Tert-Butyl Group

Definition

The tert-butyl group is a bulky, three-dimensional substituent composed of three methyl groups attached to a central carbon atom. It is a common functional group in organic chemistry and has a significant impact on the conformations and reactivity of molecules.

5 Must Know Facts For Your Next Test

1. The tert-butyl group is a bulky, three-dimensional substituent that can significantly influence the conformations of cyclohexane rings.
2. In disubstituted cyclohexanes, the tert-butyl group prefers to occupy the equatorial position due to minimized steric interactions.
3. The presence of a tert-butyl group can accelerate the rate of E2 elimination reactions by stabilizing the transition state through hyperconjugation.
4. The tert-butyl group can be used as a protecting group in organic synthesis due to its steric bulk and resistance to certain reaction conditions.
5. The deuterium isotope effect is often observed in E2 reactions involving tert-butyl-containing substrates, as the breaking of the C-H bond is the rate-determining step.

Review Questions

• Explain how the tert-butyl group influences the conformations of cyclohexane rings.
  • The tert-butyl group is a bulky, three-dimensional substituent that can significantly impact the preferred conformations of cyclohexane rings. Due to its size, the tert-butyl group prefers to occupy the equatorial position on a cyclohexane ring, as this minimizes steric interactions with the other substituents and hydrogens on the ring. The equatorial orientation allows for the least amount of steric crowding, making it the more stable and favored conformation.
• Describe the role of the tert-butyl group in E2 elimination reactions.
  • The presence of a tert-butyl group can accelerate the rate of E2 elimination reactions. This is because the tert-butyl group can stabilize the transition state of the reaction through hyperconjugation, where the sigma bonds of the methyl groups interact with the developing pi bond in the transition state. This stabilization lowers the activation energy required for the reaction, leading to a faster rate of E2 elimination. Additionally, the bulky tert-butyl group can also provide steric hindrance that facilitates the anti-periplanar alignment of the leaving group and the beta-hydrogen, which is a key requirement for the E2 mechanism.
• Analyze the relationship between the tert-butyl group and the deuterium isotope effect in E2 reactions.
  • In E2 reactions involving tert-butyl-containing substrates, a pronounced deuterium isotope effect is often observed. This is because the breaking of the C-H bond is the rate-determining step in the E2 mechanism, and the presence of the tert-butyl group can further emphasize this step. When a hydrogen atom is replaced with the heavier deuterium isotope, the rate of the reaction is slowed down due to the increased bond strength and decreased bond vibration frequency. This isotope effect is more pronounced in the case of the tert-butyl group because the C-H bond cleavage is a crucial part of the rate-determining step, leading to a larger observable difference in reaction rates between the hydrogen and deuterium-containing substrates.

Related terms

Steric Hindrance: The repulsive force between large substituents that can influence the preferred conformation of a molecule.

Equatorial Position: The orientation of a substituent on a cyclohexane ring where it is positioned between the two hydrogen atoms on the same face of the ring.

Axial Position: The orientation of a substituent on a cyclohexane ring where it is positioned above or below the plane of the ring.