Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

Dipole moment is shown by (a) 2-methyl propane (b) trans-2-butene (c) cis-2-butene (d) carbon tetrachloride

Step-by-Step Solution

Answer

Cis-2-butene shows a dipole moment.

Step 1: Understand the Concept of Dipole Moment

Dipole moment is a measure of the separation of positive and negative charges in a molecule. Molecules with a net dipole moment are typically polar, due to an asymmetric distribution of electron density.

Step 2: Analyze Each Molecule for Symmetry

Examine each molecule: - **2-methylpropane**: It has a symmetrical structure which causes any dipoles to cancel out. - **trans-2-butene**: It is a symmetrical molecule about its double bond, causing its dipoles to cancel out. - **cis-2-butene**: This has a lack of symmetry which could result in a net dipole. - **carbon tetrachloride (CCl₄)**: This has a tetrahedral structure which is symmetrical, leading dipoles from the C-Cl bonds to cancel out.

Step 3: Identify the Molecule with Asymmetry

Among the given options, only **cis-2-butene** has an asymmetrical structure. In its cis configuration, the molecule has both groups oriented on the same side of the double bond, leading to a net dipole moment.

Step 4: Conclude the Solution

Since dipole moments arise due to the lack of symmetry, the only molecule with a dipole moment is **cis-2-butene** due to its asymmetrical arrangement of atoms causing a net dipole movement.

Key Concepts

Molecular Symmetry

Molecular symmetry plays a crucial role in determining the presence or absence of a dipole moment in molecules. When a molecule is symmetrical, the individual bond dipoles (the directional charges within a bond) tend to cancel each other out. In other words, if you can divide a molecule into equal halves that mirror each other completely, then it is symmetrical.
Molecules like **2-methylpropane** and **carbon tetrachloride (CCl₄)** are excellent examples of symmetrical molecules.

• In **2-methylpropane**, the structure is such that any polar bonds, such as C-H bonds, are oriented in a way that they neutralize each other.
• **Carbon tetrachloride** has a tetrahedral geometry. Each Cl atom is equidistant and oriented symmetrically around the central carbon atom, causing the polar C-Cl bonds to cancel each other out.

Thus, symmetry in molecular geometry is key to understanding why some molecules do not display dipole moments despite having polar bonds.

Polarity in Organic Molecules

Understanding polarity in organic molecules helps in predicting how these molecules will interact with different environments and substances.
Molecules with a net dipole moment are considered polar. This results from an unequal distribution of electron density, which occurs when atoms with different electronegativities form bonds.

• Polar molecules have regions with slight positive and negative charges, causing them to interact uniquely with one another and with electric fields.
• A molecule's overall polarity is determined by both the individual dipole moments of its bonds and its geometry.

The polar nature of a molecule affects its chemical properties and interactions, including solubility and reactivity. For example, polar molecules like **cis-2-butene** result in a net dipole moment due to asymmetric distribution of side groups, enhancing their interaction with polar substances.

Cis-Trans Isomerism

Cis-trans isomerism, also referred to as geometric isomerism, occurs in certain molecules with double bonds or ring structures. These isomers differ in the spatial arrangement of groups around the double bond or ring, leading to distinct physical and chemical properties.
In cis-trans isomerism:

• **Cis-isomers** have both substituents on the same side of the double bond, leading to dramatic changes in physical properties, such as dipole moments.
• **Trans-isomers** have substituents on opposite sides of the double bond, usually resulting in a more symmetrical geometry.

Consider **cis-2-butene**: In this molecule, the two methyl (CH₃) groups are on the same side of the double bond, resulting in an overall dipole moment because the electron cloud is not evenly distributed. In contrast, in **trans-2-butene**, the symmetry often leads to no net dipole moment as the effect of the dipoles cancels out. This distinct arrangement in cis-trans isomers explains differences in their physical characteristics, reactions, and interactions with external fields.