Chapter 12, Problem 12.10YT

Interpretation Introduction

Interpretation:

Consider the hydroboration and protonation steps shown here. In each case, (a) the electrophilic atom and (b) the alkene $\text{C}$ that acquires positive or partial positive charge is to be labeled. Whether the electrophilic atoms add to the same alkene $\text{C}$ in each case is to be explained.

Concept introduction:

For the reaction of the $\text{C=C}$ bond with ${\text{H-BH}}_{\text{2}}$, the electrophile is the $\text{B}$ of the ${\text{H-BH}}_{\text{2}}$ because it lacks an octet and is electron poor. A $\text{C-B}$ bond is formed from the end of $\text{C=C}$ that is less crowded, and a new $\text{C-H}$ bond is formed from the end of $\text{C=C}$ that is more sterically crowded. The partial positive charge in the transition state forms on the more substituted carbon. For the reaction of the $\text{C=C}$ with $\text{H-Cl}$, the electrophile is the $\text{H}$ of the $\text{H-Cl}$. A $\text{C-H}$ bond is formed so that the ${\text{C}}^{\text{+ }}$ that is produced is on the more stable tertiary carbon. In both cases, the electrophilic atom adds to the same carbon of the $\text{C=C}$ bond. In both cases, either the partial positive charge in the transition state or the positive charge in the carbocation intermediate forms on the carbon that is more substituted.