Question

Without looking back in the text, draw structural formulas and give the common names for the three isomers of pentane, \(\mathrm{C}_{5} \mathrm{H}_{12}\)

Short answer

The three isomers of pentane (C5H12) are: 1. Pentane: \[ H-C-C-C-C-C-H \] 2. Isopentane (or 2-methylbutane): \[ H-C-C-C-H \] |/ \| \| \ H H-C H H H 3. Neopentane (or 2,2-dimethylpropane): \[ H-C-C-H |/ \| H H-C-H | H \]

Step-by-step solution

Identify the ways to arrange the carbon atoms

First, we need to find the possible ways to arrange the carbon atoms. There are three such arrangements: 1. Arrange the five carbons in a straight chain 2. Arrange four carbons in a straight chain and attach the fifth carbon to the second carbon in the chain 3. Arrange three carbons in a straight chain and attach the two remaining carbons to the second carbon in the chain Now that we have identified the distinct arrangements of carbon atoms, we can build the structural formulas for each isomer by adding hydrogen atoms.

Draw the structural formulas and add hydrogen atoms

For each of the three arrangements, we will draw the structural formula by adding hydrogen atoms so that each carbon atom forms four bonds. 1. Isomer with five carbons in a straight chain: H H H H H | | | | | \[ H-C-C-C-C-C-H \] | | | | | H H H H H 2. Isomer with four carbons in a straight chain and the fifth carbon attached to the second carbon in the chain: H H | | H H-C H | | | | \[ H-C-C-C-H \] | | | H H H 3. Isomer with three carbons in a straight chain and the remaining two carbons attached to the second carbon in the chain: H | H H-C H | | | | \[ H-C-C-H \] | | | H H-C-H | H

Give the common names for the three isomers

Now that we have the structural formulas, we can identify and provide the common names for each isomer. 1. Isomer with five carbons in a straight chain: Common name: Pentane 2. Isomer with four carbons in a straight chain and the fifth carbon attached to the second carbon in the chain: Common name: Isopentane (or 2-methylbutane) 3. Isomer with three carbons in a straight chain and the remaining two carbons attached to the second carbon in the chain: Common name: Neopentane (or 2,2-dimethylpropane)

Key concepts

Structural Formulas

When we talk about structural formulas in chemistry, we're referring to a unique way of drawing the molecular structure of a compound to represent the atoms and their specific arrangement. The structural formula for a compound such as pentane, which has the molecular formula \( \mathrm{C}_{5} \mathrm{H}_{12} \) shows how the atoms are bonded to each other.

For example, with pentane, the structural formula will ensure each carbon atom forms four bonds to honor the tetravalent nature of carbon. Hydrogen atoms will fill the remaining bonds to complete the valency of carbon. The structural formula provides a visual representation, and from it, one can grasp the molecule's geometry easier than just looking at the molecular formula. It's the structural formula that allows us to differentiate between isomers – molecules with the same molecular formula but different arrangements of atoms.

Drawing these formulas requires you to understand the basic placement of atoms and bonds. As the step-by-step solution shows, by arranging the carbons differently, we can visualize all three isomers of pentane – which include straight-chain pentane, isopentane with a branch, and neopentane with two branches. Each structural formula will have a distinct shape and reflects how the atoms are linked in three-dimensional space.

Carbon Atom Arrangements

The arrangement of carbon atoms is pivotal in organic chemistry since it leads to the diversity of organic compounds. The carbon chain can be straight, branched, or even form rings. For hydrocarbons like pentane \( \mathrm{C}_{5} \mathrm{H}_{12} \), variations in the carbon skeleton lead to the formation of isomers.

In the context of pentane isomers, the exercise emphasizes distinguishing the arrangements: a straight chain, a chain with one branch, and a chain with two side branches. The first isomer has a straight chain of five carbon atoms, showing that carbons can connect in a continuous unbranched line. The second isomer, where one carbon is connected to the second in a chain, illustrates a simple branching, introducing more complexity into the structure. Finally, the third isomer reveals a central carbon connected to four others, which is an even more branched and compact arrangement.

These different arrangements lead to different three-dimensional shapes and physical properties, even though the molecular formula remains the same. It’s important to note that each carbon atom will form four single covalent bonds either with other carbon atoms or with hydrogen atoms, satisfying the carbon’s valency.

Hydrocarbon Nomenclature

Naming organic compounds, such as hydrocarbons, follows a systematic approach known as nomenclature. The nomenclature helps chemists communicate efficiently the structure of molecules. In our pentane example, understanding the nomenclature involves recognizing the base name 'pentane' that indicates a five-carbon alkane chain.

Adjusting this name with prefixes and suffixes helps us distinguish the isomers. An 'iso-' prefix suggests a branch at the end of an alkane chain, thus 'isopentane' refers to a chain of four carbons with a methyl group on the second carbon. The name 'neopentane' implies an even more branched structure, in this case, two methyl groups on the second carbon. This systematic naming encodes structural information and helps prevent confusion.

Using the International Union of Pure and Applied Chemistry (IUPAC) naming system, the names become more descriptive: '2-methylbutane' and '2,2-dimethylpropane', explaining exactly where the methyl groups are located on the carbon chain. Learning to decode these names allows the prediction of a molecule’s structure and is a crucial skill in organic chemistry. Knowing the nomenclature rules, one can identify the number of carbons in the longest chain, the position of branches, the types of bonds (single, double, triple), and the presence of functional groups, leading to precise and universally understood names for any organic molecule.