Question

In which of the following statement(s), hydrogen bonding is a dominating phenomenon? (A) $p$-hydroxy benzoic acid is less acidic than $o$-hydroxy benzoic acid (B) Secondary alkyl amine is more basic than tertiary amine in aqueous solution (C) Boiling point of $\mathrm{HF}$ is higher than boiling point of HI (D) Maleic acid is a stronger acid than fumaric acid

Short answer

Hydrogen bonding is a dominating phenomenon in all statements A, B, C, and D. In statement A, hydrogen bonding affects the acidity of o-hydroxy benzoic acid and p-hydroxy benzoic acid. In statement B, hydrogen bonding impacts the basicity of secondary and tertiary alkyl amines in aqueous solution. In statement C, hydrogen bonding causes the boiling point of HF to be higher than that of HI. In statement D, hydrogen bonding contributes to the acidic strength differences between maleic acid and fumaric acid.

Step-by-step solution

1. Understanding hydrogen bonding

Hydrogen bonding is a type of strong intermolecular force (dipole-dipole interaction) that occurs between molecules containing a highly electronegative atom (such as fluorine, oxygen, or nitrogen) bonded to a hydrogen atom. It results from the attraction of a hydrogen atom in a molecule with the lone pair of electrons on an electronegative atom in another molecule. Now let's apply this understanding to each of the given statements:

2. Statement A

Statement A states that p-hydroxy benzoic acid is less acidic than o-hydroxy benzoic acid. The position of the hydroxy group in the molecule affects the acidity of the molecule. In o-hydroxy benzoic acid, there is an intramolecular hydrogen bond that forms between the hydroxy group and the nearby carbonyl group. This hydrogen bond reduces the acidity by stabilizing the hydrogen. In the case of p-hydroxy benzoic acid, the hydrogen bonding is less significant, and as a result, it is less acidic compared to o-hydroxy benzoic acid. So, hydrogen bonding dominates in statement A.

3. Statement B

Statement B states that secondary alkyl amine is more basic than tertiary amine in aqueous solution. In the case of amines, hydrogen bonding dominates the solubility of primary and secondary amines, as they have hydrogen atoms to form a hydrogen bond with a water molecule. However, tertiary amines lack a hydrogen atom to form this hydrogen bond and, as a result, are less soluble in water. In aqueous solution, the solubility of amines is essential for their basicity. Therefore, hydrogen bonding is important in statement B.

4. Statement C

Statement C states that the boiling point of HF is higher than the boiling point of HI. Boiling points of molecules are highly influenced by the strength of intermolecular forces. In the case of hydrogen fluoride (HF), there is a strong hydrogen bond between fluorine (highly electronegative) and hydrogen, leading to a high boiling point. However, HI does not have hydrogen bonding, so the boiling point of HI is lower than that of HF. Thus, hydrogen bonding is significant in statement C.

5. Statement D

Statement D says that maleic acid is a stronger acid than fumaric acid. Maleic acid has a cis configuration, while fumaric acid has a trans configuration. The cis configuration in maleic acid allows it to form an intramolecular hydrogen bond, causing the two COOH groups to be close together. This hydrogen bonding leads to a more reactive double bond and a more acidic character. In the case of fumaric acid, due to its trans configuration, no hydrogen bonding occurs, and it becomes a weaker acid compared to maleic acid. So, hydrogen bonding plays a role in statement D. In conclusion, hydrogen bonding is a dominating phenomenon in all statements A, B, C, and D.

Key concepts

Intermolecular Forces

An important concept in chemistry is the understanding of intermolecular forces, which are the attractions between molecules that affect numerous physical properties. One of the strongest types of these forces is hydrogen bonding. Hydrogen bonding occurs when a hydrogen atom is covalently bonded to a highly electronegative atom like nitrogen, oxygen, or fluorine, and this positively charged hydrogen atom is then attracted to the lone pair of electrons on another electronegative atom within a different molecule or a different part of the same molecule (in the case of intramolecular hydrogen bonding).

For instance, in the comparison between the boiling points of HF and HI, the effect of hydrogen bonding in HF results in a higher boiling point. This type of attraction is crucial to chemical behaviors and properties such as solubility and boiling points.

Acidity of Organic Compounds

The acidity of organic compounds is often influenced by the presence of substituents that can either donate or withdraw electrons. In the context of hydrogen bonding, the acidity can also be affected. Organic acids, such as carboxylic acids, can form hydrogen bonds which can stabilize the negative charge on the conjugate base that forms after an acid loses a proton.

For example, the acidity of hydroxy benzoic acids is influenced by the position of the hydroxy group. When in close proximity to the carboxyl group, as in o-hydroxy benzoic acid, hydrogen bonding occurs within the molecule stabilizing the negative charge on the conjugate base thus enhancing acidity. Whereas in p-hydroxy benzoic acid, intramolecular hydrogen bonding is not possible due to the relative positions of the functional groups, hence it's less acidic.

Boiling Point Comparison

Boiling point comparison is a fascinating topic directly related to intermolecular forces. The stronger the intermolecular forces within a substance, the higher its boiling point, as more energy is required to break the attractions between the particles. Hydrogen bonding significantly raises the boiling points of compounds due to its strength compared to other dipole-dipole interactions or Van der Waals forces.

As observed in our exercise, HF has a higher boiling point than HI because the hydrogen bond in HF entails a stronger attraction between molecules than the weaker Van der Waals forces present among HI molecules. This principle is fundamental for understanding the physical characteristics of substances, particularly when comparing compounds like alcohols, acids, and amines with differing tendencies to hydrogen bond.

Amine Basicity in Aqueous Solution

Amine basicity in aqueous solutions is greatly influenced by the molecule's ability to participate in hydrogen bonding with the solvent, which in most laboratory contexts is water. The basicity of an amine refers to its propensity to accept protons - a behavior essen­tial in many biological and chemical processes. Amines can be primary, secondary, or tertiary, depending on the number of carbon-containing groups attached to the nitrogen atom.

Secondary amines, for instance, possess hydrogen atoms that can form hydrogen bonds with water, thus enhancing their solubility and, consequently, their ability to act as bases in water. Tertiary amines lack this hydrogen, minimizing such bonding and making them less basic in aqueous solutions. Understanding these interactions is crucial in organic chemistry and pharmaceutical sciences, where the solubility and reactivity of amines affect the design and function of many medications.