The computed (HF / 6-31 G^*) rotational profiles for acetone (2-propanone), 2 -butanone, and 3 -

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The computed (HF / 6-31 G^*) rotational profiles for acetone...

The computed $\left(\mathrm{HF} / 6-31 \mathrm{G}^{*}\right)$ rotational profiles for acetone (2-propanone), 2 -butanone, and 3 -methyl-2-butanone are given in Figure $2.6 \mathrm{P}$. Draw Newman projections corresponding to each clear maximum and minimum in the curves for each compound. Analyze the factors that stabilize/destabilize each conformation and discuss the differences among them. Fig. 2.6P. Rotational profile for acetone (A, solid line), 2-butanone (B, dashed line), and 3-methyl-2-butanone (C, dotdashed line). Reproduced by permission of the American Chemical Society.

The computed $\left(\mathrm{HF} / 6-31 \mathrm{G}^{*}\right)$ rotational profiles for acetone (2-propanone), 2 -butanone, and 3 -methyl-2-butanone are given in Figure $2.6 \mathrm{P}$. Draw Newman projections corresponding to each clear maximum and minimum in the curves for each compound. Analyze the factors that stabilize/destabilize each conformation and discuss the differences among them.
Fig. 2.6P. Rotational profile for acetone (A, solid line), 2-butanone (B, dashed line), and 3-methyl-2-butanone (C, dotdashed line). Reproduced by permission of the American Chemical Society.

Key Concepts

Torsional Strain

Torsional strain arises when bonds on adjacent atoms are forced into an eclipsed or near-eclipsed conformation, leading to repulsive interactions between electron clouds. This strain typically makes eclipsed conformations higher in energy compared to staggered conformations, where such repulsions are minimized.

Rotational Profile

A rotational profile is a plot of the potential energy of a molecule as a function of the dihedral angle around a specific bond. It reveals the energy barriers associated with rotating about that bond, highlighting the conformations that are more or less stable due to factors such as steric interactions, electronic effects, and torsional strain.

Newman Projection

The Newman projection is a two-dimensional representation used to visualize the conformations of a molecule from the perspective of looking straight down a carbon-carbon bond. This projection helps in identifying staggered and eclipsed conformations and is essential for analyzing steric hindrance, torsional strain, and other interactions that affect conformational stability.

Conformational Analysis

Conformational analysis is the study of the different spatial arrangements of atoms in a molecule that result from rotation around single bonds. By evaluating these conformations, one can understand how various factors, such as torsional strain, steric effects, and electronic interactions, contribute to the overall stability and reactivity of the molecule.

Steric Hindrance

Steric hindrance refers to the repulsive interaction that occurs when atoms or groups within a molecule are brought too close together, leading to an increase in the overall energy of that conformation. Large or bulky substituents can impose steric constraints that make certain rotational conformations less favorable.

Hyperconjugation

Hyperconjugation is a stabilizing interaction that results from the delocalization of electrons from a filled bonding orbital (usually a sigma bond) into an adjacent empty or partially filled non-bonding orbital, typically an antibonding sigma orbital. This electron delocalization can lower the overall energy of a particular conformation, helping to stabilize it.

Electronic Effects

Electronic effects include a variety of influences such as dipole-dipole interactions, inductive effects, and charge distribution within a molecule. These effects can either stabilize or destabilize a conformation depending on how substituents interact electronically with the rest of the molecule, and they often play a crucial role alongside steric and hyperconjugative influences in determining conformational preferences.

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