Problem 29 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following will have the least hindered rotation around carbon- carbon bond? (a) ethane (b) ethylene (c) acetylene (d) hexachloroethane

Step-by-Step Solution

Answer

Ethane has the least hindered rotation around the carbon-carbon bond.

Step 1: Understanding Hindered Rotation

Hindered rotation around a carbon-carbon bond refers to the difficulty in rotating around the bond due to steric hindrance or pi-bond restrictions. For single bonds such as in ethane, rotation is generally less hindered compared to double or triple bonds.

Step 2: Evaluating Single Bond (Ethane)

Ethane ( ext{C}_2 ext{H}_6) has a carbon-carbon single bond. This bond allows for free rotation because there is no significant hindrance from bulky groups or multiple bonds.

Step 3: Evaluating Double Bond (Ethylene)

Ethylene ( ext{C}_2 ext{H}_4) has a carbon-carbon double bond. The presence of the double bond does not allow free rotation around this bond because it would break the ext{pi}-bond, making it a considerably hindered rotation.

Step 4: Evaluating Triple Bond (Acetylene)

Acetylene ( ext{C}_2 ext{H}_2) has a carbon-carbon triple bond. This bond consists of one sigma bond and two ext{pi}-bonds, which severely restrict rotation around the bond, making it even more hindered than a double bond.

Step 5: Evaluating Substituted Single Bond (Hexachloroethane)

Hexachloroethane ( ext{C}_2 ext{Cl}_6) has a carbon-carbon single bond like ethane, but the presence of large chlorine substituents around the bond could hinder rotation to some degree due to steric bulk.

Step 6: Comparison and Conclusion

Comparing the compounds, ethane's single bond allows for the least hindered rotation because it lacks bulky groups or multiple bonds, unlike the other options. Ethylene's double bond, acetylene's triple bond, and the steric hindrance in hexachloroethane contribute to greater hinderance in rotation.

Key Concepts

Carbon-Carbon Bond

Carbon-carbon bonds are the backbone of organic chemistry. They represent the connection between two carbon atoms. You'll often find these bonds classified into different types, such as single, double, or triple bonds. But why are they important? Well, the type of carbon-carbon bond determines the level of rotation possible around that bond, greatly affecting the molecule's structural flexibility. In simpler terms:

• Single bonds allow for rotation, giving molecules like ethane flexibility.
• Double and triple bonds limit rotation due to their structural makeup.

When thinking about hindered rotation, the type of carbon-carbon bond is a key factor to consider.

Steric Hindrance

Steric hindrance is like traffic congestion in a molecule. It refers to the way bulky groups attached to a carbon-carbon bond can block or hinder easy rotation of the bond. Imagine trying to spin a door fully open, but there's a big box on one side stopping it from going all the way. That's similar to what happens when large atoms or groups are near a bond. Important points about steric hindrance:

• It doesn't change the bond type, but affects the ease of rotation.
• Bulky groups, like large atoms or multiple atoms together, cause more hindrance.

So, even a single bond, like in hexachloroethane, can have hindered rotation if steric hindrance is significant.

Pi-Bond

Pi-bonds are a special type of bond that occurs in double and triple bonds. They are formed from the sideways overlap of p-orbitals. But why do they matter for rotation? Because breaking a pi-bond means altering the molecular structure substantially. Key characteristics:

• Pi-bonds are present in both double and triple bonds.
• They prevent free rotation because breaking them is energetically costly.

Thus, when you talk about restricted rotation, pi-bonds are the major players, providing much of the rotational resistance in double and triple-bonded structures like ethylene and acetylene.

Single Bonds

Single bonds are the simplest type of carbon-carbon bond and allow for unrestricted rotation. Picture the hands of a clock—they can turn freely. That's what single carbon-carbon bonds do; they act like a hinge that permits free rotation of the connected atoms. A few things to remember:

• Single bonds are found in saturated compounds, like alkanes.
• They offer maximum flexibility due to the lack of additional bonding constraints.

Ethane is a great example of a molecule with a single bond, showcasing the least hindered rotation due to the absence of pi-bonds or significant steric hindrance.

Double Bonds

Double bonds add another layer of complexity to molecular structures. They comprise one sigma bond and one pi-bond. This configuration prevents the atoms bonded through a double bond from rotating freely. Think of it as a rigid rod instead of a free-spinning swivel. Attributes of double bonds:

• They restrict rotation due to the presence of a pi-bond.
• Are common in unsaturated hydrocarbons like alkenes.

In molecules like ethylene, the double bond means that rotating the atoms around this bond would require breaking the pi-bond, which the molecule typically avoids.