Which of the following reactions are both stereospecific and regioselective ?

Options

A. cis-But-2-ene + `D_2 overset (Pt) rarr`
B. cis-But-2-ene + `OsO_4 + NaHSO_3 rarr`
C. cis-But-2-ene + `Br_2 rarr`
D. trans-But-2-ene + `PhCO_3 H//H_2O rarr`.

Explanation

To determine which of the given reactions are both stereospecific and regioselective, we need to understand the definitions of these terms and analyze each reaction accordingly.

### Step-by-Step Solution:

1. **Understanding Stereospecificity and Regioselectivity:**
- **Stereospecificity** refers to a reaction that leads to different stereoisomeric products from different stereomeric reactants. This means that the configuration of the reactant influences the configuration of the product.
- **Regioselectivity** means that in a reaction, one product is formed preferentially over others. This indicates that there is a major product that is favored in the reaction.

2. **Analyzing the Reactions:**
- **Option A:**
- Reaction: cis-buto-2-ene + D2 (in presence of Pt)
- Type of addition: Syn addition
- Product: Both deuterium atoms are added on the same side, resulting in a product with a plane of symmetry (optically inactive).
- Conclusion: This reaction is both stereospecific and regioselective.

- **Option B:**
- Reaction: cis-but-2-ene + OsO4 + NaHSO3
- Type of addition: Syn addition
- Product: Two OH groups are added on the same side, leading to a product with a plane of symmetry (optically inactive).
- Conclusion: This reaction is also both stereospecific and regioselective.

- **Option C:**
- Reaction: cis-but-2-ene + Br2
- Type of addition: Anti addition
- Product: Bromine atoms are added on opposite sides, resulting in two products (mirror images).
- Conclusion: This reaction is stereospecific but not regioselective since it produces two different products.

- **Option D:**
- Reaction: trans-but-2-ene + PHCO3H + H2O
- Type of addition: Anti addition
- Product: The addition results in two OH groups on the same side, leading to a product with a plane of symmetry (optically inactive).
- Conclusion: This reaction is both stereospecific and regioselective.

3. **Final Conclusion:**
- The reactions that are both stereospecific and regioselective are:
- Option A
- Option B
- Option D