What halides would undergo E2 dehydrohalogenation to give the following pure alkenes?
e. 4-methylcyclohexene

Verified step by step guidance

1. Identify the structure of 4-methylcyclohexene. It is a cyclohexene ring with a double bond and a methyl group attached to the 4th carbon. The goal is to determine which alkyl halides can undergo E2 elimination to form this specific alkene.
2. Recall the E2 elimination mechanism. It requires a strong base and a β-hydrogen (a hydrogen atom on a carbon adjacent to the carbon bearing the halide). The elimination occurs in a single step, forming a double bond between the α-carbon (the carbon bonded to the halide) and the β-carbon.
3. Determine the possible positions of the halide on the cyclohexane ring that would lead to the formation of 4-methylcyclohexene. The halide must be on a carbon adjacent to the carbon where the double bond will form. Additionally, the β-hydrogen and the halide must be anti-periplanar (in opposite planes) for the E2 mechanism to proceed.
4. Consider the stereochemistry of the cyclohexane ring. In a chair conformation, the β-hydrogen and the halide must be in axial positions to achieve the required anti-periplanar geometry. Analyze the possible chair conformations to identify suitable halide positions.
5. Conclude that the halides that can undergo E2 elimination to form 4-methylcyclohexene are those where the halide is on the 3rd or 5th carbon of the cyclohexane ring, relative to the methyl group on the 4th carbon. These positions allow for the formation of the double bond between the 3rd and 4th carbons or the 4th and 5th carbons, resulting in the desired product.

Key Concepts

E2 Mechanism

The E2 mechanism is a type of elimination reaction where a base removes a proton from a β-carbon, leading to the simultaneous departure of a leaving group, typically a halide, from the α-carbon. This concerted process results in the formation of a double bond. Understanding the stereochemistry and the requirement for anti-periplanar geometry is crucial for predicting the outcome of E2 reactions.

Dehydrohalogenation

Dehydrohalogenation refers to the elimination of a hydrogen halide (HX) from an alkyl halide, resulting in the formation of an alkene. This reaction is often facilitated by strong bases and is a key step in synthesizing alkenes from alkyl halides. The choice of base and the structure of the halide influence the reaction pathway and the stability of the resulting alkene.

Alkene Stability and Substitution

The stability of alkenes is influenced by their degree of substitution; more substituted alkenes are generally more stable due to hyperconjugation and the inductive effect. In the context of E2 reactions, the formation of more stable alkenes is favored, which often leads to the Zaitsev's rule, where the more substituted alkene is the major product. Recognizing the structure of the desired alkene helps in selecting the appropriate halide for the reaction.