Question

There are two ethylheptanes (compounds with a seven-carbon chain and one ethyl substituent). Draw the structures, and name these compounds. Is either isomer chiral?

Short answer

2-ethylheptane and 3-ethylheptane; only 2-ethylheptane is chiral due to carbon 2.

Step-by-step solution

Understand Ethylheptane

Ethylheptane is a compound with a seven-carbon (heptane) chain and one ethyl group (-C2H5) attached somewhere along the chain. Our job is to determine where this ethyl group could be placed to form distinct compounds.

Determine Possible Structures

For a seven-carbon chain, the carbons can be numbered from 1 to 7. The ethyl group can be attached to any carbon. However, distinct compounds only arise from non-terminal attachments. Thus, attach the ethyl group to carbon 2 or carbon 3 to create different compounds, as attaching it to carbon 1 would just extend the chain without forming an isomer.

Draw Structural Formulas

The first isomer can be drawn by attaching the ethyl group to the second carbon of heptane, resulting in 2-ethylheptane. The second isomer has the ethyl group attached to the third carbon, resulting in 3-ethylheptane. Refer to structural drawings, or use line-angle formulas to visualize these clearly.

Determine Chirality

To determine chirality, identify the presence of a carbon atom with four different groups attached. For both 2-ethylheptane and 3-ethylheptane, check each carbon atom. In 2-ethylheptane, carbon 2 has four different groups: an ethyl group, a methyl group (from the heptane chain), a hydrogen atom, and the rest of the heptane chain, making it chiral. In contrast, 3-ethylheptane lacks such carbon that can carry four different groups.

Key concepts

Isomerism

Isomerism in organic chemistry can be quite fascinating! It occurs when compounds have the same molecular formula but different structures, leading to distinct properties. In our example of ethylheptanes, two isomers are possible: 2-ethylheptane and 3-ethylheptane. Both of them share the formula: \\( C_9H_{20} \).

These isomers differ in the position of the ethyl group along the heptane chain. Here's a breakdown of different types of isomerism:

• Structural Isomers: These isomers differ in the connectivity of atoms. For ethylheptanes, by attaching the ethyl group to different carbons, distinct structural isomers are produced.


• Stereoisomers: These have the same connectivity but differ in spatial orientation. However, our focus is structural isomerism with ethylheptanes on carbons 2 and 3.

Understanding isomerism is essential for mastering organic chemistry, as it explains how similar molecules can have different chemical reactions and physical properties.

Chirality

Chirality introduces a level of complexity in organic molecules. A molecule is chiral if it has a carbon atom bonded to four different groups, resulting in non-superimposable mirror images, called enantiomers.

In our step-by-step exercise with ethylheptanes, only the 2-ethylheptane exhibits chirality. This is due to its second carbon atom being bonded to:

• An ethyl group
• A methyl group (from the extended chain)
• A hydrogen atom
• The remainder of the heptane chain

Each of these attachments is unique, making the carbon atom at position 2 of 2-ethylheptane chiral. Such a structure results in a chiral center, allowing for the possibility of different enantiomers, which can have distinct biological activities and physical interactions. In contrast, 3-ethylheptane does not have a carbon atom with four different substituents, and thus, it's not chiral.

Structural Formulas

Structural formulas are a way to visualize the spatial arrangement of atoms within a molecule. They show which atoms are connected and how they are arranged in space, making it easier to understand the compound's structure.

For the ethylheptanes:

• 2-Ethylheptane: The ethyl group is attached to the second carbon of the heptane chain. This can be depicted as:
  CH$_3$-CH(C$_2$H$_5$)-CH$_2$-CH$_2$-CH$_2$-CH$_2$-CH$_3$


• 3-Ethylheptane: The ethyl group moves to the third carbon of heptane, represented as:
  CH$_3$-CH$_2$-CH(C$_2$H$_5$)-CH$_2$-CH$_2$-CH$_2$-CH$_3$

Structural formulas can be in expanded or condensed formats, but using a line-angle formula is common in organic chemistry for simplifying large hydrocarbons. Each point represents a carbon atom, and lines represent the bonds between them. This aids in visualizing complex structures efficiently.

Alkanes

Alkanes are a fundamental class of hydrocarbons characterized by single bonds between carbon atoms. They are saturated, meaning they contain the maximum number of hydrogen atoms possible for the carbon skeleton.

The general formula for alkanes is \\( C_nH_{2n+2} \). For heptane (part of ethylheptanes), with seven carbon atoms, the formula becomes \\( C_7H_{16} \). By adding an ethyl group to heptane, we arrive at ethylheptanes, which follow the structural variations we've explored.

Within organic chemistry, alkanes serve as basic building blocks:

• Simple Structure: The single bond nature makes them less reactive compared to other hydrocarbons.
• Versatility: Alkanes can undergo various reactions like combustion and substitution.
• An Essential Foundation: Understanding alkanes is critical, as they form the backbone for more complex organic compounds.

Alkanes like ethylheptane show the diversity in organic compounds simply by rearranging groups, demonstrating how remarkable chemistry can be.