For each pair, choose the haloalkane that would react most quickly in an S_N2 reaction.
(a)

Verified step by step guidance

1. Step 1: Recall the key factors that influence the rate of an SN2 reaction. The SN2 mechanism involves a single-step nucleophilic substitution, where the nucleophile attacks the electrophilic carbon and displaces the leaving group. The rate of SN2 reactions is highly dependent on steric hindrance around the electrophilic carbon.
2. Step 2: Analyze the structures of the two haloalkanes provided in the image. The first structure has a bromine atom attached to a secondary carbon (a carbon bonded to two other carbons), while the second structure has a bromine atom attached to a primary carbon (a carbon bonded to only one other carbon).
3. Step 3: Understand that steric hindrance is lower for primary carbons compared to secondary carbons. In SN2 reactions, the nucleophile needs unhindered access to the electrophilic carbon to attack effectively. Therefore, primary haloalkanes generally react faster in SN2 reactions than secondary haloalkanes.
4. Step 4: Compare the two haloalkanes. The haloalkane with bromine attached to the primary carbon (on the right) will experience less steric hindrance, making it more reactive in an SN2 reaction compared to the haloalkane with bromine attached to the secondary carbon (on the left).
5. Step 5: Conclude that the haloalkane with bromine attached to the primary carbon (on the right) would react most quickly in an SN2 reaction due to reduced steric hindrance and better accessibility for the nucleophile.

Key Concepts

Sₙ2 Reaction Mechanism

The Sₙ2 (substitution nucleophilic bimolecular) reaction is a type of nucleophilic substitution where the nucleophile attacks the electrophile simultaneously as the leaving group departs. This concerted mechanism results in the inversion of configuration at the carbon center. The rate of the Sₙ2 reaction depends on steric hindrance and the strength of the nucleophile, making primary haloalkanes more reactive than secondary or tertiary ones.

Steric Hindrance

Steric hindrance refers to the prevention of chemical reactions due to the spatial arrangement of atoms within a molecule. In the context of Sₙ2 reactions, bulky groups around the carbon atom can impede the approach of the nucleophile, slowing down the reaction. Therefore, haloalkanes with less steric hindrance, such as primary haloalkanes, will generally react more quickly than those with greater steric hindrance, like tertiary haloalkanes.

Leaving Group Ability

The ability of a leaving group to depart from a molecule is crucial in determining the rate of nucleophilic substitution reactions. Good leaving groups, such as iodide or bromide, stabilize the negative charge after leaving, facilitating the reaction. In Sₙ2 reactions, the presence of a better leaving group can significantly enhance the reaction rate, making it an important factor to consider when comparing different haloalkanes.