Question

Five alkenes have the formula \(\mathrm{C}_{7} \mathrm{H}_{14}\) and a sevencarbon chain. Draw their structures and name them.

Short answer

1-heptene, cis-2-heptene, trans-2-heptene, 3-heptene, 3-methyl-1-hexene.

Step-by-step solution

Understand the Structure of Alkenes

Alkenes are hydrocarbons with a carbon-carbon double bond. In the formula \(\mathrm{C}_n\mathrm{H}_{2n}\), \(n\) represents the number of carbon atoms. Thus, \(\mathrm{C}_7\mathrm{H}_{14}\) confirms the presence of a double bond and seven carbon atoms.

Identify the Longest Carbon Chain

For alkenes given that the chain must cover seven carbon atoms, ensure the longest chain has seven continuous carbon atoms. This provides the base name of the alkene, heptene, from the seven carbon chain.

Position the Double Bond

The double bond can be placed in different positions within the seven-carbon chain: at the first, second, or third carbon for structural isomers. For each position, the alkene is considered a different isomer (e.g., 1-heptene, 2-heptene, 3-heptene).

Create Branched Isomers

You can create additional isomers by introducing branches on the 7-carbon chain while still maintaining a single double bond. Examples include creating dimethyl- or ethyl-substituted hexenes, such as 3-methyl-1-hexene and 2-ethyl-1-pentene.

Generate Configurations for Different Molecular Structures

For the 2-heptene structure, consider producing isomers with different configurations, such as cis- and trans-2-heptene, where the substituents on the double bond are arranged differently in space.

Draw the Structures

Now, sketch the structures for each isomer: 1-heptene, 2-heptene (both cis and trans), 3-heptene, and a branched version like 3-methyl-1-hexene or 2-ethyl-1-pentene.

Verify and Name the Structures

Verify and assign the correct IUPAC names to each drawn structure, checking double bond positions and branches. Name them as: 1-heptene, cis-2-heptene, trans-2-heptene, 3-heptene, 3-methyl-1-hexene.

Key concepts

Isomerism

Isomerism is a fascinating concept in organic chemistry that occurs when compounds have the same molecular formula but different structures. In the case of alkenes, this can be seen with compounds like \(\mathrm{C}_7\mathrm{H}_{14}\), where variations in the position of the double bonds or the arrangement of carbon atoms lead to different isomers.

There are two main types of isomerism in alkenes: structural isomerism and geometric (cis-trans) isomerism.

• **Structural Isomerism:** This happens when the connectivity of atoms differs between isomers. For example, placing a double bond in different positions along the carbon chain can create distinct isomers like 1-heptene and 2-heptene.
• **Geometric Isomerism:** This type of isomerism arises from the restricted rotation around the double bond, producing isomers with substituents arranged differently in space, known as cis and trans configurations. For instance, 2-heptene can exist as either cis-2-heptene or trans-2-heptene.

Understanding isomerism helps in predicting properties and reactivity of organic compounds, making it an essential aspect of studying chemistry.

IUPAC Naming

The International Union of Pure and Applied Chemistry (IUPAC) system is used to assign names to chemical compounds, ensuring clarity and standardization in the identification of molecular structures.

• **Base Name:** The base name of the alkene is determined by the longest continuous carbon chain containing the double bond. For example, a seven-carbon chain with a double bond is named heptene.
• **Position of the Double Bond:** It's important to number the carbon chain so that the double bond gets the lowest possible number. Thus, a double bond starting at the first carbon is named 1-heptene, while one starting at the second carbon is named 2-heptene.
• **Branches and Substituents:** When branches are present, their positions and names are added as prefixes. For instance, a methyl group on the third carbon of the main chain of heptene would be named 3-methyl-1-hexene if it shortens the main chain by one carbon atom.

Careful application of these rules facilitates accurate communication regarding chemical structures in scientific work.

Molecular Structures

Molecular structures of alkenes involve a framework of carbon atoms connected by single and double bonds. The presence of a double bond significantly influences the geometry and properties of the compound.

• **Double Bonds:** In alkenes, a double bond occurs between two carbon atoms. This bond is shorter and stronger than a single bond and results in restricted rotation, leading to the existence of geometric isomers.
• **VSEPR Theory:** According to the Valence Shell Electron Pair Repulsion (VSEPR) theory, the geometry around each carbon in a double bond is trigonal planar, with bond angles close to 120°.
• **Branching:** Introducing branches to the carbon chain can alter the molecule's physical properties, such as melting and boiling points. Branching often leads to a more compact structure, sometimes reducing intermolecular interactions.

Understanding these structural elements is crucial for predicting the behavior and reactivity of alkenes.

Carbon Chains

Carbon chains form the backbone of organic compounds like alkenes. They can be straight or branched, and they influence the compound's properties significantly.

A linear carbon chain means that the carbon atoms are connected in a straight sequence. In alkenes like heptene, the main chain consists of seven carbon atoms, as indicated by the prefix 'hept-'.

• **Chain Length:** The length of the carbon chain can affect boiling and melting points. Typically, longer chains have higher boiling and melting points due to increased van der Waals forces.
• **Branching:** Branches in the chain are created by adding carbon groups to the primary chain. This can reduce the boiling and melting points slightly, as branched chains tend to be more compact, affecting how the molecules pack together.
• **Flexibility and Rigidity:** The single bonds in carbon chains allow for rotation, imparting flexibility, whereas double bonds restrict this rotation, leading to rigidity in parts of the molecule.

The characteristics of carbon chains impact the physical and chemical properties of alkenes, making their study important in organic chemistry.