Hydrogen bonding plays a central role in which of the following phenomena?

Options

A. Ice floats in water
B. Higher Lewis basicity of primary than tertiary amines in aqueous solutions
C. Formic acid is more acidic than acetic acid
D. Dimersation of acetic acid in benzene .

Explanation

To determine the role of hydrogen bonding in various phenomena, we will analyze each option provided in the question step by step.

### Step 1: Analyze the first phenomenon - Ice floats in water
- **Explanation**: Ice floats on water because its density is less than that of liquid water. The lower density of ice is due to its unique cage-like structure formed by hydrogen bonding. In ice, water molecules are arranged in a way that creates open spaces, making it less dense than liquid water.
- **Conclusion**: Hydrogen bonding is responsible for the cage-like structure in ice, which leads to its lower density. Therefore, this phenomenon is due to hydrogen bonding.

### Step 2: Analyze the second phenomenon - Higher Lewis basicity of primary than tertiary amines in aqueous solution
- **Explanation**: In aqueous solutions, primary amines (RNH2) can form stable cations (RNH3+) through hydrogen bonding with water, which enhances their basicity. In contrast, tertiary amines (R3N) do not form stable cations in the same way, leading to lower basicity.
- **Conclusion**: Hydrogen bonding plays a crucial role in stabilizing the cation formed from primary amines, thus this phenomenon is also due to hydrogen bonding.

### Step 3: Analyze the third phenomenon - Formic acid is more acidic than acetic acid
- **Explanation**: The acidity of an acid is determined by the stability of its conjugate base. Formic acid (HCOOH) is more acidic than acetic acid (CH3COOH) because the formate ion (HCOO-) is more stable than the acetate ion (CH3COO-). The stability of the acetate ion is reduced due to the +I effect of the methyl group (CH3), which destabilizes the anion.
- **Conclusion**: Hydrogen bonding does not play a role in the acidity comparison between formic acid and acetic acid. Therefore, this phenomenon is not due to hydrogen bonding.

### Step 4: Analyze the fourth phenomenon - Dimerization of acetic acid in benzene
- **Explanation**: Acetic acid (CH3COOH) can form dimers through hydrogen bonding in a non-polar solvent like benzene. The hydrogen bonding between two acetic acid molecules leads to the formation of a dimer, which is stabilized by these intermolecular interactions.
- **Conclusion**: The dimerization of acetic acid in benzene is indeed due to hydrogen bonding.

### Final Conclusion
Based on the analysis:
- The phenomena where hydrogen bonding plays a central role are:
- A) Ice floats in water
- B) Higher Lewis basicity of primary than tertiary amines in aqueous solution
- D) Dimerization of acetic acid in benzene
- The correct answer is A, B, and D.

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