Question

A hydrocarbon with the formula \(\mathrm{C}_{5} \mathrm{H}_{10}\) can be either an alkene or a cycloalkane. (a) Draw a structure for each of the possible isomers for \(\mathrm{C}_{5} \mathrm{H}_{10},\) assuming it is an alkene. Six isomers are possible. Give the systematic name of each isomer you have drawn. (b) Draw a structure for a cycloalkane having the formula \(\mathrm{C}_{5} \mathrm{H}_{10}\)

Short answer

There are six alkene isomers of C5H10: 1-pentene, 2-pentene, 3-methyl-1-butene, 2-methyl-1-butene, 2-methyl-2-butene, and 3-methyl-2-butene. The cycloalkane is cyclopentane.

Step-by-step solution

Identify the Structure of Alkenes

The molecular formula for alkenes is given by \(\mathrm{C}_n \mathrm{H}_{2n}\). For \(\mathrm{C}_5\mathrm{H}_{10}\), the possible structures must contain a double bond. This means one carbon will form a double bond with another carbon atom, fulfilling the requirement of being an alkene.

Draw Possible Alkene Structures

Draw all possible structural isomers of \(\mathrm{C}_5\mathrm{H}_{10}\) where the double bond can vary in position. They are:1. 1-pentene: a double bond between the first and second carbon atoms.2. 2-pentene: a double bond between the second and third carbon atoms.3. 3-methyl-1-butene: a branching on the second carbon with a methyl group on the third carbon.4. 2-methyl-1-butene: a branching on the second carbon with a double bond after it.5. 2-methyl-2-butene: a branching on the third carbon with a double bond after it.6. 3-methyl-2-butene: a branching on the second carbon with a methyl group on the third carbon along the main chain.

Name Each Alkene Isomer

Assign the IUPAC names to each of the drawn structures based on the location of the double bond and any branching: 1. 1-pentene 2. 2-pentene 3. 3-methyl-1-butene 4. 2-methyl-1-butene 5. 2-methyl-2-butene 6. 3-methyl-2-butene.

Identify the Structure of Cycloalkanes

For cycloalkanes, use the formula \(\mathrm{C}_n \mathrm{H}_{2n}\), similar to alkenes. Create a ring structure with five carbons, forming cyclopentane, a clear isomer of \(\mathrm{C}_5\mathrm{H}_{10}\), where the hydrogen atoms complete the tetrahedral configuration around each carbon.

Key concepts

Isomer Identification

Isomer identification involves distinguishing different compounds that share the same molecular formula but differ in structure. In organic chemistry, isomers can be structural or stereoisomers.
Structural isomers, or constitutional isomers, vary in the connectivity of atoms, leading to various skeletal arrangements. For extup{C}_5 extup{H}_{10}, both alkenes and cycloalkanes must be considered.
Recognizing these differences is essential, as the variations often result in distinct chemical and physical properties.

Alkenes and Cycloalkanes

Alkenes are hydrocarbons characterized by the presence of at least one carbon-carbon double bond. This double bond is a key feature defining alkene properties and reactivity. The general formula for alkenes is extup{C}_n extup{H}_{2n}, implying extup{C}_5 extup{H}_{10} fits this category.
Cycloalkanes, on the other hand, are saturated hydrocarbons forming a closed ring. They also follow the extup{C}_n extup{H}_{2n} formula but lack the double bond, instead having a circular structure like a ring.
For extup{C}_5 extup{H}_{10}, cyclopentane is a typical representation of such a structure, where the carbon atoms form a five-membered ring.

IUPAC Nomenclature

IUPAC nomenclature is the standardized system for naming chemical compounds. Alkenes are named based on the longest carbon chain containing the double bond with the suffix '-ene' marking the presence of the double bond.
Numbering is crucial: the chain is numbered from the end closest to the double bond, ensuring the smallest possible number for the double bond's position.

• 1-pentene: Double bond between first and second carbon atoms.
• 2-pentene: Double bond between second and third carbon atoms.
• 3-methyl-1-butene: Chain branching on the second carbon.
• 2-methyl-1-butene: Methyl group and double bond both originate from the second carbon.
• 2-methyl-2-butene: Double bond after the third carbon with branching.
• 3-methyl-2-butene: Methyl group at the third carbon and a double bond starting from the second.

Each name gives insight into the structure of the molecule, ensuring clarity in scientific communication.

Structural Isomers

Structural isomers such as those in alkenes are fascinating due to their varied connectivity even with an identical molecular formula. For the extup{C}_5 extup{H}_{10} alkenes, we encounter six structural isomers.
Each isomer provides a unique way to satisfy the valency of carbon atoms while integrating a double bond: 1-pentene, 2-pentene, and the four methyl-substituted butenes illustrate this diversity.
The structural differences influence properties like boiling points, densities, and even reactivity, demonstrating why this is an important area of study. Recognizing and drawing these structures helps in understanding their distinct characteristics and potential uses.