Problem 75 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which molecules are non-polar? (i) \(\mathrm{NH}_{3}\) (ii) \(\mathrm{CO}_{2}\) (iii) \(\mathrm{H}_{2} \mathrm{O}\) (iv) \(\mathrm{CF}_{4}\) (v) \(\mathrm{Br}_{2}\) (vi) \(\mathrm{BF}_{3}\) (vii) \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) (viii) \(\mathrm{H}_{2} \mathrm{O}_{2}\) (a) \(\mathrm{ii}, \mathrm{iv}, \mathrm{v}, \mathrm{vi}\) (b) \(\mathrm{i}, \mathrm{iv}, \mathrm{vi}\), viii (c) iii, \(\mathrm{iv}, \mathrm{v}, \mathrm{vi}\) (d) i, iii, vii, viii

Step-by-Step Solution

Answer

The correct answer is (a) ii, iv, v, vi.

Step 1: Identify Molecular Geometry

First, we need to determine the molecular geometry of each given molecule. For instance, \(\mathrm{NH}_{3}\) has a trigonal pyramidal structure, \(\mathrm{CO}_{2}\) is linear, \(\mathrm{H}_{2} \mathrm{O}\) is bent, \(\mathrm{CF}_{4}\) is tetrahedral, \(\mathrm{Br}_{2}\) is linear, \(\mathrm{BF}_{3}\) is trigonal planar, \(\mathrm{CH}_{2} \mathrm{Cl}_{2}\) is tetrahedral, and \(\mathrm{H}_{2} \mathrm{O}_{2}\) is bent.

Step 2: Determine Polarity Based on Geometry and Electronegativities

Evaluate each molecule's polarity by considering its molecular geometry and the electronegativity difference between atoms. Non-polar molecules have a symmetrical distribution of electrical charges. For example, \(\mathrm{CO}_{2}\)'s linear shape results in dipoles that cancel out, making it non-polar. \(\mathrm{CF}_{4}\) has a symmetrical tetrahedral shape, \(\mathrm{Br}_{2}\) has no difference in electronegativity (being a diatomic molecule of the same element), and in \(\mathrm{BF}_{3}\), the trigonal planar shape ensures the dipoles cancel out, making all these molecules non-polar.

Step 3: Select the Non-Polar Molecules Based on Analysis

Based on the above analysis, the non-polar molecules are \(\mathrm{CO}_{2}\), \(\mathrm{CF}_{4}\), \(\mathrm{Br}_{2}\), and \(\mathrm{BF}_{3}\). Thus, the correct answer is option (a).

Key Concepts

Molecular Geometry

Molecular geometry refers to the three-dimensional arrangement of atoms within a molecule. This structural configuration is determined by the number of bonds and the lone pairs of electrons associated with the central atom. Understanding the geometry is crucial, as it directly affects the physical and chemical properties of the molecule, including its polarity.
Some common geometries include:

• Linear: This shape occurs when there are two bonding pairs and generally no lone pairs on the central atom, like in \(CO_{2}\), where it allows for the cancellation of dipoles, resulting in a non-polar molecule.
  
• Tetrahedral: Characteristic of molecules like \(CF_{4}\), where four bonded atoms around a central atom create a shape that often leads to non-polarity when all outer atoms are identical.
  
• Trigonal Planar: Involves three bonds in one plane, as seen in \(BF_{3}\), where the symmetry aids in dipole cancellation, leading to a non-polar outcome.
  
• Bent: Often contributes to molecular polarity, as in \(H_{2}O\), due to its unsymmetrical shape which prevents dipole cancellation.
  

The geometry is a defining factor in polarity, helping determine if molecules like \(CH_{2}Cl_{2}\), with a tetrahedral shape, are polar or non-polar depending on spatial arrangement and symmetry.

Electronegativity

Electronegativity is a chemical property that describes an atom's ability to attract and hold onto electrons in a chemical bond. This concept is significant in determining the polarity of molecules. Varying electronegativity among atoms causes uneven sharing of electrons, or dipoles, within a molecule.
Key aspects to consider:

• High Electronegativity: Atoms with a high electronegativity, like fluorine, oxygen, and nitrogen, are more likely to pull electrons towards themselves, creating polar bonds.
  
• Difference in Electronegativity: The larger the difference between the electronegativities of bonded atoms, the more polar the bond. For instance, in \(NH_{3}\), nitrogen is more electronegative than hydrogen, resulting in a polar structure.
  
• Symmetrical Non-Polarity: When the dipoles within a molecule cancel each other out due to symmetry, the molecule can become non-polar even if it contains polar bonds, such as in \(CO_{2}\) where the linear shape leads to dipole cancellation.
  

Understanding electronegativity is pivotal; it reveals why molecules such as \(Br_{2}\) (with no electronegativity difference) remain non-polar while others might not.

Non-Polar Molecules

Non-polar molecules are characterized by an even or symmetrical distribution of electron charge, which results in a net dipole of zero. The polarity of these molecules is often determined by the molecular geometry and electronegativity of the atoms involved.
Key features include:

• Symmetrical Electron Distribution: When the electron distribution is even, as in \(CF_{4}\) and \(BF_{3}\), the symmetrical shape allows any dipoles to cancel out, resulting in non-polarity.
  
• Identical Atoms: Molecules like \(Br_{2}\) consist of identical atoms and zero electronegativity difference, making them intrinsically non-polar.
  
• Shape-Induced Non-Polarity: Even though a molecule may contain polar bonds, its symmetrical shape can lead to overall non-polarity, as seen in \(CO_{2}\).
  

In a classroom scenario, recognizing these characteristics helps in predicting and understanding non-polar molecules, reinforcing the significance of symmetrical and balanced arrangements.

Symmetrical Molecules

Symmetrical molecules have a balanced, uniform structure where the individual polarities of bonds or atoms cancel out the effects of each other. This symmetry is key in defining their lack of overall polarity, making the molecule non-polar.
Importance of symmetry includes:

• Balanced Shape: In symmetric molecules like \(CF_{4}\), the tetrahedral arrangement balances out any possible dipole moments, ensuring the molecule remains non-polar.
  
• Dipole Cancellation: Symmetrical geometry ensures that even with polar bonds, like in \(BF_{3}\), the dipole effects negate each other, resulting in no net dipole.
  
• Predictability in Reactions: These molecules are often less reactive due to their non-polar nature, impacting their behavior in chemical reactions and intermolecular interactions.
  

Grasping the concept of symmetry helps in identifying non-polarity in molecules, equipping students with a powerful tool in assessing molecular properties in both theoretical and practical chemistry contexts.