Question

Which of the following will have the least hindered rotation around carbon- carbon bond? (a) Ethane (b) Ethylene (c) Acetylene (d) Hexachloroethane

Short answer

Ethane (a) has the least hindered rotation.

Step-by-step solution

Define Hindered Rotation

Hindered rotation refers to the resistance to rotation around a bond, typically measured around carbon-carbon double and triple bonds. This resistance occurs due to the obstacles present, usually in the form of steric hindrance or pi bonding.

Identify Types of Bonds

Examine each molecule to identify the types of bonds: - Ethane (a) has a single bond (C-C). - Ethylene (b) has a double bond (C=C). - Acetylene (c) has a triple bond (C≡C). - Hexachloroethane (d) has single bonds but with chlorine atoms causing steric hindrance.

Analyze Rotation Possibility

Evaluate rotation possibility based on bond type: - Single bonds like in ethane can freely rotate, as there are no pi bonds restricting them. - Double and triple bonds, like in ethylene and acetylene, have pi bonds, which restrict rotation due to overlapping p-orbitals. - Hexachloroethane, although having single bonds, has additional steric hindrance due to the presence of chlorine atoms.

Compare Hindrance in Each Option

Compare the hindrance in each option: - Ethane (a) can rotate freely due to having a simple C-C single bond. - Ethylene (b) and Acetylene (c) have hindered rotation by virtue of their pi bonds. - Hexachloroethane (d) is hindered due to chlorine atoms' steric effects even though it might have single bonds.

Determine Least Hinderance

The option with the least hindered rotation is the one with simple single bonds and minimal steric effects. Ethane (a) has this characteristic as it involves only a C-C single bond without steric complications.

Key concepts

Carbon-Carbon Bonds

Carbon-carbon bonds are fundamental links between carbon atoms, forming the backbone of organic molecules. These bonds can be single, double, or triple. Each bond type affects the rotation around the bond differently:

• *Single bonds* (C-C), like in ethane, allow for free rotation, as they only involve sigma bonds that form a direct and strong overlap of orbitals.
• *Double bonds* (C=C), found in ethylene, consist of both sigma and pi bonds. While sigma bonds allow for free rotation, pi bonds prevent it due to their overlapping p-orbitals which create a planar structure, fixing the orientation of the atoms.
• *Triple bonds* (C≡C), as in acetylene, contain one sigma and two pi bonds, restricting rotation even more tightly.

When analyzing molecules, it's important to know the type of carbon-carbon bond to anticipate the possible rotations and understand the molecule's 3D shape.

Steric Hindrance

Steric hindrance is a phenomenon where the size and spatial orientation of atoms or groups within a molecule hinder certain chemical processes, such as free rotation around bonds. In hexachloroethane, chlorine atoms are large and occupy significant space around the carbon atoms. Their size and electron cloud create a physical barrier, preventing nearby atoms from moving freely. As a result, even though hexachloroethane has single carbon-carbon bonds that could theoretically rotate, the bulky chlorine atoms block this motion, thus increasing steric hindrance.
When evaluating molecular rotation, consider if bulky substituents exist around the bond in question. They can create steric hindrance that limits movement even when other conditions—like the bond type—suggest possible rotation.

Pi Bonding

Pi bonds are a type of covalent bond arising from the sideways overlap of p-orbitals of the involved atoms, found in double and triple carbon-carbon bonds.

• **In double bonds,** like those in ethylene, one pi bond exists alongside a sigma bond.
• **In triple bonds,** such as in acetylene, two pi bonds are present alongside one sigma bond.

These pi bonds significantly constrain rotation because they fix the atoms in a planar, rigid structure. Breaking the pi bond is necessary for atoms to rotate freely around the bond, typically requiring energy input, like heat. Understanding pi bonding is crucial in stereochemistry and determining a molecule's geometry because these bonds create regions of electron density that affect the molecule's reactivity and physical characteristics.

Rotation in Molecular Structures

In molecular structures, rotation around bonds is a key factor in the molecule's overall shape and function. Free rotation is most common around single bonds due to their simple sigma bond nature, as observed in ethane. This free rotation allows the molecule to adopt various conformations, affecting properties like reactivity and interactions.
However, in molecules with double or triple bonds, like ethylene and acetylene, rotation is restricted. The presence of pi bonds creates a planar structure that locks rotation, making the molecule's shape fixed. These restrictions impact how the molecules participate in chemical reactions and interact with other molecules.
In some cases, steric hindrance can further limit rotation, even around single bonds, as seen in hexachloroethane with its bulky chlorine atoms. This emphasizes the importance of both the bond type and surrounding atoms or groups in understanding molecular flexibility and structure.