In which of the following molecule, vacant orbital is not necessary for dimer formation :-

Options

A. `BH_(3)`
B. `AlCl_(3)`
C. `CH_(3)COOH`
D. `BeCl_(2)(s)`

Answer

To determine which molecule does not require a vacant orbital for dimer formation, we will analyze each given option step by step.

### Step 1: Analyze BH3
- **Molecule**: BH3 (Boron Trifluoride)
- **Structure**: Boron has only three valence electrons and forms three bonds with hydrogen atoms, leading to an incomplete octet.
- **Dimer Formation**: To complete its octet, BH3 molecules can dimerize to form B2H6 (Diborane). This process involves the formation of additional bonds, which requires vacant orbitals on boron.

**Conclusion**: Vacant orbitals are necessary for dimer formation in BH3.

### Step 2: Analyze AlCl3
- **Molecule**: AlCl3 (Aluminum Chloride)
- **Structure**: Aluminum has three valence electrons and forms three bonds with chlorine atoms, also resulting in an incomplete octet.
- **Dimer Formation**: AlCl3 can dimerize to form Al2Cl6. Similar to BH3, this dimerization requires the presence of vacant orbitals on aluminum.

**Conclusion**: Vacant orbitals are necessary for dimer formation in AlCl3.

### Step 3: Analyze CH3COOH
- **Molecule**: CH3COOH (Acetic Acid)
- **Structure**: Acetic acid has a hydroxyl group (-OH) and a carbonyl group (C=O). The molecule can form hydrogen bonds with another acetic acid molecule.
- **Dimer Formation**: The dimerization occurs through hydrogen bonding between the -OH group of one acetic acid and the carbonyl oxygen of another. This process does not require vacant orbitals.

**Conclusion**: Vacant orbitals are NOT necessary for dimer formation in CH3COOH.

### Step 4: Analyze BeCl2
- **Molecule**: BeCl2 (Beryllium Chloride)
- **Structure**: Beryllium has two valence electrons and forms two bonds with chlorine atoms, leading to an incomplete octet.
- **Dimer Formation**: BeCl2 can dimerize to form Be2Cl4. This process also requires the presence of vacant orbitals on beryllium.

**Conclusion**: Vacant orbitals are necessary for dimer formation in BeCl2.

### Final Answer
The molecule in which a vacant orbital is NOT necessary for dimer formation is **CH3COOH (Acetic Acid)**.

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