Question

Propose a structure for an alkyl halide that gives only (E)-3-methyl2-phenyl-2-pentene on E2 elimination. Make sure you indicate the stereochemistry.

Short answer

Use 2-bromo-3-methyl-2-phenylpentane to achieve (E)-3-methyl-2-phenyl-2-pentene via E2 elimination.

Step-by-step solution

Understand the Target Product

The product of the reaction is (E)-3-methyl-2-phenyl-2-pentene. The (E) designation indicates that the two highest priority groups on each carbon atom of the double bond are on opposite sides. This means we need to choose substituents carefully to ensure trans configuration after elimination.

E2 Elimination Mechanism Overview

In an E2 elimination, a proton is abstracted from a β-carbon and a leaving group (halide) departs from the adjacent α-carbon simultaneously. The reaction requires anti-periplanar alignment of the proton and the leaving group for the elimination to produce a double bond with specific stereochemistry.

Determine β-Hydrogen Location

To form a (E)-3-methyl-2-phenyl-2-pentene, we need the β-hydrogen and the leaving group to be anti-periplanar. Locate the carbon atoms in the product where the double bond forms: C-2 and C-3 in the pentene chain.

Choose Suitable Halide Precursor

Choose an alkyl halide where the bromine is on the carbon that will become C-2 of the double bond. For this task, select 2-bromo-3-methyl-2-phenylpentane as your substrate. This ensures that the bromine is in position for the elimination reaction to occur between C-2 and C-3.

Ensure Anti-Periplanar Geometry

Draw the Newman projection or use a model to ensure the β-hydrogen (at C-3) and the bromine (at C-2) are anti-periplanar. The substituents need to be oriented such that when the elimination occurs, the highest priority groups based on CIP rules are trans to each other on the final double bond.

Validate Product Stereochemistry

Ensure that when elimination occurs, the groups attached to the double bond in the final structure align with the (E)-stereochemistry. This occurs when the methyl group and phenyl group appear on opposite sides of the double bond in the end product.

Key concepts

(E)-stereochemistry

(E)-stereochemistry is an important aspect in organic chemistry, particularly when looking at the arrangement of substituents around a double bond. In (E)-isomers, the highest priority groups on each carbon of the double bond are located on opposite sides. This trans configuration contrasts with the (Z)-isomer, where these groups are on the same side. To determine (E)- or (Z)-stereochemistry, Cahn-Ingold-Prelog (CIP) priority rules are applied to rank substituents. These rules help in identifying which groups have more bulk or higher atomic number, influencing their priority ranking. Understanding (E)-stereochemistry is essential when predicting the outcomes of reactions such as E2 eliminations, where the spatial arrangement directly affects the structure and properties of the resultant alkenes. Correctly identifying and achieving (E)-stereochemistry ensures the desired product is obtained, which is crucial in synthesis and real-world applications like pharmaceuticals.

anti-periplanar geometry

Anti-periplanar geometry is a key feature of the E2 elimination mechanism. This alignment refers to the spatial arrangement where a leaving group (like a halide) is opposite to a hydrogen atom on adjacent carbons, both lying in the same plane. Such geometry facilitates the elimination process, allowing for the formation of a double bond. The anti-periplanar structure is particularly crucial because it minimizes steric hindrance and maximizes orbital overlap, leading to an efficient pi bond formation. Visualizing this geometry can be done using Newman projections, which help show how substituents are aligned in space. This projection method simplifies identifying whether the necessary conditions for anti-periplanar alignment are met. Understanding anti-periplanar geometry ensures successful E2 reactions, facilitating the formation of alkenes with specific stereochemistry.

alkyl halide

Alkyl halides are organic compounds characterized by one or more halogen atoms (such as chlorine, bromine, or iodine) attached to an alkyl group. They play a vital role in many substitution and elimination reactions, including E2 elimination, due to the ability of the halogen to act as a good leaving group. In E2 reactions, the presence of an alkyl halide, such as 2-bromo-3-methyl-2-phenylpentane, is crucial for forming desired alkenes. The halogen's electronegative property allows it to stabilize the negative charge as it leaves, thus facilitating the elimination process. The choice of alkyl halide substrate impacts the reaction pathway and the stereochemical outcome. Ensuring the correct positioning of the halogen on the alkyl chain is essential, as it influences whether the reaction will yield the correct (E)-stereochemistry in the product. Alkyl halides, thus, are indispensable in achieving targeted organic synthesis.