Reaction involving anti addition is

Options

A. `CH_(2)=CH_(2) overset(H^(+)//H_(2)O)(to)`
B. `CH_(3) CH= CH_(2) overset(HX)(to)`
C. `CH_(3)CH=CH_(2)overset(Hg(OA_(c))_(2)H_(2)O)underset(NaBH_(4))(to)`
D. `CH_(2)=CH_(2)overset(B_(2)H_(6)//THF)underset(H_(2)O_(2)//OH)(to)`

Explanation

To determine which reaction involves anti-addition, we need to analyze the given options and identify the mechanism of each reaction. Anti-addition refers to a process where two substituents add to opposite sides of a double bond, resulting in a trans configuration.

### Step-by-Step Solution:

1. **Identify the Reactions**: We have four reactions to analyze. The first step is to write down the reactions clearly and identify the reactants and products.

2. **Analyze Each Reaction**:
- **Reaction 1**: CH2=CH2 + H+ → CH3-CH2+ (carbocation formation) + H2O (nucleophile attack).
- This reaction does not show anti-addition because the nucleophile can attack from either side of the carbocation.

- **Reaction 2**: CH3-CH=CH2 + H+ → CH3-CH(+)-CH2 (carbocation formation) + X- (nucleophile attack).
- Similar to the first reaction, the nucleophile can attack from either side of the carbocation, indicating no anti-addition.

- **Reaction 3**: CH3-CH=CH2 + Hg(OAc)2 + H2O → Non-classical carbocation formation followed by nucleophile attack.
- In this reaction, after the formation of the non-classical carbocation, water attacks from the opposite side of the leaving group (HgOAc), which is characteristic of anti-addition.

- **Reaction 4**: B2H6 addition to alkenes.
- This reaction is known for syn-addition, where both substituents add to the same side of the double bond.

3. **Conclusion**: Based on the analysis, the reaction that involves anti-addition is the third one (mercuration-demercuration).

### Final Answer:
The reaction involving anti-addition is the third option: the reaction of CH3-CH=CH2 with Hg(OAc)2 followed by water.

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