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) \(1,3,2\)

Short answer

The correct sequence is (c) 2, 1, 3.

Step-by-step solution

Analyzing H2S2O6

The structure of \(\mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{6}\) involves a dithionic acid, where each sulfur is bonded to two oxygen atoms and shares a bond with another sulfur. Among the oxygen atoms attached to each sulfur, one forms a double bond, contributing one \(\pi\) bond per sulfur. Thus, the molecule contains 2 \(\pi\) bonds.

Analyzing H2SO3

The structure of \(\mathrm{H}_{2} \mathrm{SO}_{3}\) is that of sulfurous acid. In this molecule, sulfur is bonded to three oxygen atoms, where one is double-bonded and the other two are single-bonded. Therefore, \(\mathrm{H}_{2} \mathrm{SO}_{3}\) contains 1 \(\pi\) bond from this double bond.

Analyzing H2S2O5

The structure of \(\mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{5}\) involves two sulfur atoms each bonded to oxygen atoms. Typically, each sulfur will have one oxygen double-bonded to it, contributing a total of 2 \(\pi\) bonds in the molecule, similar to the structure of pyrosulfuric acid.

Ordering by Increasing Pi Bonds

Now that we've determined the number of \(\pi\) bonds in each molecule: \(\mathrm{H}_{2} \mathrm{SO}_{3}\) has 1 \(\pi\) bond, \(\mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{6}\) has 2 \(\pi\) bonds, and \(\mathrm{H}_{2} \mathrm{S}_{2} \mathrm{O}_{5}\) also has 2 \(\pi\) bonds. Arranging in increasing order yields: 2, 1, 3.

Key concepts

Pi Bonds

Chemical bonding is an essential part of understanding molecular structures. One important type of bond is the pi bond, which forms when parallel p-orbitals overlap. It is different from a sigma bond, which forms through head-on overlapping of orbitals. In molecules, pi bonds are generally found in double and triple bonds.

They contribute to a molecule's strength and stability, and influence its shape, reactivity, and color. A single bond in a molecule (like a C-C or S-S bond) consists of one sigma bond, and any additional bonds (double or triple bonds) introduce pi bonds.

For example, a C=C (carbon-carbon double bond) comprises one sigma bond and one pi bond. This additional pi bond increases electron density above and below the internuclear axis, affecting how molecules react with others. In our exercise, assessing the number of pi bonds helps us understand how arranging molecules by their bonding can show different structural properties.

Dithionic Acid

Dithionic acid, with the chemical formula \(\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{6}\), is a molecule featuring sulfur and oxygen atoms. It can be challenging to visualize, but imagine two sulfur atoms linked directly to each other. Each sulfur atom then connects to two oxygen atoms.

Specifically, one of these two oxygen atoms forms a double bond with sulfur. This results in two \(\pi\) bonds in the entire molecule—as each sulfur brings one \(\pi\) bond to the shared structure.

Notably, dithionic acid appears less frequently in discussions than compounds like sulfur dioxide. However, due to its structure involving these distinctive \(\pi\) bonds, it highlights the diversity of sulfur-oxygen compounds and their unique bonding properties.

Sulfurous Acid

Sulfurous acid, represented by the formula \(\mathrm{H}_{2} \mathrm{SO}_{3}\), is an important chemical often found in discussions about sulfur compounds. Sulfurous acid is an example of a molecule where sulfur forms a double bond with one of the oxygen atoms, resulting in a total of one \(\pi\) bond in the compound.

The structure consists of a single sulfur atom bound to three oxygen atoms. In an aqueous solution, sulfurous acid decomposes, and it is a medium-strong acid used in various applications, like water treatment and as a preservative.

Understanding its bonding critical, as it demonstrates how a single \(\pi\) bond can influence the behavior and stability of a molecule in both chemical reactions and physical interactions.

Pyrosulfuric Acid

Known as Oleum in industrial settings, pyrosulfuric acid has the formula \(\mathrm{H}_{2} \mathrm{~S}_{2} \mathrm{O}_{5}\). In pyrosulfuric acid, the presence of two sulfur atoms gives rise to complexity in its structure.

Each sulfur links to oxygen atoms, mirroring the structure found in dithionic acid where each sulfur forms a double bond with one oxygen. Consequently, pyrosulfuric acid contains two \(\pi\) bonds.

In industry, pyrosulfuric acid, or oleum, has a role in producing sulfuric acid in a highly concentrated form. Its structural intricacies underscore the importance of understanding chemical bonding and its effects on the properties and uses of a compound.