Problem 102 - A Complete Resource Book in Chemistry for JEE Main

Question

Identify the correct sequence of increasing number of \(\pi\) bonds in the structures of the following molecules 1\. \(\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{6}\) 2\. \(\mathrm{H}_{2} \mathrm{SO}_{3}\) 3\. \(\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{5}\) (a) \(1,2,3\) (b) \(2,3,1\) (c) \(2,1,3\) (d) \(1,3,2\)

Step-by-Step Solution

Answer

Correct sequence is (b) 2, 3, 1.

Step 1: Identify the Number of Pi Bonds in H2S2O6

The molecule H2S2O6 is known as dithionic acid. In this compound, there is a direct S-S bond with each sulfur atom double-bonded to two oxygen atoms. This results in the presence of two S=O bonds for each sulfur, amounting to a total of four  bonds.

Step 2: Identify the Number of Pi Bonds in H2SO3

H2SO3 refers to sulfurous acid. This structure typically consists of sulfur being double-bonded to one oxygen and single-bonded to two hydroxyl groups (OH). There is only one  bond present originating from the single S=O bond.

Step 3: Identify the Number of Pi Bonds in H2S2O5

H2S2O5 is known as disulfurous acid. In this molecule, there is an S-O-S linkage wherein each sulfur atom is double-bonded to one oxygen. Therefore, this molecule contains a total of two  bonds.

Step 4: Sequence the Molecules by Increasing Number of Pi Bonds

Using the pi bond counts from the previous steps: H2SO3 has 1  bond, H2S2O5 has 2  bonds, and H2S2O6 has 4  bonds. Arranging these in increasing order gives the sequence: H2SO3, H2S2O5, H2S2O6.

Key Concepts

Dithionic Acid

Dithionic Acid, with the chemical formula \(H_2S_2O_6\), is a fascinating compound known for its structure involving sulfur atoms. In this molecule, two sulfur atoms are linked directly by a sulfur-sulfur (S-S) bond. Each sulfur atom then forms two double bonds with oxygen, creating the sulfur-oxygen (S=O) bonds. Altogether, this provides the compound with four pi bonds.

Understanding the structure of dithionic acid helps in recognizing its chemical behavior. These multiple pi bonds play a crucial role in determining the compound’s stability and reactivity. The double bonds between sulfur and oxygen contribute significantly to the molecule's resonance and overall molecular geometry.

• Structure: Direct S-S linkage and S=O double bonds.
• Pi Bonds: Four in total from double-bonded oxygens.

This unique feature of having multiple pi bonds in dithionic acid affects its chemical interactions, influencing properties like solubility and acidity.

Sulfurous Acid

Sulfurous Acid, represented as \(H_2SO_3\), is another sulfur-containing compound, differing significantly from dithionic acid. Here's how: in sulfurous acid, the sulfur atom is bonded to one oxygen atom through a double bond, leading to the presence of a single pi bond. This is the only pi bond in the molecule, making it simpler than dithionic acid.

Sulfurous acid also contains two hydroxyl (OH) groups bonded to the sulfur atom. This arrangement influences its acidity and is crucial for understanding acid-base reactions involving sulfurous acid. The presence of hydroxyl groups means there is an opportunity for hydrogen bonding, which affects its physical properties such as boiling point and solubility.

• Structure: Single S=O double bond and two hydroxyl groups.
• Pi Bonds: One from the S=O double bond.

The simplicity of sulfurous acid's structure, with only one pi bond, makes it distinct and vital in distinguishing it from its sulfur-containing counterparts like dithionic and disulfurous acids.

Disulfurous Acid

Disulfurous Acid, known chemically as \(H_2S_2O_5\), presents itself with a unique arrangement among sulfur and oxygen atoms with an S-O-S linkage. Each sulfur in disulfurous acid is bonded to an oxygen atom through a double bond, yielding a total of two pi bonds in the entire molecule.

The presence of these pi bonds, along with the bridging oxygen, affects the reactivity and stability of disulfurous acid. The linkage and pi bonds contribute to the rigidity and shape of the molecule, impacting its interaction with other compounds.

• Structure: S-O-S linkage with S=O double bonds for each sulfur.
• Pi Bonds: Two pi bonds from the double-bonded oxygens.

Disulfurous acid, containing an intermediate number of pi bonds compared to dithionic and sulfurous acids, showcases different chemical properties and reactivities that are critical for understanding its role in chemical reactions.