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

Verified step by step guidance

1. Step 1: Recall the key factors that influence the rate of an Sₙ2 reaction. The Sₙ2 mechanism is a one-step process where the nucleophile attacks the electrophilic carbon and the leaving group departs simultaneously. The rate of the reaction depends on steric hindrance, the strength of the nucleophile, and the quality of the leaving group.
2. Step 2: Focus on steric hindrance. In an Sₙ2 reaction, the nucleophile must approach the electrophilic carbon from the opposite side of the leaving group. Therefore, primary haloalkanes react faster than secondary haloalkanes, and tertiary haloalkanes are generally unreactive due to steric hindrance.
3. Step 3: Compare the structures of the two haloalkanes in the pair. Identify whether the carbon attached to the halogen is primary, secondary, or tertiary. A primary haloalkane will react faster in an Sₙ2 reaction than a secondary haloalkane.
4. Step 4: Consider the leaving group. A better leaving group (e.g., I⁻ > Br⁻ > Cl⁻ > F⁻) will facilitate a faster reaction. If the two haloalkanes have different leaving groups, the one with the better leaving group will react more quickly.
5. Step 5: Combine your analysis of steric hindrance and leaving group quality to determine which haloalkane in the pair will react most quickly in an Sₙ2 reaction. The haloalkane with less steric hindrance and a better leaving group will be the faster-reacting compound.

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 a single transition state and is characterized by a second-order reaction rate, dependent on both the nucleophile and the substrate. The steric hindrance around the electrophilic carbon significantly influences the reaction rate, with less hindered substrates reacting more quickly.

Haloalkanes

Haloalkanes, or alkyl halides, are organic compounds containing carbon, hydrogen, and halogen atoms. The reactivity of haloalkanes in Sₙ2 reactions is influenced by the type of halogen and the structure of the carbon chain. Primary haloalkanes are generally more reactive than secondary or tertiary haloalkanes due to less steric hindrance, making them more accessible for nucleophilic attack.

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, increased steric hindrance around the electrophilic carbon atom can slow down or inhibit the nucleophilic attack. Therefore, when comparing haloalkanes, those with less steric hindrance (such as primary haloalkanes) will typically react more quickly than those with greater steric hindrance (such as tertiary haloalkanes).