Question

Draw a structure for each alkane. \begin{equation}\begin{array}{l}{\text { a. } 2,2 \text { -dimethylpentane }} \\\ {\text { b. } 3 \text { -isopropylheptane }} \\ {\text { c. } 4 \text { -ethyl- } 2,2 \text { -dimethylhexane }} \\ {\text { d. } 4,4 \text { -diethyloctane }}\end{array}\end{equation}

Short answer

For each alkane: a) Draw a five-carbon chain with two methyl groups on the second carbon; b) Draw a seven-carbon chain with an isopropyl group on the third carbon; c) Draw a six-carbon chain with an ethyl group on the fourth carbon and two methyl groups on the second carbon; d) Draw an eight-carbon chain with ethyl groups on both fourth carbons.

Step-by-step solution

Identifying the Main Chain

First, identify the longest continuous chain of carbon atoms in each molecule, which determines the base name of the alkane. The number of carbon atoms in this chain gives the prefix (e.g., pentane, heptane, hexane, octane).

Locating the Substituents

Determine the position and identity of any substituents (alkyl groups) attached to the main chain. Positions are indicated by numbers, and the branches are named by their alkyl group names (e.g., methyl, ethyl, propyl, etc.).

Drawing 2,2-Dimethylpentane

For 2,2-dimethylpentane, draw a chain of five carbon atoms (pentane). Add two methyl groups on the second carbon atom. Both methyl groups are on the same carbon, so the carbon is labeled with two '2's.

Drawing 3-Isopropylheptane

Draw a seven-carbon main chain (heptane). At the third carbon atom, attach an isopropyl group, which consists of a central carbon atom attached to two additional carbon atoms and six hydrogen atoms in a Y-shape.

Drawing 4-Ethyl-2,2-Dimethylhexane

For 4-ethyl-2,2-dimethylhexane, sketch a six-carbon main chain (hexane). Attach an ethyl group on the fourth carbon and two methyl groups on the second carbon.

Drawing 4,4-Diethyloctane

Make an eight-carbon main chain (octane). Add an ethyl group to each of the fourth carbon atoms.

Key concepts

Alkyl Substituent Identification

In organic chemistry, identifying the correct alkyl substituents in alkanes is an essential foundational skill. Alkyl substituents are simply side chains or branches off the main carbon chain of an alkane. These substituents are used to alter the properties of a molecule and are named based on the number of carbon atoms they contain with a 'yl' ending. For example, a one-carbon substituent is a methyl group, and a two-carbon substituent is an ethyl group.

When we look at complex molecules such as 3-isopropylheptane or 4-ethyl-2,2-dimethylhexane, identifying these substituents helps in understanding the molecule's structure. The first step in identifying these groups involves finding the longest carbon chain, which serves as the parent alkane. Subsequently, look at the branches coming off this chain. The position of each substituent on the main chain is indicated by a number, which corresponds to the carbon atom it is attached to. Additionally, if there are multiple identical substituents, prefixes such as di-, tri-, or tetra- are used to signify how many there are.

Identifying alkyl substituents is not just about naming, but understanding their shape and how they connect to the main chain. For example, isopropyl is a three-carbon group with a specific Y-shaped structure. Recognizing these can significantly help in predicting the physical and chemical properties of the molecule.

IUPAC Nomenclature

The International Union of Pure and Applied Chemistry (IUPAC) provides systematic guidelines for naming organic compounds, ensuring that each compound has a unique and universally understood name. When dealing with alkanes, IUPAC nomenclature describes their structure based on the number of carbons in the longest chain, the locations of substituents, and the identity of these substituents.

Steps for Naming Alkanes:

• Identify the Longest Chain: This gives the base name of the alkane, such as pentane or hexane.
• Number the Chain: Assign numbers to the carbon atoms in the main chain starting from the end nearest to the first substituent, ensuring the lowest possible numbers for the substituents.
• Name the Substituents: Each alkyl substituent gets a name based on its own number of carbons, and if there are multiples, use a prefix (di-, tri-, etc.).
• Combine the Names: Write the full name by listing the substituents in alphabetical order, each with its corresponding number, and end with the base name of the main chain.

These rules allow for consistent and clear communication regarding the structure of organic molecules among chemists.

Organic Molecule Representation

Representing organic molecules accurately is key to conveying their structure clearly. The representation of alkanes and their alkyl substituents should provide a visual overview of the molecule’s complete structure, including the main chain, branches, and their positions. This visualization holds great importance not only for proper identification and naming but also for understanding reactions and interactions within organic chemistry.

To represent an organic molecule such as an alkane, you may start with a skeletal line drawing or a more detailed structural formula. Skeletal drawings outline the carbon backbone with lines and assume hydrogen atoms wherever needed without showing them explicitly, while structural formulas depict every atom and bond, making it easier for beginners to follow. For the given example, 4-ethyl-2,2-dimethylhexane, a structural formula would show all carbon and hydrogen atoms with their bonds, indicating clearly the main hexane chain, the ethyl substituent, and the two methyl groups at the second carbon.

Additionally, three-dimensional perspective can be used to represent the spatial arrangement of groups within molecules, which can be vital for understanding sterics and the shape-dependent properties of the molecule. In any representation, consistency and clarity are vital to accurately conveying the molecule’s identity.