Question

Draw a structure for each alkane. (a) 2 -methylbutane (b) 3 -ethyl-2-methylhexane (c) 3 -isopropylheptane (d) 2,5-dimethyloctane

Short answer

The structural formulas for the alkanes are as follows: (a) 2-methylbutane: a four-carbon chain with a methyl group on the second carbon. (b) 3-ethyl-2-methylhexane: a six-carbon chain with an ethyl group on the third carbon and a methyl on the second. (c) 3-isopropylheptane: a seven-carbon chain with an isopropyl group on the third carbon. (d) 2,5-dimethyloctane: an eight-carbon chain with methyl groups on the second and fifth carbons.

Step-by-step solution

- Identify the main chain for 2-methylbutane

For 2-methylbutane start by identifying the main chain. Butane means a four-carbon alkane. The prefix '2-methyl' indicates that there is a methyl group (one carbon) attached to the second carbon of the butane chain.

- Draw the structure for 2-methylbutane

Draw a straight chain of four carbon atoms in a line to represent butane. Then add a methyl group to the second carbon atom from either end.

- Identify the main chain for 3-ethyl-2-methylhexane

For 3-ethyl-2-methylhexane, hexane is the main chain which contains six carbon atoms. The '3-ethyl' and '2-methyl' prefixes indicate that there is an ethyl group (two carbons) and a methyl group (one carbon) on the third and second carbon of the hexane, respectively.

- Draw the structure for 3-ethyl-2-methylhexane

Draw a straight chain of six carbon atoms. Attach a methyl group to the second carbon and an ethyl group to the third carbon.

- Identify the main chain for 3-isopropylheptane

For 3-isopropylheptane, the main chain is heptane, which means a seven-carbon chain. The prefix '3-isopropyl' indicates that there is an isopropyl group (a three carbon group with a branch at the first carbon) linked to the third carbon of the heptane chain.

- Draw the structure for 3-isopropylheptane

Draw a straight chain of seven carbon atoms to represent heptane. Then, on the third carbon atom, add an isopropyl group which consists of a central carbon atom attached to two additional carbon atoms.

- Identify the main chain for 2,5-dimethyloctane

For 2,5-dimethyloctane, the main chain is octane which has eight carbon atoms. The prefix '2,5-dimethyl' suggests that there are methyl groups attached to the second and the fifth carbon atoms of the octane chain.

- Draw the structure for 2,5-dimethyloctane

Draw a straight chain of eight carbon atoms to represent octane. Attach a methyl group to both the second and fifth carbon atoms of the chain.

Key concepts

Organic Chemistry

Organic chemistry is a branch of chemistry which studies the structure, properties, composition, reactions, and synthesis of organic compounds that contain carbon. Organic molecules form the basis of life and are found in a vast array of products, from medications to plastics. Understanding organic chemistry is fundamental when it comes to grasping how various molecules interact in living organisms and how they can be synthesized or modified in the laboratory.

When studying alkanes, the simplest family of organic compounds, we dive into the core of organic chemistry. Alkanes are hydrocarbons, meaning they consist only of hydrogen and carbon atoms, linked by single bonds. Learning about their structure and how to represent them is essential for students as it provides a foundation for more complex organic chemistry concepts.

Molecular Structure

The molecular structure refers to the three-dimensional arrangement of atoms within a molecule. Understanding this structure is crucial for predicting the physical and chemical properties of compounds. In the case of alkanes, which are saturated hydrocarbons, they have a tetrahedral geometry around each carbon atom, due to sp3 hybridization. The bond angles in a perfect tetrahedral structure are approximately 109.5 degrees.

The molecular structure of alkanes can be represented in different ways, including structural formulas and skeletal formulas. These representations help chemists to visualize molecules, understand their properties, and predict their reactivity. Although alkanes are considered to be quite unreactive due to their strong C-H and C-C bonds, grasping their structure is key to understanding more complex reactions involving organic molecules.

Hydrocarbon Nomenclature

Hydrocarbon nomenclature is the systematic method of naming hydrocarbon compounds based on their structure. The International Union of Pure and Applied Chemistry (IUPAC) has developed the rules for naming these compounds to provide consistency and clarity. For alkanes, the nomenclature depends on the number of carbon atoms in the longest continuous chain and the presence of any branches or substituents.

• Linear alkanes are named according to the number of carbon atoms in the chain, with a suffix '-ane'. For example, 'butane' has four carbon atoms.
• When alkyl groups are attached to the main chain, prefix such as 'methyl-' or 'ethyl-' indicate the type and position of the branch.
• Numbering the carbon atoms starts from the end nearest to the first branch to provide the lowest possible numbers for the substituents, such as '2-methylbutane' not '3-methylbutane'.

Understanding these naming conventions is critical for effective communication in chemistry and for deciphering the structure of molecules from their names.

Skeletal Formula

The skeletal formula is a shorthand representation of a molecule's structure which simplifies the drawing by omitting hydrogen atoms bonded to carbon. This type of formula is widely used in organic chemistry because it makes complex structures more manageable and focuses on the arrangement of the carbon skeleton. In this system, every bend and end of a line represents a carbon atom.

When drawing skeletal formulas for alkanes:

• Lines represent carbon-carbon bonds, and the ends of lines indicate carbon atoms that continue to be bonded to hydrogen atoms until each carbon adheres to the tetravalent rule, having four bonds in total.
• Functional groups and branches are usually represented by their respective chemical symbols or abbreviations at the position where they attach to the main carbon chain.
• Understanding how to interpret and draw skeletal formulas is vital for students to quickly and correctly represent complex organic molecules.

For example, the skeletal formula for 2-methylbutane depicts a main chain of three line segments (representing four carbon atoms), with an additional branch (representing the methyl group) attached to the second carbon.