Question

Draw structures for the following compounds. (a) 3 -ethylpentane (b) 2,3 -dimethylpentane (c) 2,4 -dimethylpentane (d) 2,2 -dimethylpentane

Short answer

Draw each pentane with specified substituents at indicated positions.

Step-by-step solution

Identify the Parent Chain

For each compound, identify the parent chain, which is the longest continuous chain of carbon atoms. For pentane, the parent chain has 5 carbon atoms.

Determine the Substituents

Check the substituents and their positions according to the names of the compounds. (a) In 3-ethylpentane, there is an ethyl group at the 3rd position. (b) In 2,3-dimethylpentane, there are methyl groups at the 2nd and 3rd positions. (c) In 2,4-dimethylpentane, there are methyl groups at the 2nd and 4th positions. (d) In 2,2-dimethylpentane, there are two methyl groups at the 2nd position.

Sketch the Parent Chain

For each compound, draw a straight chain of 5 carbon atoms for pentane.

Add the Substituents

Add the substituents to the parent chain as determined in Step 2. (a) Add an ethyl group (-C2H5) at the 3rd carbon of pentane. (b) Add a methyl group (CH3-) at both the 2nd and 3rd carbons of pentane. (c) Add a methyl group (CH3-) at both the 2nd and 4th carbons of pentane. (d) Add two methyl groups (CH3-) at the 2nd carbon of pentane.

Key concepts

Structural Isomers

Structural isomers are molecules with the same molecular formula but different structural arrangements. This means they have different connectivity between atoms.

They can look quite different even if they share the same number of each type of atom. Structural isomers often have distinct chemical and physical properties, which makes them interesting in chemistry.

For example, consider 3-ethylpentane and 2,3-dimethylpentane. Both have the same number of carbon atoms, but the arrangement of the atoms is different, leading to isomerism. Understanding structural isomers involves visualizing different "shapes" that the same collection of atoms can take.

Alkanes

Alkanes are a class of hydrocarbons containing only single bonds between carbon atoms. They are also known as saturated hydrocarbons.

The simplest alkanes follow the formula \( C_nH_{2n+2} \), where \( n \) is the number of carbon atoms. For example, pentane has five carbon atoms and follows the formula \( C_5H_{12} \).

Alkanes are characterized by their relative inertness and are mostly found as gases or liquids, depending on their molecular weight. Understanding alkanes is essential for learning about their isomers and how they interact with other organic compounds.

Hydrocarbon Substituents

Hydrocarbon substituents are groups of atoms containing carbon and hydrogen that are attached to the main carbon chain of a molecule.

They play a critical role in defining the identity of isomers. Common substituents include methyl \( (CH_3) \), ethyl \( (C_2H_5) \), and propyl \( (C_3H_7) \).

In the example compounds, substituents are added to the pentane chain at specific positions. The position is critical as it changes the compound's properties and name. Properly identifying hydrocarbon substituents and assessing their positions is a key step in nomenclature.

IUPAC Nomenclature

The IUPAC nomenclature is a standardized method of naming chemical compounds. This system ensures that each compound has a unique and descriptive name.

For naming alkanes with substituents, the longest carbon chain is identified first. Then, substituents are numbered and named according to their position on this chain.

For instance, 2,4-dimethylpentane indicates that there are two methyl groups on the 2nd and 4th carbons of a five-carbon chain. This system helps in avoiding confusion and enables chemists worldwide to easily understand the structure from the name alone.