Question

Explain why two different products are formed from disrotatory ring closure of (2E,4Z,6Z)-octatriene, but only one product is formed from disrotatory ring closure of (2E,4Z,6E)-octatriene.

Short answer

The disrotatory ring closure of (2E,4Z,6Z)-octatriene leads to the formation of a trans isomer that can exist as a pair of enantiomers.

The disrotatory ring closure of (2E, 4Z, 6E)-octatriene leads to the formation of a cis isomer that is a meso compound and does not possess a superimposable mirror image.

Step-by-step solution

Disrotatory ring closure

An electrocyclic reaction comes from the creation of a new sigma bond. This bond forms when the p orbitals at the end of the conjugated system rotate in order to overlap in a head-to-head fashion.

If the orbitals rotate in the opposite direction, the ring closure is termed disrotatory.

Explanation of why different products are formed from the disrotatory ring closures of (2E,4Z,6Z)-octatriene and (2E, 4Z,6E)-octatriene

According to the Woodward-Hoffmann rules, the disrotatory ring closure of (2E,4Z,6Z)-octatriene leads to a trans isomer that can exist as a pair of enantiomers.

One enantiomer is formed if the top lobes of the p orbital rotate toward each other. The other enantiomer is formed if the bottom lobes of the p orbital rotate toward each another.

The disrotatory ring closure of (2E, 4Z, 6E)-octatriene leads to the formation of a cis isomer that is a meso compound and does not possess a superimposable mirror image.

Hence, the same compound is formed from the top lobes of the p orbital rotating toward each other and from the bottom lobes of the p orbital rotating toward one other.