Problem 43 - Advanced Problems in Organic Chemistry for Competitive Examinations

Question

Which statements are true for \(\mathrm{S}_{\mathrm{N}} 2\) reaction of alkyl halides? (i) Both of the alkyl halide and nucleophile are involved in the transition state. (ii) Reaction proceeds with inversion of configuration at the substitution centre. (iii) Reaction proceeds with retention of configuration at the substitution centre. (iv) The order of reactivity is \(3^{\circ}>2^{\circ}>1^{\circ}\). (v) The nucleophile must have an unshared electron pair and bear a negative charge. (vi) The greater the nucleophilicity of the nucleophile, the greater the rate of reaction. (a) \(\mathrm{i}, \mathrm{ii}, \mathrm{v}, \mathrm{vi}\) (b) \(\mathrm{i}, \mathrm{iii}, \mathrm{v}, \mathrm{vi}\) (c) \(\mathrm{i}, \mathrm{i} \mathrm{i}, \mathrm{iv}, \mathrm{v}\) (d) \(\mathrm{i}, \mathrm{ii}, \mathrm{vi}\)

Step-by-Step Solution

Answer

Option (d) contains the true statements: (i), (ii), and (vi).

Step 1: Understanding SN2 Mechanism

In an SN2 reaction, the nucleophile attacks the electrophilic carbon atom of the alkyl halide. This results in a transition state where both the nucleophile and the alkyl halide are involved. Thus, statement (i) is true.

Step 2: Inversion vs. Retention of Configuration

The mechanism involves a backside attack by the nucleophile, leading to inversion of configuration at the substitution center of the alkyl halide. Therefore, statement (ii) is true, while statement (iii) is false.

Step 3: Reactivity Order of Alkyl Halides

In SN2 reactions, steric hindrance plays a significant role. Primary alkyl halides are more reactive than secondary, which are more reactive than tertiary. Hence, the order of reactivity is the opposite of choice (iv) and is actually: \(1^{\circ} > 2^{\circ} > 3^{\circ}\). Therefore, statement (iv) is false.

Step 4: Nucleophile Characteristics

For an SN2 reaction, the nucleophile should have an unshared electron pair but does not necessarily need to bear a negative charge. Many good nucleophiles are neutral species. Therefore, statement (v) is partially false, as a neutral nucleophile can also participate in SN2 reactions.

Step 5: Effect of Nucleophilicity on Reaction Rate

Increased nucleophilicity generally increases the rate of SN2 reactions since the nucleophile attacks the substrate. Therefore, statement (vi) is true.

Step 6: Identifying Correct Answer

The true statements for an SN2 reaction are (i), (ii), and (vi). Therefore, the correct option is (d).

Key Concepts

Nucleophilic Substitution

Nucleophilic substitution is a fundamental type of reaction in organic chemistry where a nucleophile replaces a leaving group bonded to a carbon atom. In SN2 reactions, this process happens in one concerted step. "S" in SN2 stands for substitution and "N" for nucleophilic, while "2" indicates that two molecular entities, the nucleophile and the electrophile, take part in the rate-determining step, meaning both are involved in the transition state.
This type of reaction is particularly notable because it leads to changes in the three-dimensionality of the molecule, which we will explore further in the stereochemistry section.

• The reaction occurs when a nucleophile donates its pair of electrons to form a new chemical bond with the substrate.
• The leaving group's bond to the substrate is simultaneously broken, allowing it to depart as a leaving ion or molecule.
• Success in these reactions often depends on the strength and structure of the nucleophile, the nature of the leaving group, and the steric environment of the substrate.

Reaction Mechanism

The reaction mechanism of an SN2 process is a single-step event characterized by a backside attack, where the nucleophile approaches the substrate opposite the leaving group. In this case, the nucleophile displaces the leaving group as both the nucleophile and the leaving group are attached to the carbon atom in the transition state.
This mechanism has a high preference for substrates that are less hindered, such as primary alkyl halides, because they offer less steric obstruction. Secondary and tertiary alkyl halides, burdened with additional groups around the reacting center, are less suited for SN2 reactions due to increased steric hindrance.

• The simultaneous bond forming and breaking lead to a transition state that is neither stable nor isolatable; it represents the peak of the potential energy barrier.
• Common factors that influence SN2 mechanisms include nucleophile strength, leaving group ability, and the solvent used.

Stereochemistry

Stereochemistry in SN2 reactions involves a significant change to the original configuration of the molecule. Due to the backside attack of the nucleophile, the configuration at the carbon atom where the substitution occurs is inverted - a situation often described as an "umbrella flip." This inversion is because the nucleophile displaces the leaving group by attacking from the opposite side, hence entering a state known as "Walden inversion."
This pivotal change is crucial in chiral molecules where enantiomers can form.

• "Inversion of configuration" means that if you started with a molecule with a known stereochemical orientation, the product would have the opposite orientation after the reaction.
• The stereochemical outcome is predictable and makes use of the stereochemical relationships to determine the stereochemistry of the products formed.

Alkyl Halides

Alkyl halides, also known as haloalkanes, are organic compounds containing carbon, hydrogen, and halogen (such as fluorine, chlorine, bromine, iodine). They serve as pivotal electrophiles in SN2 reactions due to the polarized carbon-halogen bond.
The carbon atom in alkyl halides is electron-poor (electrophilic) because the halogen atom pulls electron density towards itself, making it susceptible to attack by a nucleophile seeking a positive center.

• Primary alkyl halides, with less steric hindrance, are the most reactive in SN2 reactions, followed by secondary. Tertiary alkyl halides rarely undergo SN2 reactions due to steric clashes.
• Effective SN2 reactivity also demands a good leaving group - typically a halide ion, due to its ability to stabilize the negative charge upon departure.
• The nature of the halogen affects the leaving ability, with iodine being the best leaving group compared to fluorine, which is the least efficient.