Which of the following are true regarding hydroboration of alkene `?(` More than one correct statements . `)`

Options

A. Reaction proceeds through ionic intermediat and syn`-`addition take place.
B. Reaction proceeds through cyclic intermediate transition state and syn`-` addition takes place.
C. Reagent diborane in ether `(THF)` exists as `:`
.
D. Reagent exists in `THF` as borane `-THF` complex.

Answer

To determine which statements are true regarding the hydroboration of alkenes, let's analyze the process step by step.

### Step 1: Understanding Hydroboration
Hydroboration is a reaction where an alkene reacts with borane (BH3) to form an organoborane compound. The general reaction can be represented as:
\[ RCH=CH2 + BH3 \rightarrow RCH2-CH2-BH2 \]

### Step 2: Mechanism of Hydroboration
1. **Formation of Pi Complex**: The alkene first forms a pi complex with borane. The empty p-orbital of boron interacts with the pi electrons of the alkene.
- **Hint**: Look for the interaction between the alkene's pi electrons and boron's empty p-orbital.

2. **Cyclic Transition State**: This pi complex leads to a cyclic four-center transition state. This transition state involves the formation of a new bond between boron and the alkene.
- **Hint**: Identify the four atoms involved in the cyclic transition state.

3. **Formation of Alkyl Borane**: After the transition state, the complex dissociates to form an alkyl borane.
- **Hint**: Consider how the transition state breaks down to form the final product.

### Step 3: Analyzing the Statements
Now, let's evaluate the statements provided:

1. **Statement A**: "Reaction proceeds through ionic intermediate and same addition takes place."
- This statement is **false**. Hydroboration does not proceed through an ionic intermediate; instead, it involves a cyclic transition state.

2. **Statement B**: "Reaction proceeds through cyclic intermediate transition state and same addition takes place."
- This statement is **true**. The reaction does involve a cyclic transition state, and it exhibits syn addition.

3. **Statement C**: "Diborane in ether exists as ionic entities."
- This statement is **false**. Diborane (B2H6) does not exist as ionic entities in ether; it exists as a molecular species.

4. **Statement D**: "Diborane in ether exists as a molecular species."
- This statement is **true**. In non-polar solvents like ether, diborane exists as a molecular species rather than ionic entities.

### Conclusion
Based on the analysis:
- The true statements regarding hydroboration of alkenes are **B and D**.

### Summary of True Statements:
- **B**: Reaction proceeds through cyclic intermediate transition state and syn addition takes place.
- **D**: Diborane in ether exists as a molecular species.