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 exhibits a dipole moment.

Step 1: Identify the Molecular Structures

Analyze the given molecules to understand their geometry and possible charge distribution. For (a) 2-methylpropane, it is a symmetric molecule, (b) trans-2-butene is also symmetric, (c) cis-2-butene is not symmetric, and (d) carbon tetrachloride is symmetric around the carbon atom.

Step 2: Determine the Symmetry of Molecules

Check each molecule to see if it has a center of symmetry. Molecules with symmetric structures, like 2-methylpropane, trans-2-butene, and carbon tetrachloride, often have their dipoles cancel out, resulting in a net dipole moment of zero. Cis-2-butene lacks such symmetry due to the arrangement of its substituents.

Step 3: Assess Polarity

Evaluate whether the molecules are polar or non-polar. Non-polar molecules do not have net dipole moments, while polar molecules like cis-2-butene typically have unequal sharing of electrons that leads to a net dipole moment.

Step 4: Choose the Correct Answer

Based on the assessments, select the molecule that has a dipole moment. Since cis-2-butene is asymmetric with polar bonds that do not cancel out, it exhibits a dipole moment.

Key Concepts

Molecular Symmetry

Molecular symmetry refers to a balanced spatial arrangement of atoms within a molecule. When a molecule possesses symmetry, it means that its geometric shape is consistent and regular, allowing various parts to be superimposed onto each other.
Symmetrical molecules, such as carbon tetrachloride (CCl extsubscript{4}) and 2-methylpropane, have molecular structures where polar bonds, if present, tend to cancel each other out, often resulting in a non-polar molecule.
This occurs because the individual bond dipoles, which are directional, are equal and opposite, effectively balancing and nullifying any overall dipole moment.

• Symmetric molecules are often non-polar.
• Symmetry causes dipole moments from individual bonds to cancel out.
• This can lead to a net dipole moment of zero.

Understanding molecular symmetry helps predict the dipole moment and polarity of molecules.

Polarity

Polarity in chemistry refers to the distribution of electric charge across a molecule. Polarity arises from the differences in electronegativity between atoms, leading to an uneven distribution of electron density.
A polar molecule has a net dipole moment because the positive and negative charges within the molecule do not coincide. In simpler terms, polar molecules tend to have a part that is relatively more positive and another that is more negative.
For example, cis-2-butene exhibits polarity due to its asymmetrical structure, causing the dipole moments from its C-H and C=C bonds to not cancel out perfectly.

• Polarity is determined by the molecule's geometry and charge distribution.
• Polar molecules have a net dipole moment.
• Polarity influences molecule interactions, such as solubility and boiling point.

Recognizing whether a molecule is polar or non-polar is crucial for understanding its chemical behavior.

Organic Molecules

Organic molecules are compounds primarily composed of carbon and hydrogen atoms, along with a few others like nitrogen, oxygen, and halogens. They form the basis of organic chemistry, centered around the study of carbon-containing compounds.
These molecules can range from simple structures like methane ( CH extsubscript{4}), to complex polymers like proteins and DNA. The nature of how organic molecules interact depends heavily on molecule geometry, polar bonds, and functional groups present.
Organic molecules can be either planar or three-dimensional in structure, which in turn affects their chemical properties such as polarity and reactivity.

• Organic molecules are versatile with a variety of shapes and sizes.
• Their properties are influenced by elements like carbon and hydrogen.
• Understanding the structure helps predict behavior and reactivity.

Gaining insights into organic molecules can illustrate essential biological processes and industrial applications.

Asymmetry in Molecules

Asymmetry in molecules refers to an uneven spatial arrangement of atoms within a molecule, lacking symmetry. Asymmetric molecules often have unequal charge distribution, leading to a net dipole moment.
This imbalance affects the molecule's physical and chemical properties, including polarity. Cis-2-butene is a classic example of asymmetry in molecules; its substituents on the double-bonded carbon atoms differ, resulting in a permanent dipole moment.
Asymmetry is crucial because it often dictates how a molecule interacts with its environment, influencing reactions and interactions with other molecules.

• Asymmetric molecules typically exhibit polarity.
• They possess unbalanced charge distributions.
• Asymmetry plays a significant role in molecular functionality and reactivity.

Understanding asymmetry allows better predictions and manipulations of molecular interactions in various chemical contexts.