Electrocyclic reactions are types of Pericyclic Reactions in which a molecule with a system of pi electrons undergoes a ring-opening or ring-closing process. This transformation happens smoothly, without forming intermediates, and follows specific rules based on the number of pi electrons and whether heat or light is used. ^[1-4]

Examples ^[1-4]

i. Cyclobutene to Butadiene

One of the earliest examples of an electrocyclic reaction is the ring opening of cyclobutene to give butadiene. Researchers also found that butadiene could undergo a reverse reaction, forming cyclobutene through ring closure. As the reaction progresses, the electron clouds in the molecule arrange themselves in a specific way, resulting in new bond formation and old bond cleavage. The following bond changes occur during the forward and reverse reactions:

Forward Reaction: One C–C pi bond and one C–C sigma bond are broken, while two C–C pi bonds are formed. 

Reverse Reaction: Two C–C pi bonds break, leading to the formation of a C–C pi bond and a C–C sigma bond.

However, ring strain generally makes the reverse reaction unfavorable.

Cyclobutene and butadiene’s ability to interconvert suggests that electrocyclic ring-opening and ring-closing proceed through a shared transition state, establishing an equilibrium between the two molecules at elevated temperatures.

Woodward–Hoffmann Rule

The Woodward–Hoffmann rule predicts how molecules change shape based on the number of π electrons and reaction conditions. For a 4 pi electron system undergoing a ring-opening or ring-closing reaction, the following rules apply.

• Heat (Thermal Conditions): The molecule changes by rotating both ends in the same direction, either clockwise or counterclockwise. It is known as conrotatory motion.

• Light (Photochemical Conditions): The molecule rotates in opposite directions at both ends, one clockwise and the other counterclockwise. It is known as disrotatory motion.

ii. 3,4-Dimethylcyclobutene to 2,4-Hexadiene

Substituted cyclobutenes also undergo electrocyclic reactions. When exposed to heat or light, the molecule 3,4-dimethylcyclobutene can open into 2,4-hexadiene. Electrocyclic reactions are stereospecific processes, meaning the stereochemistry of the starting material directly influences the stereochemistry of the product and follows the Woodward–Hoffmann rules.

Ring Opening

Heat

• Cis-3,4-dimethylcyclobutene opens up to cis,trans-2,4-hexadiene.
• Trans-3,4-dimethylcyclobutene opens up to trans,trans-2,4-hexadiene.

Light

• Cis-3,4-dimethylcyclobutene opens up to trans,trans-2,4-hexadiene.
• Trans-3,4-dimethylcyclobutene opens up to cis,trans-2,4-hexadiene.

Ring Closure

Although the ring closure of 2,4-hexadiene is unfavorable due to strain, it follows a converse pathway with the same Woodward–Hoffmann rules.

Heat

• Cis,trans-2,4-hexadiene closes to cis-3,4-dimethylcyclobutene.
• Trans,trans-2,4-hexadiene closes to trans-3,4-dimethylcyclobutene.

Light

• Cis,trans-2,4-hexadiene closes to trans-3,4-dimethylcyclobutene.
• Trans,trans-2,4-hexadiene closes to cis-3,4-dimethylcyclobutene.

Applications ^[1-4]

• Vitamin D Production: Our body makes vitamin D when sunlight reacts with a molecule in the skin called 7-dehydrocholesterol. This photochemical electrocyclic reaction helps produce previtamin D3, an important step in vitamin D formation.  
• Medicines: Chemists use electrocyclic reactions to create complex drug molecules. These reactions help them make medicines that respond to light, which can be useful for targeted drug delivery and disease treatment. 
• Photochromic Lenses and Smart Windows: Certain materials undergo color changes upon light exposure due to electrocyclic reactions. For example, photochromic sunglasses darken in sunlight and become clear indoors. The same idea is used in smart windows, which control how much light enters a room.  
• OLED: Electrocyclic reactions help make organic light-emitting diodes (OLEDs), which are used on phone and TV screens. They also help create flexible electronics and improve materials that conduct electricity.
• Sensors: Scientists use light-driven electrocyclic reactions to make molecular machines, tiny devices that move when exposed to light. These reactions are also useful in sensors, which help detect chemicals or changes in the environment.