Problem 12 - A Complete Resource Book in Chemistry for JEE Main

Question

Which of the following has zero dipole moment? (a) 1 -butene (b) cis-2-butene (c) trans-2-butene (d) 2 methyl-1-propene

Step-by-Step Solution

Answer

Trans-2-butene has a zero dipole moment.

Step 1: Identify Molecular Symmetry

To determine which compound has a zero dipole moment, first consider the molecular structure and symmetry of each compound. Symmetrical molecules often have zero dipole moments. The dipole moment is a vector quantity; if it cancels out from all regions, then the net dipole moment is zero.

Step 2: Analyze 1-butene

1-butene is a linear alkene with a terminal double bond. It is not symmetrical because the ends of the molecule are different, leading to a non-zero dipole moment due to the uneven distribution of charge.

Step 3: Examine cis-2-butene

Cis-2-butene has both of its substituents on the same side of the double bond, creating a situation where dipole moments do not cancel out. This asymmetry results in a non-zero dipole moment.

Step 4: Examine trans-2-butene

In trans-2-butene, the substituents on the double bond are opposite each other. This configuration creates symmetry along the horizontal axis, and the dipole moments cancel each other out, resulting in a zero dipole moment.

Step 5: Consider 2-methyl-1-propene

2-methyl-1-propene is asymmetric due to the presence of the methyl group. This asymmetry means that its dipole moments do not cancel out, leading to a non-zero dipole moment.

Key Concepts

Molecular Symmetry

When discussing molecular symmetry, we're focusing on the balance and arrangement of molecules in a way that affects their physical properties like dipole moment. Molecules that exhibit high symmetry are often non-polar. This is because the symmetrical arrangement allows the dipole moments from bonds within the molecule to cancel each other out.
In our exercise, we're looking at different organic molecules. To determine if a molecule has a zero dipole moment, we must consider its symmetry. For example:

• A symmetrical molecule will have dipole moments that are equal in magnitude but opposite in direction, leading to a net dipole moment of zero.
• An asymmetrical molecule will have an uneven distribution of charge, resulting in a non-zero dipole moment.

Understanding molecular symmetry helps chemists predict physical and chemical properties, which is crucial in synthetic chemistry and material science.

Alkenes

Alkenes are a class of hydrocarbons characterized by at least one double bond between carbon atoms. This double bond gives alkenes unique chemical properties and affects their geometry.
Double bonds in alkenes restrict rotation, leading to different geometric isomers; examples in this exercise include cis and trans isomers:

• Cis-isomer: Substituents are on the same side of the double bond, often resulting in an unsymmetrical structure. This leads to a non-zero dipole moment.
• Trans-isomer: Substituents are on opposite sides, creating a more symmetrical arrangement, contributing to a zero dipole moment.

These structural differences between isomers affect their chemical behavior and physical properties, explaining why some alkenes have zero dipole moments while others do not.

Dipole Moment Cancellation

Dipole moment cancellation occurs when the individual dipoles in a molecule negate each other, resulting in a net dipole of zero. For a molecule to have zero dipole moment, it must have symmetry that enables this cancellation. This typically happens in molecules where substituents are symmetrically placed.
Here's how it works:

• In the exercise, trans-2-butene is an excellent example where the substituents are on opposite sides, perfectly canceling any dipole generated by the double bond.
• Contrastingly, in cis-2-butene and 1-butene, the asymmetry prevents dipole cancellation, resulting in a non-zero dipole moment.

Dipole moment cancellation is crucial in molecular design, especially when developing materials with specific dielectric properties or when determining intermolecular forces.

Organic Chemistry

Organic chemistry is the study of carbon-containing compounds and their structures, properties, and reactions. It forms the backbone of biochemistry, pharmaceuticals, and numerous other fields.
Key concepts in organic chemistry that relate to this exercise include:

• Functional Groups: The reactive part of molecules, heavily influence the dipole moment and chemical reactivity.
• Isomerism: As seen with alkenes, isomerism can significantly affect a molecule's symmetry and dipole moment.
• Molecular geometry: Determines how atoms are arranged in space, crucial for understanding reactivity and interaction with other molecules.

By understanding these principles, students can predict and rationalize the behavior of organic compounds, crucial for solving exercises related to dipole moments and structural analysis.