Problem 24 - A Complete Resource Book in Chemistry for JEE Main

Question

The most stable conformation of \(\mathrm{n}\)-butane is (a) eclipsed (b) gauche (c) anti-staggered (d) skew-boat

Step-by-Step Solution

Answer

The most stable conformation of n-butane is anti-staggered.

Step 1: Understand n-butane structure

n-Butane is a straight-chain alkane with the molecular formula C₄H₁₀. It has four carbon atoms connected in a row, with hydrogen atoms filling the remaining valencies of carbon.

Step 2: Identify possible conformations

In n-butane, there are different possible spatial arrangements due to rotation about the carbon-carbon bonds. These conformations include eclipsed, gauche, anti-staggered, and skew forms.

Step 3: Evaluate energy stability of conformations

In terms of energy, the eclipsed conformation is the least stable due to steric hindrance. The anti-staggered conformation is the most stable because it has the least steric interference, with the two largest groups (methyl groups) being as far apart as possible at 180°.

Step 4: Compare anti-staggered vs. other conformations

The gauche conformation is less stable than anti-staggered because it has the methyl groups 60° apart, leading to more steric hindrance. The skew-boat is not a commonly discussed conformation for n-butane.

Step 5: Determine most stable conformation

Given that the anti-staggered conformation minimizes steric strain, it is the most stable conformation of n-butane.

Key Concepts

n-butane structure

n-Butane, with the chemical formula C₄H₁₀, belongs to the family of alkanes, which are simple hydrocarbons composed solely of carbon and hydrogen. In n-butane, four carbon (C) atoms are linked in a straight chain:

• C-C-C-C
• Each carbon is bonded to the appropriate number of hydrogen atoms to complete its valence shell.

This linear arrangement defines n-butane as a straight-chain molecule. For each carbon atom not at the end of the chain, it is bonded to two hydrogens and two other carbons, while the terminal carbon atoms each connect to three hydrogen atoms. This setup allows n-butane to explore different spatial configurations as the carbon atoms rotate around single bonds. This property marks its ability to form various conformations.

energy stability of conformations

The concept of conformation in molecules like n-butane refers to the distinct shapes the molecule can take as its atoms rotate around the single bonds connecting them. These varying shapes impact the molecule’s energy stability.

• Within n-butane, different positions of atoms arise due to rotation around the central carbon-carbon bond.
• Conformations such as eclipsed, gauche, and anti-staggered are possible, each having unique qualities.

The energy associated with each conformation is determined primarily by the spatial arrangement of atoms and the resulting interactions, primarily steric interactions.

• Eclipsed conformation: Atoms or groups are perfectly aligned, increasing repulsion and resulting in higher energy and less stability.
• Anti-staggered conformation: Atoms or groups are spaced 180° apart, minimizing repulsion and thus, lowering energy and providing stability.
• Gauche conformation: Atoms are 60° apart, increasing steric interaction slightly more than in anti-staggered.

Among these, the anti-staggered conformation is the most energetically favorable due to its extended structure, minimizing steric hindrance.

steric hindrance

Steric hindrance is a concept that encapsulates the physical presence and spatial arrangement of atoms or groups within a molecule that can influence its stability and reactivity. It plays a crucial role in determining the energy stability of various conformations of n-butane.

• As atoms or groups approach each other, their electron clouds begin to interact adversely, creating repulsion.
• This interaction is more pronounced in conformations where large groups are in proximity, such as in the eclipsed form.

The anti-staggered conformation is stable largely because it provides the maximum distance between bulky groups, minimizing steric hindrance. Conversely, in the gauche conformation, groups are closer together, leading to increased steric interactions and hence higher energy.

• Eclipsed conformation is typically avoided due to maximum steric strain, destabilizing the molecule.

Understanding steric hindrance helps predict and explain the stability of molecular conformations based on spatial arrangements, particularly emphasizing why molecules like n-butane prefer certain conformations over others to maintain lower energy states.