Chapter 4, Problem 73IP

Interpretation Introduction

Interpretation:

The compound that has the largest heat of combustion has to be identified.

Concept introduction:

Heat of combustion (alkanes): For an alkane, the heat of combustion is the negative of the change in enthalpy $\left({\text{-ΔH}}^{\text{o}}\right)$ associated with the complete combustion of 1mol of alkane in the presence of Oxygen. Heat of combustion can be measured experimentally and used to compare the stability of isomeric alkanes; are way to determine the relative stability of compounds.

Heat of combustion for rings of different sizes: Comparing the rings of different sizes by dividing the heat of combustion by the number of ${\text{CH}}_2$ groups in the compound, giving a heat of combustion per ${\text{CH}}_2$ group.

Ring strain: A type of instability that exists when bonds in a molecule from angles that are abnormal (deviate from ideal bond angles).

Generally, the bonds of larger cycloalkanes can position themselves three dimensionally so as to achieve a conformation that minimizes the total energy of the compound.

Conformations: Rotation about C-C single bonds allows a compound to adopt a variety of possible three-dimensional shapes.

Newman projections: The new conformations of compounds can be drawn and analyzed by Newman projections. A Newman projection visualizes different conformations of Carbon-carbon chemical bond from front to back with the front carbon represented as a black dot and the back represented as a circle.

The angle between two hydrogens of a Newman projection is called as dihedral angle or torsional angle. This dihedral angle changes as the C-C bond rotates. Two conformations with special attentions are staggered and eclipsed conformation. Staggered conformation is the lowest in energy and the eclipsed conformation is the highest in energy.

For example,

Anti-conformation: The conformation with a dihedral angle of ${180}^{\circ }$ is called anti-conformation.

The two methyl groups achieve maximum separation from each other. In other, methyl groups are closer to each other; their electron clouds are repelling each other, causing an increase in energy. This unfavorable interaction is called gauche interaction.