Question

The correct order for ortho xylene and para xylene is/are (A) \((\sigma \text {-bond })_{\text {oxylene }}>(\sigma \text {-bond })_{p \text {-xylene }}\) (B) \((\pi \text {-bond })_{o \text {-xylene }}<(\pi \text {-bond })_{p \text {-xylene }}\) (C) \(\mathrm{IHD}_{o \text {-xylene }}>(\pi \text {-bond })_{p \text {-xylene }}\) (D) \((\sigma \text {-bond })_{o \text {-xylene }}>(\pi \text {-bond })_{p \text {-xylene }}\)

Short answer

The correct order for ortho-xylene and para-xylene is: (D) \((\sigma \text{-bond})_{o \text{-xylene}}>(\pi \text{-bond})_{p \text{-xylene}}\)

Step-by-step solution

Draw the structures of ortho-xylene and para-xylene.

Ortho-xylene and para-xylene are both isomers of dimethylbenzene, with two methyl groups attached to a benzene ring. Ortho-xylene has the two methyl groups attached to adjacent carbon atoms, while para-xylene has the two methyl groups attached opposite each other on the benzene ring.

Calculate the number of sigma-bonds and pi-bonds in ortho-xylene and para-xylene.

In both ortho-xylene and para-xylene, there are 12 sigma bonds originating from the benzene ring, 6 sigma bonds originating from the methyl groups, and 3 pi bonds as part of the aromatic ring, totalling 18 sigma-bonds and 3 pi-bonds.

Calculate the IHD for ortho-xylene and para-xylene.

The Index of Hydrogen Deficiency (IHD) is a measure of the degree of unsaturation in a molecule and can be calculated using the formula: \[ \mathrm{IHD} = \frac{(2n + 2) - X}{2} \] where n is the number of carbon atoms and X is the number of hydrogen atoms. For both ortho-xylene and para-xylene, they have 8 carbon atoms and 10 hydrogen atoms. Substituting the values to the equation, we get: \[ \mathrm{IHD} = \frac{(2 \cdot 8 + 2) - 10}{2} = \frac{16}{2} = 8 \] The IHD for both ortho-xylene and para-xylene is 8.

Compare the values and determine the correct statement.

We have calculated the following values for ortho-xylene and para-xylene: Sigma-bonds (σ-bond): 18 Pi-bonds (π-bond): 3 IHD: 8 Now let's look at the given inequalities: (A) The sigma-bond values for ortho-xylene and para-xylene are the same (18), therefore this statement is FALSE. (B) The pi-bond values for ortho-xylene and para-xylene are the same (3), therefore this statement is FALSE. (C) The IHD value for ortho-xylene and para-xylene is 8, not greater than the pi-bond value (3), therefore this statement is FALSE. (D) The sigma-bond values for ortho-xylene and para-xylene are greater (18) than the pi-bond values (3), therefore this statement is TRUE. Based on our analysis, the correct order for ortho-xylene and para-xylene is: (D) \((\sigma \text {-bond })_{o \text {-xylene }}>(\pi \text {-bond })_{p \text {-xylene }}\)

Key concepts

ortho-xylene and para-xylene

Both ortho-xylene and para-xylene are isomers of a compound known as dimethylbenzene. Understanding how these isomers differ is crucial to grasp their unique chemical behaviors. Here, both compounds share the same molecular formula, but differ in how their atoms are arranged.

**Ortho-xylene** has two methyl groups (–CH₃) that are attached to consecutive carbon atoms on a benzene ring. Imagine this as two friends standing shoulder to shoulder. In comparison, **para-xylene** has its methyl groups positioned directly across from each other on the benzene ring. Think of them as two friends facing each other from opposite sides of a circular table.

This positioning alters how the molecules interact with each other and their environment. Ortho-xylene's closely placed methyl groups can cause more steric hindrance, whereas the para positioning in para-xylene can offer more even distribution of any additional reactions or forces.

sigma and pi bonds

In chemistry, understanding bonds between atoms helps explain a molecule’s properties. Two common types of chemical bonds are sigma (σ) bonds and pi (π) bonds.

**Sigma bonds** are the strongest type of covalent chemical bond. They form when two orbitals overlap along the axis connecting two atomic nuclei, allowing maximum overlap and forming a stable structure. In both ortho-xylene and para-xylene, there are 18 sigma bonds. These strong and stable bonds contribute to the overall structural integrity of the molecules.

On the other hand, **pi bonds** are formed when orbitals overlap across two different lobes above and below the atomic nucleus' axis. They are usually found in double or triple bonds within a molecule. Aromatic rings, like those found in ortho-xylene and para-xylene, have 3 pi bonds that are crucial for the ring’s stability and its unique aromatic properties. These bonds, though weaker and more reactive than sigma bonds, contribute to the distinct characteristics of aromatic compounds.

isomers of dimethylbenzene

The concept of isomers relates to compounds having the same molecular formula but a different arrangement of atoms, leading to distinct properties.

**Dimethylbenzene** is an example of such isomerism, with ortho-xylene and para-xylene being its well-known isomers. Each arrangement of the methyl groups affects not only the physical properties, such as melting and boiling points, but also the chemical reactivity of these compounds.

The presence of different isomers allows chemists to tailor specific characteristics for desired applications. Whether one seeks a solvent with a specific evaporation rate or a material with certain aromatic properties, isomers offer a way to achieve these goals through their nuanced structural differences. Thus, studying isomers like those found in dimethylbenzene expands our ability to harness chemical properties for various scientific, industrial, and commercial uses.