Which alkyl halide is more reactive in an S_N2 reaction with a given nucleophile?
a.
b.

Verified step by step guidance

1. Step 1: Understand the SN2 reaction mechanism. SN2 reactions are bimolecular nucleophilic substitution reactions where the nucleophile attacks the electrophilic carbon and displaces the leaving group in a single step. The rate of the reaction depends on steric hindrance and the quality of the leaving group.
2. Step 2: Analyze the structures in part (a). The first structure has a primary alkyl bromide, while the second structure has a secondary alkyl bromide. Primary alkyl halides are less sterically hindered compared to secondary alkyl halides, making them more reactive in SN2 reactions.
3. Step 3: Analyze the structures in part (b). The first structure has a chlorine atom as the leaving group, while the second structure has an iodine atom. Iodine is a better leaving group than chlorine because it is larger and more polarizable, which stabilizes the negative charge after leaving.
4. Step 4: Compare the reactivity of the alkyl halides in part (a). The primary alkyl bromide is more reactive in an SN2 reaction than the secondary alkyl bromide due to reduced steric hindrance.
5. Step 5: Compare the reactivity of the alkyl halides in part (b). The alkyl iodide is more reactive in an SN2 reaction than the alkyl chloride because iodine is a superior leaving group compared to chlorine.

Key Concepts

SN2 Mechanism

The SN2 (substitution nucleophilic bimolecular) mechanism involves a single concerted step where a nucleophile attacks an electrophile, leading to the simultaneous displacement of a leaving group. This reaction is characterized by a backside attack, which results in inversion of configuration at the carbon center. The rate of the reaction depends on both the nucleophile and the substrate, making it bimolecular.

Leaving Group Ability

The ability of a leaving group to depart from a substrate is crucial in determining the reactivity of alkyl halides in SN2 reactions. Good leaving groups, such as bromide (Br-) and iodide (I-), stabilize the negative charge after leaving, facilitating the reaction. In contrast, poor leaving groups, like chloride (Cl-), hinder the reaction, making the substrate less reactive.

Steric Hindrance

Steric hindrance refers to the spatial arrangement of atoms around a reactive center that can impede the approach of a nucleophile. In SN2 reactions, primary alkyl halides are more reactive than secondary or tertiary ones due to less steric hindrance. The presence of bulky groups around the electrophilic carbon can significantly slow down or prevent the nucleophilic attack.