Question

1-Chloro-1,2-diphenylethane can undergo E2 elimination to give either cis- or trans-1,2-diphenylethylene (stilbene). Draw Newman projections of the reactive conformations leading to both possible products, and suggest a reason why the trans alkene is the major product.

Short answer

Trans-stilbene is more stable due to less steric hindrance.

Step-by-step solution

Identify the Substrate and Products

The substrate is 1-Chloro-1,2-diphenylethane. The elimination leads to 1,2-diphenylethylene (stilbene), which can exist as either cis or trans isomers.

Understand the E2 Elimination Mechanism

E2 eliminations involve the simultaneous removal of a proton and a leaving group, often needing an antiperiplanar arrangement for optimal overlap in the transition state.

Draw the Start of the Newman Projection

Begin by drawing the front carbon (C1) as a circle with the chlorine atom bonded to it. The back carbon (C2) will have two phenyl groups and a hydrogen.

Position for Cis Isomer Formation

For the formation of cis-stilbene, one phenyl group and the chlorine should be antiperiplanar, allowing the leaving of Cl and formation of a double bond resulting in two phenyl groups on the same side.

Position for Trans Isomer Formation

For the formation of trans-stilbene, the hydrogen and the chlorine in a Newman projection should be antiperiplanar to facilitate proper overlap and elimination resulting in phenyl groups on opposite sides.

Comparison and Rationale for Trans Major Product

The anti-periplanar orientation required for E2 means the reaction favors formation of trans isomers, where the bulky phenyl groups are farthest apart, minimizing steric hindrance and leading to greater thermodynamic stability.

Key concepts

Newman projections

Newman projections are a valuable tool in understanding conformational and stereochemical aspects of organic molecules. When looking at a molecule through a specific carbon-carbon bond, Newman projections represent this view by displaying the front carbon as a dot and the back carbon as a circle. This perspective helps simplify and visualize the three-dimensional spatial arrangement of atoms connected to these carbons.
For the case of 1-Chloro-1,2-diphenylethane, drawing a Newman projection assists in identifying the alignment and interactions between substituents around the two carbon atoms involved. In E2 elimination reactions, using a Newman projection can clearly show which groups are antiperiplanar, a crucial arrangement where a hydrogen atom and a leaving group like chlorine need to be on opposite sides for the reaction to proceed efficiently.

cis-trans isomerism

Cis-trans isomerism, also known as geometric isomerism, occurs in alkenes where two distinct substituents are attached to each carbon of the double bond. In the E2 elimination of 1-Chloro-1,2-diphenylethane, the creation of cis or trans isomers comes into play. When the two phenyl groups are positioned on the same side of the double bond, the resulting isomer is called *cis*. Conversely, if the phenyl groups are on opposite sides, the isomer is *trans*.
The formation of the trans isomer is favored due to the spatial relationship of the larger substituents. Trans isomers generally have lower steric strain because the larger groups are further apart, reducing repulsive interactions. This is significant because the product distribution in elimination reactions not only depends on kinetics but also on the energetic favorability of the resulting structures.

stereochemistry

Stereochemistry involves the study of the arrangement of atoms in molecules and the impact of this arrangement on the chemical properties and reactions of those molecules. In the context of E2 eliminations, understanding stereochemistry is critical for predicting the outcome of the reaction.
For this particular molecule, stereochemistry dictates how the substituents will align to allow the proper antiperiplanar setup required for E2 eliminations. This means that the chlorine and hydrogen on adjacent carbon atoms must be positioned in such a way that they are directly opposite each other maintaining a straight line. This ensures the effective overlapping of orbitals to form the stereospecific trans product, which is typically more stable due to fewer steric clashes between large groups.

thermodynamic stability

Thermodynamic stability relates to the energy state of molecules; the lower the energy, the more stable the molecule. In reactions like E2 eliminations, achieving a stable product is often a driving factor. For 1,2-diphenylethylene (stilbene), the trans isomer is more thermodynamically stable than the cis isomer. This is because the bulky phenyl groups are positioned apart from one another in the trans configuration, leading to minimized steric hindrance and decreased strain energy.
Such considerations are vital as they affect the eventual yield of a reaction. Even though both cis and trans products can form, thermodynamically, the system favors the pathway leading to the more stable, lower energy trans isomer, making it the predominant product in E2 reactions involving 1-Chloro-1,2-diphenylethane.