Rotational Barrier

Definition

The rotational barrier is the energy required to rotate around a carbon-carbon single bond in an alkane molecule. This term is particularly relevant in the context of understanding the conformations and flexibility of alkane structures.

5 Must Know Facts For Your Next Test

1. The rotational barrier in ethane is approximately 12 kJ/mol, which represents the energy required to rotate the two methyl groups relative to each other.
2. The rotational barrier arises from the repulsive interactions between the hydrogen atoms on adjacent carbon atoms as the molecule rotates.
3. Alkanes with longer carbon chains generally have higher rotational barriers due to the increased number of repulsive interactions between hydrogen atoms.
4. The rotational barrier affects the flexibility and conformational preferences of alkane molecules, with lower barriers allowing for more rapid interconversion between conformations.
5. Understanding the rotational barrier is crucial for predicting the stability and reactivity of alkane conformations, which is important in organic chemistry.

Review Questions

• Explain the relationship between the rotational barrier and the conformations of ethane.
  • The rotational barrier in ethane is responsible for the molecule's ability to adopt different conformations. The 12 kJ/mol barrier allows the two methyl groups to rotate relative to each other, enabling the molecule to exist in both the staggered (lower energy) and eclipsed (higher energy) conformations. The rotational barrier determines the ease with which ethane can interconvert between these conformations, with the staggered conformation being more stable due to the reduced steric strain.
• Describe how the rotational barrier affects the conformations of other alkanes beyond ethane.
  • As the carbon chain length of alkanes increases, the rotational barrier also tends to increase. This is due to the greater number of repulsive interactions between the hydrogen atoms as the molecule rotates. The higher rotational barrier in longer-chain alkanes means that the interconversion between conformations is less facile, and the molecules may exhibit a greater preference for specific conformations. Understanding the relationship between the rotational barrier and conformations is crucial for predicting the stability and reactivity of alkane structures in organic chemistry.
• Analyze the role of the rotational barrier in determining the overall flexibility and reactivity of alkane molecules.
  • The rotational barrier is a key factor in determining the flexibility and reactivity of alkane molecules. A lower rotational barrier, as seen in ethane, allows for rapid interconversion between conformations, making the molecule more flexible and potentially more reactive. Conversely, higher rotational barriers in longer-chain alkanes restrict the molecule's ability to adopt different conformations, reducing its flexibility. This conformational rigidity can influence the reactivity of the alkane, as certain conformations may be more or less favorable for specific chemical reactions. Therefore, the rotational barrier is a critical consideration in understanding the structural and reactivity properties of alkane molecules in organic chemistry.

Related terms

Conformations:

The different spatial arrangements that a molecule can adopt while maintaining the same connectivity between atoms.

Torsional Strain:

The strain experienced by a molecule due to unfavorable interactions between atoms as it rotates around a carbon-carbon bond.

Newman Projection:

A graphical representation used to depict the three-dimensional arrangement of atoms around a carbon-carbon bond.