Question

Equal quantities of \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) and \(\mathrm{CH}_{3} \mathrm{Cl}\) are treated with iodide ion in acetone. Which reacts faster? (a) both react at almost equal rate (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) reacts faster (c) \(\mathrm{CH}_{3} \mathrm{Cl}\) reacts faster (d) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) doest not reacts at all

Short answer

Answer: (b) CH3OCH2Cl reacts faster.

Step-by-step solution

Identify the reaction type

The reaction taking place in this problem is nucleophilic substitution, where the iodide ion (I-) acts as the nucleophile, attacking the carbon atom that is bonded to the chlorine atom in both the given compounds.

Determine the mechanism

For a nucleophilic substitution reaction, there are two common mechanisms: SN1 and SN2. In this case, both compounds are primary alkyl halides (primary carbon bound to a halogen). Primary alkyl halides typically undergo the SN2 mechanism due to steric reasons, as SN1 mechanism is favored by more substituents around the reactive site.

Compare the leaving groups

In both cases, the leaving group is a chlorine atom. The leaving group ability for both compounds is the same, as they both have chlorine attached to them.

Compare the nucleofuges

The nucleofuge is the part of the molecule that is not involved in the nucleophilic substitution but helps stabilize the transition state or facilitate the reaction. In the case of \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\), the nucleofuge is the OCH2 group, whereas, in the case of \(\mathrm{CH}_{3} \mathrm{Cl}\), the nucleofuge is simply a hydrogen atom. For SN2 reactions, the stability of the transition state is important, and the presence of an electron-withdrawing group such as OCH2 makes the transition state more stable and facilitates the reaction.

Determine the faster-reacting compound

Based on the analysis, it is clear that the compound with the OCH2 group as the nucleofuge, \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\), will have a more stable transition state and will likely react faster via the SN2 mechanism. Therefore, the correct answer is: (b) \(\mathrm{CH}_{3} \mathrm{OCH}_{2} \mathrm{Cl}\) reacts faster.

Key concepts

SN1 and SN2 mechanisms

Nucleophilic substitution reactions are fundamental in organic chemistry, and understanding the mechanisms behind these reactions is crucial. The two primary types of mechanisms are SN1 and SN2. The SN1 mechanism involves a two-step process. First, the leaving group departs, forming a carbocation intermediate. This intermediate is then attacked by the nucleophile.

• SN1 reactions are favored by tertiary alkyl halides because the carbocation can be stabilized by surrounding alkyl groups.
• Rate of reaction: SN1 reactions are not dependent on the concentration of the nucleophile but depend on the stability of the formed carbocation.

On the other hand, the SN2 mechanism is a single-step reaction where the nucleophile attacks the substrate from the opposite side, displacing the leaving group.

• SN2 reactions are favored by primary alkyl halides because there is minimal steric hindrance.
• Rate of reaction: SN2 reactions are bimolecular, meaning both the nucleophile and the substrate influence the reaction rate.

In the given exercise focusing on primary alkyl halides and iodide ions, an SN2 mechanism is at play because of minimal steric hindrance, which allows the nucleophile to effectively attack the electrophilic carbon.

primary alkyl halides

Primary alkyl halides have a halogen atom bonded to a primary carbon atom, which is only connected directly to one other carbon atom. These are less crowded around the reactive site compared to secondary and tertiary alkyl halides.

• Because of the low steric hindrance, primary alkyl halides are highly favorable for SN2 reactions.
• The lack of surrounding bulky groups provides an open pathway for the nucleophile to approach the electrophilic carbon.

When a reaction involves \[\text{CH}_3\text{OCH}_2\text{Cl} \]or \[\text{CH}_3\text{Cl},\]we recognize them as primary alkyl halides. The simple nature and accessibility of the carbon atom make them ideal candidates for SN2, where the one-step mechanism allows for a simultaneous invasion by the nucleophile and ejection of the leaving group. Other reaction mechanisms, like SN1, which require the formation of a carbocation, are less favorable due to the less stabilized intermediate. Hence, in the given scenario, \[\text{CH}_3\text{OCH}_2\text{Cl}\]with an additional functional group (OCH2) would further stabilize the transition state, making it react faster.

transition state stability

Stability of the transition state is a critical factor in determining the rate of a reaction, especially in SN2 mechanisms. The transition state is a high-energy state that exists momentarily during the conversion of reactants to products.

• In SN2, the transition state involves partial bonds forming and breaking as the nucleophile approaches the electrophilic carbon.
• Any electron-withdrawing groups present can stabilize this transitional state by spreading out the electronic charge.

For example, in the case of \[\text{CH}_3\text{OCH}_2\text{Cl},\]there is an OCH2 group. This group can help stabilize the transition state via its electron-withdrawing ability, which helps disperse charge accumulation during the transition phase. Meanwhile, \[\text{CH}_3\text{Cl}\]lacks such stabilizing groups. The difference in these stabilizing interactions between the two compounds explains why \[\text{CH}_3\text{OCH}_2\text{Cl}\]reacts faster: it has a more stable transition state under SN2 conditions.