Final answer:
Anti-periplanar in E2 reactions refers to the orientation of the leaving group and the hydrogen atom on the adjacent carbon in the transition state. It allows for efficient overlap of orbitals, leading to a lower-energy transition state and a faster reaction rate. In an example reaction, the chlorine and hydrogen atoms are anti-periplanar to each other.
Step-by-step explanation:
In E2 reactions, anti-periplanar refers to the orientation of the leaving group and the hydrogen atom on the adjacent carbon in the transition state. Specifically, it means that the two groups are on opposite sides of the carbon-carbon double bond, with a 180-degree dihedral angle between them.
This orientation is important because it allows for the most efficient overlap of the orbitals involved in the reaction, leading to a lower-energy transition state and a faster reaction rate. It also ensures the formation of the most stable product.
For example, in the reaction of 2-chloropropane with a strong base like sodium ethoxide, the chlorine atom and the hydrogen atom are anti-periplanar to each other. The base attacks the hydrogen atom from the opposite side of the chlorine atom, resulting in the elimination of the chloride ion and the formation of propene.