Chapter 9, Problem 9.24P

Interpretation Introduction

Interpretation:

It is to be determined which of the two substrates shown - C and D - will undergo an E1 reaction faster.

Concept introduction:

An E1 reaction starts the same way as an ${\text{S}}_{\text{N}}1$ reaction. The laving group breaks off from the substrate, along with its bond pair of electrons, leaving behind a carbocation. In the next step of the E1 reaction, a proton from an adjacent carbon atom is extracted by a base. The $\text{C}-\text{H}$ then moves toward the carbocation, forming the $\text{π}$ bond between the two carbon atoms in the product. The rate of the reaction depends on the stability of the carbocation as this is the rate determining step of the E1 mechanism. All carbocations are high energy, unstable species by virtue of the charge. However, the stability of a carbocation can be increased by resonance. The presence of an electron rich atom like oxygen or nitrogen or an electron-rich double or triple bond stabilizes a carbocation by delocalizing the charge over more than one atom.

The more stable the carbocation, the faster the rate at which it forms. This in turn also increases the rate of the elimination (or substitution) reaction.