Question

Hydrogen bonding plays a central role in the following phenomena: (\Lambda) Ice floats in water. (B) Higher Lewis basicity of primary amines than tertiary amines in aqueous solutions. (C) Formic acid is more acidic than acetic acid. (D) Dimerisation of acetic acid in benzene.

Short answer

Ice floats because it is less dense due to its hydrogen-bonded lattice structure. Primary amines are more basic due to less steric hindrance allowing for better hydrogen bonding with water. Formic acid is more acidic than acetic due to lack of an electron-donating group stabilizing the anion. Acetic acid forms dimers through hydrogen bonding in non-polar solvents like benzene.

Step-by-step solution

Understanding Ice Floating

Examine the structure of ice and how molecules are arranged. Ice has a lattice structure that is less dense than liquid water due to hydrogen bonding, which holds the water molecules apart in a fixed arrangement, resulting in a solid with a lower density.

Comparing Lewis Basicity of Amines

Analyze the ability of primary and tertiary amines to donate an electron pair. Primary amines have more available lone pair of electrons for hydrogen bonding with water molecules which makes them more soluble and thus higher Lewis basicity compared to tertiary amines, where the lone pair is more sterically hindered.

Analyzing Acidity of Carboxylic Acids

Examine the molecular structure of formic acid and acetic acid. Formic acid lacks an electron-donating alkyl group, which makes the carboxylate anion more stable through hydrogen bonding. This means formic acid can more easily lose a proton, making it more acidic.

Dimerisation of Acetic Acid

Understand the formation of dimers through hydrogen bonding. In a non-polar solvent like benzene, acetic acid molecules form dimers linked by strong hydrogen bonds, which minimize polar characteristics of acetic acid in the non-polar environment.

Key concepts

Ice Lattice Structure

Water is a remarkable substance, and its unique properties become especially evident in its solid form – ice. At a microscopic level, ice forms a lattice structure due to hydrogen bonding. Unlike in its liquid state, where water molecules are closer together, in ice, each water molecule forms a fixed pattern with four others creating a hexagonal shape.

This geometry spaces the molecules further apart than when they are in a liquid state, accounting for ice's lower density and its ability to float on water. The hydrogen bonds are responsible for keeping the structure rigid, which also makes ice less dense than liquid water. It's essential to appreciate that this phenomenon plays a critical role in the environment, such as allowing aquatic life to survive under frozen surfaces during winter.

Lewis Basicity of Amines

Amines are organic compounds that have a nitrogen atom with a lone pair of electrons, which makes them act as Lewis bases. This lone pair can form a coordinate bond with a hydrogen atom, a process often referred to as hydrogen bonding.

Primary amines exhibit greater Lewis basicity than tertiary amines because the lone pair of electrons on the nitrogen is less sterically hindered and more available to interact with water molecules. In comparison, the added alkyl groups in tertiary amines create steric bulk that hinders the accessibility of the lone pair. This difference in basicity can affect the solubility and reactivity of the amines in aqueous solutions.

Acidity of Carboxylic Acids

Carboxylic acids are acidic due to their ability to donate a proton. The strength of an acid is often determined by how stable the resulting anion is. For instance, formic acid is more acidic than acetic acid because the formate anion can stabilize itself through hydrogen bonding without any electron-donating alkyl groups to diminish this effect.

In contrast, acetic acid has a methyl group that donates electrons by induction, which stabilizes the charged anion but also makes the proton less likely to dissociate. Therefore, even though both acids can form hydrogen bonds, formic acid is more willing to give up its proton, thus being more acidic.

Dimerisation of Acetic Acid

Acetic acid has an inclination to pair up, or 'dimerise', particularly in solutions where polarity is less pronounced, like benzene. The hydrogen bonding between the carboxyl groups of two acetic acid molecules leads to the formation of a dimer, which is essentially two molecules linked together.

This dimerization process is notable because it minimizes the polar characteristics of acetic acid in the non-polar environment, which is a fine illustration of the compounds' adaptive nature to different surroundings. This behavior has important implications for the acid's properties in various chemical environments.