Problem 18 - Chemistry: The Central Science in SI Units, Global Edition

Question

Give the name or condensed structural formula, as appropriate: (a) 3-phenylpentane (b) 2,3-dimethylhexane (c) 3,3-dimethyloctane (d) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}\) e.

Step-by-Step Solution

Answer

(a) ``` C6H5 | CH3-CH2-CH-CH2-CH3 ``` (b) ``` CH3 CH3 | | CH3-CH-C-CH2-CH2-CH3 ``` (c) ``` CH3 | CH3-CH2-CH-C-CH2-CH2-CH2-CH3 | CH3 ``` (d) 3,5-dimethyl-6-ethylhexane

Step 1: (a) 3-phenylpentane

For 3-phenylpentane, we need to draw the structural formula. We know that it is a pentane molecule (5 carbon atoms in a chain) with a phenyl group (C6H5) attached at the third carbon atom. The structural formula can be drawn as: ``` C6H5 | CH3-CH2-CH-CH2-CH3 ```

Step 2: (b) 2,3-dimethylhexane

For 2,3-dimethylhexane, we need to draw the structural formula. We know that it is a hexane molecule (6 carbon atoms in a chain) with methyl groups (CH3) attached at the second and third carbon atoms. The structural formula can be drawn as: ``` CH3 CH3 | | CH3-CH-C-CH2-CH2-CH3 ```

Step 3: (c) 3,3-dimethyloctane

For 3,3-dimethyloctane, we need to draw the structural formula. We know that it is an octane molecule (8 carbon atoms in a chain) with methyl groups (CH3) attached at the third carbon atom, twice. The structural formula can be drawn as: ``` CH3 | CH3-CH2-CH-C-CH2-CH2-CH2-CH3 | CH3 ```

Step 4: (d) $\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2}$

For the given condensed structural formula, we need to write the IUPAC name. This is a hexane molecule (6 carbon atoms in a chain): ``` CH3-CH2-CH-CH2-CH-CH3 | | CH3 CH3 CH3 ``` To name it, identify the substituents attached to the main hexane chain. We have a methyl group at carbons 3 and 5, and two methyl groups at carbon 6. Therefore, the IUPAC name for this molecule is 3,5-dimethyl-6-ethylhexane.

Key Concepts

IUPAC Nomenclature

The IUPAC Nomenclature is the universally accepted method for naming organic chemical compounds and is essential for students in organic chemistry. Its purpose is to provide a unique and understandable name for every distinct compound, ensuring that chemists around the world can communicate clearly. This systematic approach involves several rules and conventions, such as identifying the longest carbon chain as the main structure and noting any substituents attached to it.

For example, when naming "3-phenylpentane," the base chain "pentane" is identified first, which indicates a five-carbon chain. The group "phenyl" (a benzene ring minus one hydrogen) is attached to the third carbon in this chain, which is reflected in its naming.

It is vital for students to become familiar with these conventions, as it allows them to accurately describe complex molecules and understand various chemical reactions and processes.

Structural Formula

A structural formula represents the spatial arrangement of atoms within a molecule, providing important insights into its properties and behavior. Structural formulas go beyond the molecular formula by illustrating how atoms are connected in a compound. This makes it easier to understand how the molecule might react chemically.

Let's consider 2,3-dimethylhexane, where the base molecule, hexane, consists of six carbon atoms in a straight chain. The structural formula shows methyl groups attached to the second and third carbon atoms. This visual arrangement makes it easy to comprehend how these groups interact within the molecule.

• They help in identifying isomers, which are compounds with the same molecular formula but different structures.
• Provide a clear picture of the molecule for predicting physical and chemical properties.

Mastering the ability to draw and interpret structural formulas is an important skill in organic chemistry.

Alkane Naming

Alkanes are the simplest hydrocarbons, consisting only of carbon and hydrogen atoms connected by single bonds, forming saturated molecules. They are named based on the number of carbon atoms in their longest chain, with a "-ane" suffix.

The example of "3,3-dimethyloctane" showcases how alkane naming works. The prefix "octa-" indicates eight carbon atoms in the main chain. The term "dimethyl" signals the presence of two methyl ( CH_3) substituents positioned on the third carbon.

To name an alkane:

• Identify the longest continuous carbon chain.
• Number the chain, giving the lowest numbers to substituents.
• Name and position all substituents before the base name.

Understanding the rules of alkane naming is crucial to depicting chemical compounds accurately.

Hydrocarbon Chains

Hydrocarbons are compounds that exclusively contain carbon and hydrogen. Among them, hydrocarbon chains are a common feature, forming the backbone of various organic molecules. These chains can vary in length and branching, influencing the compound's properties and reactivity.

In the exercise for naming, focusing on hydrocarbon chains like in the molecule represented as \( \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right) \mathrm{CH}_{2} \mathrm{CH}\left(\mathrm{CH}_{3}\right)_{2} \), helps in recognizing the main chain, which in this case is a hexane. The compound showcases additional branching with methyl groups, impacting its nomenclature and chemistry.

• The length of the chain usually determines the base name.
• Branching and substituents affect the properties and stability of the compound.
• It serves as a foundation in more complex compounds.

Focusing on hydrocarbon chains provides the basis for understanding more intricate substances encountered in organic chemistry.