Question

The compound which can show faster rate of \(\mathrm{S}_{\mathrm{N}} 2\) mechanism than \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{Cl}\) in identical condition is

Short answer

The compound that can show a faster rate of SN2 mechanism than CH3CH2CH2Cl in identical conditions is CH3Cl (methyl chloride) due to its lower steric hindrance, allowing nucleophiles to approach more easily and promote a faster reaction rate.

Step-by-step solution

Identify the given compound

The given compound is CH3CH2CH2Cl, which is also known as 1-chloropropane.

Identify the factors affecting the SN2 reaction rate

In an SN2 reaction, a nucleophile attacks the substrate and a leaving group is displaced. The rate of this reaction depends on two major factors: 1. Steric hindrance: Bulky substituents on the substrate hinder the nucleophile from approaching the target atom, slowing down the reaction. Primary substrates (like the given compound) have less steric hindrance and react faster in an SN2 reaction compared to secondary or tertiary substrates. 2. Leaving group ability: A good leaving group is weakly basic and can stabilize the negative charge when leaving. Better leaving groups generally lead to faster SN2 reaction rates.

Compare compounds for favorable factors

We need to find a compound that reacts faster in an SN2 reaction compared to 1-chloropropane. To achieve this, we can consider compounds with even less steric hindrance or with a better leaving group. Some options could include: 1. CH3Cl (methyl chloride): It has even less steric hindrance than 1-chloropropane as it is a methyl group with only one carbon, which provides less steric hindrance for nucleophiles. 2. CH3CH2CH2Br (1-bromopropane) or CH3CH2CH2I (1-iodopropane): In these compounds, the leaving group is bromide or iodide, which are better leaving groups than chloride due to their larger size and weaker bonds. This makes the reaction rate faster for SN2 mechanisms.

Choose the compound that will have a faster reaction rate

Based on the discussion in Step 3, there are two compounds we found that can show faster SN2 reaction rates than 1-chloropropane, CH3Cl (methyl chloride) and CH3CH2CH2Br (1-bromopropane) or CH3CH2CH2I (1-iodopropane). Although any of these compounds could be a correct answer, we can choose CH3Cl (methyl chloride) due to its even lower steric hindrance compared to the given compound, making it the best candidate for a faster SN2 reaction rate.

Key concepts

Steric Hindrance

Steric hindrance is a critical factor affecting the rate of the SN2 reaction mechanism. It refers to the physical obstacle created by larger groups of atoms within a molecule that restricts the access of a nucleophile to the reaction site. Picture a crowded entrance at a concert; the fewer people there, the faster you can move through. Similarly, in chemistry, a nucleophile is the 'guest' trying to reach the 'door,' which is the central carbon atom.

In the given compound, 1-chloropropane, one could see the effect of steric hindrance as it contains a straight chain of three carbon atoms. This structure has minimal hindrance, thus generally allowing for a quicker SN2 reaction compared to compounds with more crowded carbon chains, such as secondary or tertiary carbon centers. Students should appreciate that in evaluating reaction rates, the goal is to minimize crowdedness at the reaction site to ensure the nucleophile has clear access to reach and react with the central carbon atom.

Leaving Group Ability

The leaving group's ability is another cornerstone of the SN2 reaction mechanism. A leaving group is an atom or group of atoms that can take a pair of electrons and exit the molecule when new bonds are formed during the reaction. The smoother the exit, the quicker the curtain falls on the act of the reaction.

In the context of the exercise, the chloro group in 1-chloropropane is a competent leaving group, but other halogens like bromine or iodine from 1-bromopropane or 1-iodopropane can do an even better job. They are larger and less electronegative, meaning they can accommodate a negative charge with more ease as they 'leave,' akin to larger doors easing the exit in a crowded room. Students should remember that efficient leaving groups tend to be weaker bases, which is a counterintuitive but essential aspect of nucleophilic substitution reactions.

Nucleophilic Substitution

Nucleophilic substitution is the heart of the SN2 reaction where a nucleophile - which is electron-rich - replaces a leaving group in a single, concerted step. Imagine a dance floor where a dancer (the nucleophile) swiftly swaps places with another (the leaving group), without pausing the music. The 'concerted' nature means that the bond-forming and bond-breaking occur simultaneously.

This process relies heavily on the nucleophile's ability to approach and form a bond with the central carbon atom of the substrate while the leaving group departs. For those studying SN2 mechanisms, it's critical to understand that the nucleophile and the substrate must be properly aligned for this 'swap' to occur; therefore, less hindered, more accessible carbon atoms are preferred for a rapid and successful exchange.

Reaction Rate Factors

The rate at which SN2 reactions occur is influenced by various factors, including the nature of the nucleophile and the solvent, in addition to steric hindrance and leaving group ability already discussed. A more potent nucleophile equals a quicker reaction, much like a more persuasive individual can more quickly convince others. Solvent effects are akin to the environment of the reaction; for instance, polar aprotic solvents enhance the reaction rate by stabilizing ionic intermediates without hindering the nucleophile.

When analyzing reaction rates, students should also consider factors such as concentration and temperature. Higher concentrations of reactants can increase the likelihood of collisions, hence speeding up the reaction, while elevated temperatures contribute additional energy, allowing reactants to surpass energy barriers with greater frequency. Grasping all these components enables students to predict and manipulate the outcomes of chemical reactions with precision and insight.