Problem 120 - A Complete Resource Book in Chemistry for JEE Main

Question

The molecules that will have dipole moment (1) 2,2 -dimethylpropane (2) trans-1-pent-2-ene (3) cis-hex-3-ene (4) \(2,2,3,3\)-tetramethylbutane (a) 2,3 (b) 1,3 (c) \(1,2,3\) (d) all of these

Step-by-Step Solution

Answer

The correct answer is (a) 2,3.

Step 1: Understand Dipole Moment

A dipole moment occurs when there is an uneven distribution of electron density in a molecule, leading to a separation of electrical charge. Molecules with a symmetrical shape often have no net dipole moment due to their symmetry causing the charges to cancel out.

Step 2: Analyze 2,2-Dimethylpropane

2,2-Dimethylpropane, also known as neopentane, is a symmetrical molecule with all equivalent C-H bonds. Its symmetrical shape and balanced distribution of its constituent elements mean it has no net dipole moment.

Step 3: Analyze trans-1-pent-2-ene

Trans-1-pent-2-ene is a type of alkene with a double bond between the second and third carbon atoms. Due to the trans configuration, the molecule is nearly symmetrical across the double bond, leading to cancellation of any potential dipole moments, resulting in a negligible dipole moment.

Step 4: Analyze cis-hex-3-ene

Cis-hex-3-ene has a cis configuration around the double bond, meaning the same groups are on the same side. This asymmetric nature results in an uneven distribution of electron density, and the molecule does have a dipole moment.

Step 5: Analyze 2,2,3,3-tetramethylbutane

2,2,3,3-tetramethylbutane is also symmetrical with the methyl groups evenly distributed. This symmetry ensures cancellation of any dipole moments, rendering the molecule non-polar.

Step 6: Conclusion

From the analysis, cis-hex-3-ene is the only molecule with a significant dipole moment due to its asymmetry.

Key Concepts

Molecular Symmetry

Molecular symmetry is a key factor in determining whether a molecule has a dipole moment or not. Symmetry refers to how the parts of a molecule are arranged along different axes. When a molecule is symmetrical, its charges are balanced, which means any potential dipole moments are canceled out. For example, 2,2-dimethylpropane and 2,2,3,3-tetramethylbutane are both highly symmetrical. This symmetry ensures that the electron density is evenly distributed, resulting in no net dipole moment.

Symmetrical molecules often appear visually balanced. You could imagine them as being able to be sliced into mirror images along certain planes. Because of this, these molecules tend to be non-polar. Understanding symmetry thus allows chemists to predict which molecules might interact strongly with others through polar attractions.

Electron Density Distribution

Electron density distribution refers to how electrons are spread across a molecule. It is a critical concept in understanding the formation of dipole moments. In molecules, different atoms have varied abilities to attract electrons, termed as electronegativity. When atoms with different electronegativities form bonds, the electrons tend to skew towards the more electronegative atom, creating areas of partial negative and positive charge.

In cis-hex-3-ene, the arrangement of groups around the double bond causes an uneven electron density distribution. The 'cis' configuration means similar atoms or groups are on the same side, creating an imbalance in the electron cloud. This leads to a net dipole moment, as the molecule doesn't cancel out the charges. Conversely, in molecules like trans-1-pent-2-ene, the 'trans' configuration allows more symmetrical electron distribution across the double bond, reducing the potential for a dipole moment.

Cis-Trans Isomerism

Cis-trans isomerism (also known as geometric isomerism) refers to variations in the arrangement of atoms across a double bond. In alkenes, where the geometry around the double bond is locked, this difference leads to distinct properties in isomers. The 'cis' isomer, where similar or identical groups are on the same side, often results in an asymmetric structure, as evidenced in cis-hex-3-ene. This asymmetry tends to result in a non-canceling dipole moment, as the electron distribution is uneven.

The 'trans' isomer, like in trans-1-pent-2-ene, places similar groups across the double bond, leading to a more balanced molecule. This symmetry usually results in equal pull from opposite sides, canceling out potential dipole and resulting in a non-polar molecule.

• The cis configuration can affect physical properties such as boiling points and solubility due to its polar nature.
• The trans configuration's symmetry usually leads to more linear structures, potentially lowering these physical properties.

Understanding these differences is critical in predicting both chemical reactivity and biological activity of such molecules.