Q.32 Which of the following statements are TRUE for hydrogen bonds?
Strength of hydrogen bond is
(A) low in a solvent of high dielectric constant
(B) low in a solvent of low dielectric constant
(C) lower in water as compared to organic solvents
(D) higher in water as compared to organic solvents

Hydrogen bond strength decreases in solvents with high dielectric constants, such as water, due to better stabilization of charged or polar groups, making intermolecular hydrogen bonds weaker compared to low dielectric organic solvents.​

Option Analysis

(A) low in a solvent of high dielectric constant: True. High dielectric solvents screen electrostatic interactions, reducing hydrogen bond strength by stabilizing separated donor and acceptor groups more effectively.​

(B) low in a solvent of low dielectric constant: False. Low dielectric solvents (e.g., aprotic organics like chloroform) provide poor screening, so hydrogen bonds form more readily and are stronger to minimize unfavorable charge separation.​

(C) lower in water as compared to organic solvents: True. Water (dielectric constant ~80) competes with solute hydrogen bonds and stabilizes dissociated forms better, weakening them relative to organic solvents (dielectric constants often <20).​

(D) higher in water as compared to organic solvents: False. Water’s high polarity disrupts solute hydrogen bonds, making them weaker than in less polar organics where bonds are more stable.​

Correct options: (A) and (C).​

Hydrogen bond strength in high dielectric constant solvents plays a key role in molecular interactions, especially for CSIR NET Life Sciences aspirants tackling biochemistry and physical chemistry questions. This article breaks down why hydrogen bonds weaken in polar environments like water compared to organic solvents.

Dielectric Constant Basics

Dielectric constant measures a solvent’s ability to screen electrostatic forces. High values (e.g., water at 78.5) reduce attraction between hydrogen bond donors (like O-H) and acceptors (like carbonyl O), lowering bond strength. Low dielectric organic solvents (e.g., chloroform ~5) enhance these bonds by limiting screening.​

Water vs Organic Solvents

In water, hydrogen bonds between solutes are lower strength than in organic solvents because water molecules compete, forming stronger water-solute bonds and stabilizing separated polar groups. Studies show binding free energy inversely correlates with dielectric constant, confirming weaker bonds in water.​

• Gas phase or vacuum: Strongest hydrogen bonds (no screening).

• Organic solvents: Intermediate strength.

• Water: Weakest due to solvation.​

CSIR NET Exam Insights

For questions like “Strength of hydrogen bond is low in a solvent of high dielectric constant,” options (A) and (C) hold true. Electrostatic energy E=q1q24πϵ0DrE=4πϵ0Drq1q2 drops as dielectric $D$ rises, directly impacting bond stability. Practice this for molecular biology and enzyme kinetics sections.