Problem 79 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

In the Newman projection formula of the least stable staggered form of \(\mathrm{n}\) -butane, which of the following reasons is the causes of its unstability? (a) Van der Waal's strain (b) Torsional strain (c) Combination of both (d) None of these

Step-by-Step Solution

Answer

(a) Van der Waal's strain

Step 1: Understanding Newman Projections

A Newman projection is a way to depict a molecule by looking straight down the bond connecting two carbon atoms. It helps visualize the arrangement of atoms around the bond and determine different conformations of a molecule.

Step 2: Identifying Butane's Staggered Conformation

In the case of n-butane, we focus on the bond between the second and third carbon atoms. The possible staggered conformations are anti and gauche. The least stable staggered conformation is the gauche conformation.

Step 3: Analyzing the Gauche Conformation

In the gauche conformation of n-butane, the two largest groups (methyl groups) are 60 degrees apart, leading to steric clashes. This steric hindrance mainly causes the Van der Waal's strain, which contributes to instability.

Step 4: Conclusion About Stability

The least stable staggered form of n-butane is the gauche conformation. This instability is primarily due to Van der Waal's strain from sterical hindrance, rather than torsional strain, as the torsional strain is minimal in staggered conformations.

Key Concepts

n-butane

n-Butane is a simple alkane with the chemical formula C₄H₁₀. It consists of a straight chain of four carbon atoms. The molecule can rotate around the carbon-carbon single bonds, creating different spatial configurations known as conformations. These different shapes can offer varying levels of stability due to interactions among atoms and electron clouds. Butane has multiple conformations due to the free rotation around its internal C-C bonds. Examining these conformations can be simplified using Newman projections, a specific technique for visually representing the three-dimensional structure of molecules.

gauche conformation

The gauche conformation of n-butane is one of the staggered forms, characterized by a 60-degree angle between two of the largest groups, specifically the methyl groups on the second and third carbons. This orientation is less stable compared to the anti conformation, where the methyl groups are 180 degrees apart. In the gauche conformation, these methyl groups are closer to each other, leading to crowding, also known as steric hindrance. This configuration is notably less stable than the anti conformation due to increased repulsive forces between the electron clouds of the close-proximity groups.

Van der Waal's strain

Van der Waal's strain arises from the repulsive forces experienced when atoms are brought too close together, a common issue in the gauche conformation of n-butane when using the Newman projection. As the methyl groups in n-butane are positioned at a 60-degree angle in this conformation, their electron clouds overlap more than in other conformations. This overlap creates a repulsion due to Van der Waal's forces, which are weak forces that can cause significant instability when molecules get too close, especially in the case of large substituent groups. This strain is a primary factor in the instability of the gauche conformation.

staggered conformation

A staggered conformation is one of the primary conformations for molecules viewed in Newman projections. It occurs when the atoms or groups on adjacent carbon atoms are positioned at dihedral angles, minimizing electron repulsion and therefore lowering torsional strain. In n-butane, the staggered settings can be anti or gauche. These conformations can be seen as energetically favorable over eclipsed conformations, where atoms are in direct alignment, leading to higher torsional strain. The anti staggered form is generally more stable than the gauche form due to lesser steric interactions between bulky groups, providing the molecule with its lowest energy state possible, unhampered by excessive strain.