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

(a) 2,3

Step 1: Understand the Concept of Dipole Moment

A dipole moment occurs when there is a separation of charge within a molecule, often due to differences in electronegativity between bonded atoms or due to the molecule's geometry. Symmetrical molecules often do not have a net dipole moment, while asymmetric ones do.

Step 2: Analyze 2,2-dimethylpropane

2,2-dimethylpropane is a symmetrical molecule because it is a branched alkane with identical groups around the central carbon. Since it is symmetrical, it does not have a dipole moment.

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

In the trans configuration of 1-pent-2-ene, the substituent groups are across from each other, causing the polarities to cancel out due to symmetry. As a result, trans-1-pent-2-ene has no dipole moment.

Step 4: Analyze cis-hex-3-ene

In the cis configuration of hex-3-ene, substituent groups are on the same side of the double bond, creating an asymmetric distribution of charge. This geometry results in a molecule with a dipole moment.

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

2,2,3,3-tetramethylbutane is highly symmetrical, as it has identical groups on either side. Due to this symmetry, the molecule does not have a dipole moment.

Step 6: Determine Molecules with Dipole Moments

Based on the analysis, only cis-hex-3-ene (Option 3) has a dipole moment because it lacks symmetry in its geometric arrangement. Therefore, the molecules that have dipole moments are those containing cis-hex-3-ene.

Key Concepts

Molecular Geometry

Molecular geometry is a very important concept in understanding the properties of molecules, such as their reactivity and polarity. It describes the three-dimensional arrangement of atoms within a molecule. The geometry is determined by several factors, including:

• Number of bonded atoms
• Number of lone pairs on the central atom
• The specific arrangement of these bonds and lone pairs

Understanding molecular geometry helps in predicting whether a molecule will have a dipole moment. A dipole moment occurs due to differences in electronegativity between atoms, leading to partial charges and an overall separation of charge, if the molecular geometry is conducive to such separation. Geometries can range from linear and planar to more complex shapes like tetrahedral and trigonal bipyramidal. The molecular geometry greatly impacts the physical and chemical properties of the molecule, influencing the behavior of substances in reactions.

Symmetrical Molecules

Symmetrical molecules are those that are evenly balanced in terms of their shape and charge distribution. This symmetry can occur when identical atoms or groups of atoms are arranged around a central atom, in a way that their individual polarities cancel each other out. A key characteristic of many symmetrical molecules is their lack of a net dipole moment. Without a net dipole, these molecules tend to be less polar and are generally non-polar substances. An example of a symmetrical molecule can be seen with 2,2-dimethylpropane, which, due to its branched alkane structure with identical groups, displays no net dipole moment. Such symmetry not only makes these molecules stable in certain environments but also affects their solubility and interaction with other molecules, often resulting in being hydrophobic and not easily soluble in polar solvents.

Asymmetric Molecules

Asymmetric molecules are characterized by their uneven distribution of charge due to the molecular geometry and differing electronegativities of atoms within the molecule. This lack of symmetry results in a non-cancelling of dipoles and hence, these molecules often exhibit a net dipole moment. The dipole moment makes the molecule polar, which can greatly influence its behavior in various chemical contexts. Molecules like cis-hex-3-ene demonstrate this concept well. The substituent groups in cis-hex-3-ene are on the same side of the double bond, which leads to an uneven charge distribution and results in a dipole moment. These polar molecules tend to interact strongly with other polar substances, affecting their solubility, melting and boiling points, and other physical properties. Hence, asymmetric molecules often play crucial roles in biological systems and industrial applications.

Cis-Trans Isomerism

Cis-trans isomerism is a specific type of stereoisomerism found in alkenes (and certain other compounds) where there is restricted rotation around a double bond. Cis- and trans-isomers have the same chemical formula but differ in spatial arrangement around a functional group or a double bond. In the 'cis' configuration, similar or identical groups are on the same side of the bond or functional group, resulting in an asymmetrical molecule with a possible dipole moment. This is seen in cis-hex-3-ene, where the asymmetry leads to polarity. On the other hand, the 'trans' configuration places similar or identical groups on opposite sides, making the molecule more symmetrical. This tends to cancel out any dipoles, leading to a non-polar molecule, as displayed by the trans-1-pent-2-ene. Cis-trans isomerism significantly impacts the chemical properties and biological activity of molecules. This isomerism is critical in the field of synthetic chemistry when designing compounds with specific properties, such as pharmacological activity.