Question

Draw the structure of each of the following compounds: (a) 2,2 -dimethylpentane (b) 3,3 -diethylpentane (c) 3-ethyl-2-methylpentane (d) 3 -ethylhexane

Short answer

Draw each compound by identifying the main chain and positioning the substituents as numbered.

Step-by-step solution

Understanding the Base Chain

Each compound name includes a base, which indicates the longest carbon chain in the molecule. For example, 'pentane' suggests a base chain of 5 carbon atoms. Recognize 'pentane' or 'hexane' as the base chain for the given compounds.

Identifying Substituents

Look at the prefix in each compound's name to identify the substituents: 'dimethyl', 'diethyl', or 'ethyl' tell us what groups are attached to the main carbon chain and how many there are. For example, '2,2-dimethyl' implies two methyl groups attached to the second carbon in the chain.

Locating Substituents on the Base Chain

Assign positions to each substituent based on the numbers indicated in the compound's name. For 2,2-dimethylpentane, place the two methyl groups on the second carbon of the pentane chain.

Constructing the Structure of (a) 2,2-dimethylpentane

Draw a straight chain of 5 carbons for 'pentane'. Then, add two CH₃ groups to the second carbon of the chain identifying the structure as 2,2-dimethylpentane.

Constructing the Structure of (b) 3,3-diethylpentane

Begin with a straight chain of 5 carbons for 'pentane'. Place one ethyl group (CH₂CH₃) on each side of the third carbon for the 3,3-diethyl arrangement.

Constructing the Structure of (c) 3-ethyl-2-methylpentane

Draw a 5-carbon chain for the 'pentane'. Attach an ethyl group (CH₂CH₃) to the third carbon and a methyl group (CH₃) to the second carbon to build 3-ethyl-2-methylpentane.

Constructing the Structure of (d) 3-ethylhexane

Create a chain of 6 carbon atoms for 'hexane'. Attach one ethyl group (CH₂CH₃) to the third carbon, forming the structure for 3-ethylhexane.

Key concepts

IUPAC Nomenclature

Understanding the International Union of Pure and Applied Chemistry (IUPAC) Nomenclature is paramount for naming organic compounds. This system helps in uniquely identifying molecular structures based on a set of standard rules. Each part of an IUPAC name indicates specific features of the molecule:

• The **base name** represents the longest continuous carbon chain. For instance, "pentane" means the base contains five carbon atoms, while "hexane" denotes six.


• **Substituents** are groups of atoms attached to the main chain, identified by prefixes like "methyl" or "ethyl".


• **Numerical prefixes**, such as "2,2" or "3,3," indicate the exact position of substituents on the carbon chain. For example, in "2,2-dimethylpentane," two methyl groups are on the second carbon of a pentane chain.

This naming convention allows chemists worldwide to accurately communicate the structure of compounds with ease and prevents ambiguity.

Structural Isomers

Structural isomers are fascinating because even though they have the same molecular formula, their atoms are arranged differently. This changes their physical and chemical properties. In the context of our exercise, when naming isomers:

• Each isomer has the same number of each type of atom, but the connectivity between them varies.


• "2,2-dimethylpentane" and "3,3-dimethylpentane" have identical molecular formulas, but the placement of methyl groups leads to entirely different structural arrangements.

When identifying isomers, look for differences in the carbon chain's layout and the positions of substituents. This diversity makes isomers versatile and essential in various applications like material science and pharmaceuticals.

Carbon Chain

A carbon chain forms the backbone of organic compounds and consists of connected carbon atoms in a sequence. Here's how carbon chains are fundamental to understanding organic structures:

• The **length** of a carbon chain determines the primary characteristics of the compound, such as "pentane" for a chain of five carbons or "hexane" for six.


• **Branching** occurs when substituents like methyl (CH₃) or ethyl (CH₂CH₃) groups connect to the chain, altering its shape and function.


• Chains can be **linear** or contain **branches**, leading to different structural possibilities and influencing properties like boiling point and solubility.

Recognizing the carbon chain's role gives context to other features described in IUPAC names and is key to visualizing molecular structures in organic chemistry.