen atoms associated with each carbon.

Cycloalkanes

Cycloalkanes are a subtype of alkanes wherein the carbon atoms are arranged in a ring or a cyclic structure. Unlike linear alkanes, cycloalkanes form a closed loop of carbon atoms.
The simplest example is cyclopentane, where five carbon atoms form a pentagonal ring. In skeletal formulas, each terminus of the line represents a carbon atom.
For cyclopentane, we depict it as a simple pentagon. This skeletal formula is concise and represents the ring structure efficiently without detailing every bond and atom.
Understanding the difference between linear and cyclic structures is essential in organic chemistry to predict the molecule's reactivity and properties.

Skeletal Formula

A skeletal formula is a simplified diagrammatic representation of an organic molecule. Here, carbon atoms are represented by the vertices (corners) or the ends of lines, and most hydrogen atoms are omitted for clarity.
This form of notation is particularly useful for depicting cycloalkanes and complex molecules.
For instance, with cyclopentane, a simple pentagon represents the skeletal formula. Each vertex of the pentagon stands for a carbon atom.
The skeletal formula allows chemists to quickly visualize the molecular structure without cluttering the diagram with unnecessary hydrogen atoms, making it easier to grasp and manipulate large and complex organic molecules.