Question

\(18 \mathrm{C}-\mathrm{H}\) and \(7 \mathrm{C}-\mathrm{C}\) sigma bonds are present in: (a) n-heptane (b) Cyclohexane (c) 3,3 -dimethyl pentane (d) \(2,2,3\) -trimethyl pentane

Short answer

The correct answer is (c) 3,3-dimethylpentane.

Step-by-step solution

Understand the Molecular Formula of Each Option

First, identify the molecular formula for each compound option given.- **n-heptane (a)**: It is a linear chain with 7 carbon atoms, written as \( C_7H_{16} \).- **Cyclohexane (b)**: It is a ring structure with 6 carbon atoms, written as \( C_6H_{12} \).- **3,3-dimethylpentane (c)**: A branched alkane; write the structure from the name, identifying 7 carbon atoms in total.- **2,2,3-trimethylpentane (d)**: Another branched alkane; combine names to see that there are 8 carbon atoms.

Determine the Number of Carbon-Carbon (C-C) Bonds

Count the number of \( \text{C-C} \) sigma bonds in each structure:- **n-heptane (a)**: 6 \( \text{C-C} \) bonds in a linear chain.- **Cyclohexane (b)**: 6 \( \text{C-C} \) bonds forming a ring.- **3,3-dimethylpentane (c)**: Draw to see 7 \( \text{C-C} \) bonds.- **2,2,3-trimethylpentane (d)**: Count 7 \( \text{C-C} \) bonds from its structure.

Count the Number of Carbon-Hydrogen (C-H) Bonds

Determine the number of \( \text{C-H} \) bonds in each structure:- **n-heptane (a)**: Check each carbon, leading to 16 \( \text{C-H} \) bonds.- **Cyclohexane (b)**: This forms 12 \( \text{C-H} \) bonds.- **3,3-dimethylpentane (c)**: Total 18 \( \text{C-H} \) bonds.- **2,2,3-trimethylpentane (d)**: Count 18 \( \text{C-H} \) bonds analyzing the full structure.

Match Requirements

Identify which option matches the requirement of having 18 \( \text{C-H} \) and 7 \( \text{C-C} \) bonds by reviewing bond counts from earlier:- **3,3-dimethylpentane (c)** matches with 18 \( \text{C-H} \) and 7 \( \text{C-C} \) bonds.

Key concepts

Sigma Bonds

In the realm of organic chemistry, sigma bonds are fundamental. They form the backbone of molecular structures by accounting for the first bond made between two atoms. In hydrocarbons, these bonds appear universally as either carbon-carbon (C-C) or carbon-hydrogen (C-H) bonds.
Sigma bonds are created by the head-on overlapping of orbitals, usually combining one s orbital and one p orbital. This overlap is stronger and more stable than that of other types of covalent bonds. When considering hydrocarbons, each time you connect a carbon atom to another carbon or to a hydrogen atom, you are looking at sigma bonds.

• All single bonds in organic molecules are sigma bonds;
• They allow for free rotation, which is crucial in organic compounds;
• The strength and stability of sigma bonds contribute greatly to molecular integrity.

Sigma bonds, therefore, are crucial in understanding the structure and behavior of organic molecules.

Hydrocarbons

Hydrocarbons are the simplest organic compounds, made only of carbon and hydrogen atoms. They serve as fundamental building blocks in organic chemistry. These molecules can form endless chains and rings, varying in length and complexity. Two primary types of hydrocarbons are alkanes and cycloalkanes.
- **Alkanes**: These are saturated hydrocarbons, meaning they have only single bonds. An example is **n-heptane**, a straight-chain alkane. - **Cycloalkanes**: These hydrocarbons form ring structures with single bonds, such as **cyclohexane**. In addition:

• Hydrocarbons are often used as fuels, like methane and propane;
• They are considered the raw materials for producing plastics, fibers, and other organic chemicals;
• The diversity of hydrocarbons allows for numerous structural variations, influencing properties like boiling and melting points.

Understanding the basic structure of hydrocarbons is key to mastering more advanced concepts in organic chemistry.

Structural Isomers

Structural isomers are a fascinating aspect of chemistry. They have the same molecular formula but differ in the arrangement of atoms. This variation leads to vast differences in physical and chemical properties. In hydrocarbons, structural isomerism plays a significant role.
- **Straight Chain Isomers**: These isomers have carbon atoms arranged in a continuous chain, like in **n-heptane**. - **Branched Chain Isomers**: Here, carbon chains branch off, such as in **3,3-dimethylpentane** and **2,2,3-trimethylpentane**. Why it matters:

• Structural isomers can have drastically different boiling and melting points;
• They have different levels of reactivity and interactions with other molecules;
• This concept is critical for designing and understanding pharmaceuticals and petrochemicals.

By understanding structural isomers, chemists can predict and manipulate the behavior of molecules for various applications.