Question

Draw Lewis structures and name all possible compounds that have a seven-carbon chain with one methyl substituent group. Which of these isomers has a chiral carbon center?

Short answer

2-methylheptane has a chiral carbon center.

Step-by-step solution

Understand the Structure

A seven-carbon chain suggests a structure with seven continuous carbon atoms. Considering one methyl group as a substituent, we will have an overall eight-carbon count in the structure.

Begin With the Longest Carbon Chain

Create the parent chain by drawing a straight chain of seven carbon atoms, labeled C1 to C7. This will serve as the base structure for the isomers.

Add the Methyl Substituent

Position a methyl group (-CH₃) onto one of the carbon atoms in the seven-carbon chain. The positions available for this substituent group are as follows: C2, C3, C4, C5, and C6, as placing it at C1 or C7 would still result in a seven-carbon chain being the longest.

Draw and Name the Isomers

By placing the methyl group at various positions, we derive the following structural isomers: 1. 2-methyheptane 2. 3-methyheptane 3. 4-methyheptane 5. 5-methyheptane Each structure needs to be drawn with correct single bonds and hydrogen atoms filling the remaining valences of carbon to complete their Lewis structures.

Identify Chiral Centers

A chiral carbon is tetrahedral (sp³ hybridized) and bonded to four different groups. In the listed isomers, evaluate each carbon to determine if it's bonded to four different groups, indicating chirality: - 2-methylheptane: The second carbon is a chiral center since it is bonded to a hydrogen, a methyl group, an isopropyl group, and the rest of the chain. - Other isomers do not have a chiral center.

Key concepts

Lewis Structures

The Lewis structure is a way of representing molecules on paper. It uses symbols to show atoms and dots to represent electrons. This method helps to understand the electron distribution in a molecule. In organic chemistry, Lewis structures are often used to represent hydrocarbons like alkanes, alkenes, and alkynes, among other compounds.
To draw a Lewis structure for a hydrocarbon such as a long carbon chain with a substituent like a methyl group, start by identifying and counting the valence electrons for each atom. Remember, carbon has four valence electrons, and hydrogen has one.

• Arrange the carbon atoms in a straight line or desired backbone structure.
• Add the substituent groups at their respective positions.
• Complete the structure by placing hydrogen atoms to fill the remaining bonds of carbon.

Bringing all these elements together ensures the octet rule is satisfied for each carbon atom.

Structural Isomers

Structural isomers are compounds with the same molecular formula but different connectivity of atoms, meaning the atoms are arranged differently in space. This difference can change a molecule's physical and chemical properties significantly. In our example, drawing a seven-carbon chain and adding a methyl group creates structural isomers.
For the given molecular formula of C₈H₁₈ (seven-carbon chain and a methyl substituent), possible isomers include:

• 2-methylheptane
• 3-methylheptane
• 4-methylheptane
• 5-methylheptane

Each placement of the substituent methyl group on the carbon skeleton leads to a distinct compound with slightly varying properties that are important in different applications.

Chiral Centers

Chirality in molecules is an important concept in organic chemistry because it can influence the behavior of molecules in biological systems. A chiral center is a carbon atom that is bonded to four different groups, making the molecule non-superimposable on its mirror image.
Determining chirality involves identifying which carbon atoms in the molecule have four different substituents. In our case of methyl-substituted heptane isomers, examining each carbon:

• 2-methylheptane has a chiral center on the second carbon, as it bonds to a methyl, a hydrogen, an isopropyl, and the remaining carbon chain differently.
• Other isomers do not contain a chiral center since their carbon atoms do not meet the criteria for chirality.

Recognizing chiral centers is crucial for understanding the stereochemistry of the molecule, which can affect how it interacts with other chiral molecules in physical and chemical reactions.

Methyl Substituent

A methyl substituent is a common group found in organic molecules. It consists of a carbon atom bonded to three hydrogen atoms (CH₃−) and is used widely as a branch off a main carbon chain.
In drawing structural isomers, where the methyl group is placed can change the resulting compound. For instance:

• Placing it on the second carbon gives us 2-methylheptane.
• Positioning it on the third carbon leads to 3-methylheptane, and so forth.

The position of the methyl group affects not only the name of the compound but also its potential physical and chemical behaviors.
When creating molecule structures, remember that placing a methyl group at the ends of a carbon chain (like C1 or C7 in our example) would not create a new isomer.

Hydrocarbon Chains

Hydrocarbon chains are fundamental frameworks in organic molecules, comprised solely of carbon (C) and hydrogen (H) atoms. These chains can vary in length and structure, forming linear, branched, or cyclic configurations. In our exercise, the focus is on a seven-carbon straight chain.
These hydrocarbons serve as backbones in organic molecules, influencing properties such as boiling and melting points. Here are some key points about hydrocarbon chains:

• Linear chains can easily be altered by substituents to form different functional groups.
• The length and degree of branching affect both the physical properties (e.g., solubility, boiling point) and chemical properties (e.g., reactivity) of the molecule.
• Hydrocarbon chains can also serve as frameworks for creating complex organic structures.

Knowing how to manipulate hydrocarbon chains is vital for synthesizing various organic compounds efficiently.