Final answer:
The correct answer is (D) E2 reactions involve 4 atoms with hydrogen in the transition state. The E2 mechanism involves a single, concerted step where a base removes a proton and the leaving group is expelled, forming a double bond.
Step-by-step explanation:
The correct answer to the question is D) E2 reactions involve 4 atoms with hydrogen in the transition state. An E2 reaction is a bimolecular elimination mechanism that involves the concerted movement of electrons from a base to an electrophilic atom (usually a halogen) causing the departure of the leaving group and the simultaneous elimination of a proton from the adjacent carbon atom.
This results in the formation of a double bond. In the transition state of an E2 reaction, four atoms are involved, which align linearly as the reaction proceeds. This transition state involves both the base removing a proton and the leaving group being expelled, all occurring in one concerted step.
E2 reactions are characterized by their bimolecular rate of reaction, indicating that both the substrate (the molecule possessing the leaving group) and the base (the molecule removing the proton) are involved in the rate-determining step.