Explain why the following compounds are not optically active:
a. the product obtained from the reaction of 1,3-butadiene with cis-1,2-dichloroethene

Verified step by step guidance

1. Identify the type of reaction occurring between 1,3-butadiene and cis-1,2-dichloroethene. This is a Diels-Alder reaction, which is a [4+2] cycloaddition reaction where a conjugated diene (1,3-butadiene) reacts with a dienophile (cis-1,2-dichloroethene) to form a cyclic product.
2. Analyze the stereochemistry of the reactants. Cis-1,2-dichloroethene has a plane of symmetry due to the cis arrangement of the chlorine atoms, making it achiral. 1,3-butadiene is also achiral because it is a planar molecule with no stereogenic centers.
3. Determine the stereochemistry of the product. The Diels-Alder reaction is stereospecific, meaning the stereochemistry of the dienophile (cis-1,2-dichloroethene) is preserved in the product. The product will have the substituents (chlorine atoms) on the same side of the newly formed six-membered ring, resulting in a meso compound.
4. Explain why the product is not optically active. A meso compound has an internal plane of symmetry, which makes it achiral even though it contains stereogenic centers. The presence of this symmetry prevents the compound from rotating plane-polarized light, rendering it optically inactive.
5. Conclude that the lack of optical activity in the product is due to the formation of a meso compound, which arises from the stereospecific nature of the Diels-Alder reaction and the symmetry of the reactants.

Key Concepts

Optical Activity

Optical activity refers to the ability of a chiral compound to rotate the plane of polarized light. A compound is considered optically active if it lacks an internal plane of symmetry and has a non-superimposable mirror image. This property is crucial for distinguishing between enantiomers, which are pairs of chiral molecules that are mirror images of each other.

Chirality and Symmetry

Chirality is a geometric property of some molecules that makes them non-superimposable on their mirror images. A molecule is chiral if it has at least one carbon atom bonded to four different substituents, leading to two distinct enantiomers. Conversely, if a molecule has an internal plane of symmetry, it is achiral and cannot exhibit optical activity.

Reaction Products and Stereochemistry

The stereochemistry of reaction products is essential in determining their optical activity. In the case of the reaction between 1,3-butadiene and cis-1,2-dichloroethene, the resulting product may have a symmetrical structure that leads to the cancellation of optical activity. Understanding the spatial arrangement of atoms in the product helps in assessing whether it is chiral or achiral.