Problem 121 - 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

Option (a) 2,3 is correct, as only molecule 3 has a dipole moment (cis-hex-3-ene).

Step 1: Understanding Dipole Moment

A dipole moment is a measure of the separation of positive and negative charges in a molecule. It occurs when there is an unequal distribution of electrons between different atoms in a molecule, which is typical in molecules that are polar or have asymmetrical structures.

Step 2: Analyzing 2,2-dimethylpropane

2,2-dimethylpropane is a symmetrical molecule with identical methyl groups attached to the central carbon atom. Due to its symmetrical nature, the partial charges cancel each other out, resulting in a net dipole moment of zero.

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

In trans-1-pent-2-ene, the structure is more symmetrical compared to its cis counterpart. The trans configuration allows substituents to be on opposite sides of the double bond. This causes the dipoles to cancel each other, resulting in a molecule with no net dipole moment.

Step 4: Analyzing cis-hex-3-ene

Cis-hex-3-ene has a structure where substituent groups are on the same side of the double bond. This causes a separation of charge that results in a net dipole moment, due to the lack of symmetry.

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

Like 2,2-dimethylpropane, 2,2,3,3-tetramethylbutane is symmetrical as well. The equal distribution of identical groups around the central points of the molecule results in the cancellation of any potential dipoles, leading to no net dipole moment.

Step 6: Conclusion

Of the given molecules, only cis-hex-3-ene has a dipole moment because of its asymmetrical arrangement with substituent groups on the same side. Hence, option (a) 2,3 is the correct answer as it lists the molecule with a dipole moment.

Key Concepts

Molecular Symmetry

Molecular symmetry is a crucial concept in understanding why some molecules have dipole moments and others do not. Symmetry in molecules refers to the balanced distribution of similar groups or atoms around a central point. When a molecule is symmetrical, the charges are evenly distributed, leading to cancellation of any potential dipoles.

• Example: Molecules like 2,2-dimethylpropane or 2,2,3,3-tetramethylbutane are symmetrical, and thus, exhibit no dipole moment.
• Symmetry ensures that no side of the molecule holds more negative or positive charge compared to another.
• In general, when analyzing molecular symmetry, if identical substituents are opposite each other, the molecule is likely non-polar.

Understanding this helps in analyzing molecular structures to predict dipole moments.

Polar Molecules

Polar molecules are characterized by having a net dipole moment due to uneven distribution of electrons. They arise when atoms in a molecule have differing electronegativities, causing electron clouds to skew towards the more electronegative atom.

A classic example of a polar molecule is cis-hex-3-ene. In polar molecules:

• The electrons are not shared equally, leading to partial positive and negative charges.
• These molecules tend to interact more strongly with other polar substances.
• The presence of lone pairs or differing atoms contributes to the polarity.

Non-polar molecules, on the other hand, have an even distribution of charge and are often characterized by symmetrical structures. Discerning between polar and non-polar molecules is pivotal, as it affects how substances interact chemically and physically.

Cis-Trans Isomerism

Cis-trans isomerism, also known as geometric isomerism, is a type of stereoisomerism that occurs due to the restricted rotation around a double bond. This results in different spatial arrangements of atoms or groups, leading to varying properties.

The term "cis" indicates that substituents are on the same side of the double bond, whereas "trans" refers to substituents on opposite sides.

• The cis configuration leads to a net dipole moment if the groups attached create an asymmetrical shape, as seen in cis-hex-3-ene.
• The trans configuration, like in trans-1-pent-2-ene, tends to cancel out dipole moments due to symmetry.
• Cis isomers often have higher boiling points due to stronger intermolecular attractions compared to trans isomers.

Understanding these isomeric forms is essential for predicting the physical and chemical behavior of organic compounds.