Which substitution reaction takes place more rapidly?
d. CH₃CH₂Cl + I⁻ or CH₃CH₂Br + I⁻

Verified step by step guidance

1. Step 1: Understand the type of substitution reaction involved. This problem refers to nucleophilic substitution reactions, specifically SN2 reactions, where the nucleophile (I−) attacks the electrophilic carbon bonded to the leaving group.
2. Step 2: Evaluate the leaving group ability. In SN2 reactions, the rate depends significantly on the leaving group. A better leaving group will make the reaction proceed more rapidly. Compare Cl− and Br− as leaving groups. Br− is a better leaving group than Cl− because it is larger and more polarizable, making it more stable after leaving.
3. Step 3: Consider the nucleophile. I− is a strong nucleophile due to its large size and high polarizability, which makes it effective in attacking the electrophilic carbon in both cases.
4. Step 4: Analyze the steric hindrance. Both CH3CH2Cl and CH3CH2Br are primary alkyl halides, meaning they have minimal steric hindrance, which favors the SN2 mechanism. Therefore, steric hindrance does not significantly affect the comparison in this case.
5. Step 5: Conclude based on the leaving group ability. Since Br− is a better leaving group than Cl−, the reaction involving CH3CH2Br + I− will take place more rapidly than CH3CH2Cl + I− under identical conditions.

Key Concepts

Nucleophilic Substitution Reactions

Nucleophilic substitution reactions involve the replacement of a leaving group in a molecule by a nucleophile. The rate of these reactions can depend on the nature of the leaving group and the strength of the nucleophile. In this context, the reaction speed is influenced by the bond strength between the carbon and the leaving group, which affects how easily the leaving group can depart.

Leaving Group Ability

The ability of a leaving group to depart from a molecule is crucial in determining the rate of nucleophilic substitution reactions. Bromide (Br−) is generally a better leaving group than chloride (Cl−) due to its larger size and weaker bond strength with carbon. This means that reactions involving bromides typically proceed faster than those involving chlorides, all else being equal.

SN2 Mechanism

The SN2 mechanism is a type of nucleophilic substitution that involves a single concerted step where the nucleophile attacks the carbon atom from the opposite side of the leaving group. This results in a transition state where both the nucleophile and the leaving group are partially bonded to the carbon. The sterics and the strength of the leaving group play significant roles in the reaction rate, making the SN2 pathway faster for substrates with better leaving groups.