Chapter 3, Problem 31P

(a)

Interpretation Introduction

Interpretation: Major organic product of monobromination of pentane at $125°\text{C}$ should be identified.

Concept introduction: The monobromination performed with ultraviolet light proceeds via radical chain mechanism.

Tertiary $\text{C – H}$ bonds react in halogenations more readily than secondary those in turn are faster to react than primary. This can be attributed to stability of radical formed upon $\text{C – H}$ bond cleavage via hyperconjugation.

The phenomenon of hyperconjugation refers to donation of $\text{σ}$ electrons into the partly vacant $p$ orbital lobes of radical carbon. More will be number of $\text{σ}$ orbital capable of donation of electrons more is the stabilization.

(b)

Interpretation Introduction

Interpretation: All Newman projections of major brominated products in staggered conformations should be drawn.

Concept introduction: Various interconvertible forms that result from rotation around the $\text{C–C}$ bind in ethane are termed as conformer. To depict such conformation Newman projections are written. These are obtained by conversion of hashed-wedged line structure simply when molecule is viewed in a plane along the $\text{C–C}$ as illustrated as follows:

  

Thus in Newman's projection of simple ethane molecule the “front” $\text{C}$ is depicted as intersection of three bonds with three attached $\text{H}$ . The other three bonds connect to the large circle corresponding to “back” carbon in $\text{C–C}$ bond.

(c)

Interpretation Introduction

Interpretation: Qualitative plot of potential energy against torsional angle for ${\text{C}}_{\text{2}}–{\text{C}}_{\text{3}}$ rotation in this molecule should be drawn.

Concept introduction: As rotation is carried out along $\text{C – C}$ axis, energy changes occur due to bond rotation from staggered to eclipsed conformation. This change amounts to torsional strain. Energy difference between eclipsed and staggered conformations corresponds to barrier that must be overcome for rotation. This is also termed as activation energy.

The potential energy shows peaks and falls. This corresponds to transition from staggered and eclipsed conformation molecule adopts with each $60°$ rotation as illustrated in lateral view below:

  

Staggered conformation that has substituents on both carbons farthest apart is regarded most stable due to least torsional strain, whereas the eclipsed has large amount of torsional strain due to steric repulsions. Therefore in potential-energy diagram, peak corresponds to eclipsed while the valley corresponds to stable staggered conformation.