Final answer:
Additions of Br2 to alkenes and hydroboration-oxidation are both characterized as anti additions because the substituents add to opposite sides of the double bond. The correct answer is option E) both A and B
Step-by-step explanation:
The anti addition is a characteristic feature of certain addition reactions where two substituents are added to opposite sides (or faces) of a double bond in an alkene.
Addition of Br2 (Bromine)
The addition of Br2 is one of the classic examples of an anti addition. When bromine is added to an alkene, each bromine atom attaches to different carbon atoms of the double bond from opposite sides. This occurs because the intermediate bromonium ion formed during the reaction is attacked from the back side by the bromide anion, leading to an anti-configured product.
Hydroboration-Oxidation
Hydroboration-oxidation is another addition reaction that proceeds in an anti fashion. During hydroboration, the BH3 adds across the double bond in a syn addition; however, the subsequent oxidation step with hydrogen peroxide and sodium hydroxide converts the boron-substituted alkene into an alcohol with retention of configuration, resulting in an overall anti addition relative to the original double bond.
Therefore, the correct answer to which addition reactions to alkenes occur specifically in an anti fashion is both A and B, referring to hydroboration-oxidation (A) and the addition of Br2 (B).