While not covered explicitly in this chapter, the ene reaction occurs similarly to the Diels–Alder reaction but replaces the electrons from one bond in the diene with the electrons in a C―H bond. Draw the mechanism for the following reaction. [Number the carbons and draw in the hydrogens of the product. And, of course, make a note of bonds formed and bonds broken.]

Verified step by step guidance

1. Identify the components of the ene reaction: the 'ene' (alkene with an allylic hydrogen) and the 'enophile' (an electron-deficient species). In this case, determine which part of the molecule acts as the ene and which as the enophile.
2. Number the carbon atoms in the alkene and identify the allylic hydrogen that will participate in the reaction. This hydrogen is typically adjacent to the double bond.
3. Draw the initial step of the mechanism where the allylic hydrogen is transferred to the enophile. This involves the formation of a new C-H bond and the breaking of the original C-H bond.
4. Illustrate the movement of electrons: the π electrons from the alkene double bond will form a new σ bond with the enophile, while the electrons from the C-H bond will form a new π bond in the alkene.
5. Finally, draw the product of the reaction, ensuring that all new bonds are shown, and the numbering of carbons and hydrogens is clear. Highlight the bonds that were formed and those that were broken during the reaction.

Key Concepts

Ene Reaction

The ene reaction is a type of pericyclic reaction where an alkene (ene) reacts with a compound containing a hydrogen atom (enophile) to form a new bond. It involves the transfer of electrons from a C-H bond to form a new C-C bond, similar to the Diels–Alder reaction but with different electron sources. Understanding the electron flow and bond formation is crucial for drawing the mechanism.

Pericyclic Reactions

Pericyclic reactions are a class of organic reactions that occur through a concerted process involving cyclic transition states. They are characterized by the redistribution of bonding electrons in a cyclic manner, often without intermediates. Familiarity with these reactions helps in understanding the mechanism of the ene reaction, as they share similar principles of electron movement and bond formation.

Mechanism Drawing

Drawing mechanisms involves illustrating the step-by-step process of a chemical reaction, showing how reactants transform into products. It requires identifying bonds broken and formed, numbering atoms for clarity, and indicating electron movement. This skill is essential for visualizing the ene reaction, ensuring accurate representation of the molecular changes and understanding the reaction pathway.