Problem 42 - A Complete Resource Book in Chemistry for JEE Main

Question

KCN react readily to form a cyanide with (a) chlorobenzene (b) ethyl alcohol (c) ethyl bromide (d) bromobenzene

Step-by-Step Solution

Answer

KCN reacts with ethyl bromide to form ethyl cyanide.

Step 1: Understanding the Reaction

KCN, or potassium cyanide, is more likely to participate in nucleophilic substitution reactions. We need to identify which of the given compounds can undergo a SN2 reaction with KCN.

Step 2: Assessing Chlorobenzene

Chlorobenzene contains a C-Cl bond but the carbon is sp2 hybridized and part of an aromatic ring. These factors make chlorobenzene resistant to SN2 reactions because the intermediate carbocation would be unstable.

Step 3: Evaluating Ethyl Alcohol

Ethyl alcohol has a hydroxyl group (-OH), which is a poor leaving group for SN2 reactions. Additionally, alcohols generally do not undergo such substitutions directly without further modification.

Step 4: Examining Ethyl Bromide

Ethyl bromide has a good leaving group (Br-) and a primary carbon, making it highly susceptible to SN2 reactions. KCN will easily displace the bromine, forming ethyl cyanide.

Step 5: Analyzing Bromobenzene

Similar to chlorobenzene, bromobenzene has a C-Br bond where the carbon is part of an aromatic ring. This makes bromobenzene resistant to SN2 substitution due to the same reasons as chlorobenzene.

Step 6: Conclusion

Of the options given, ethyl bromide will react with KCN to form ethyl cyanide because it has suitable conditions for a nucleophilic substitution with KCN.

Key Concepts

SN2 mechanism

In organic chemistry, the SN2 mechanism refers to a bimolecular nucleophilic substitution reaction. It is characterized by a single, concerted step where the nucleophile attacks the electrophile and pushes out the leaving group simultaneously. This type of reaction typically involves:

• Primary carbons or occasionally secondary carbons, as steric hindrance is minimized.
• A strong nucleophile, which readily donates electron pairs to form a new bond.
• A good leaving group, which can efficiently dissociate from the carbon atom.

The term SN2 stands for "Substitution Nucleophilic Bimolecular." This suggests that the concentration of both the nucleophile and the substrate affect the reaction rate. The reaction mechanism is often depicted as the nucleophile approaching the substrate from the opposite side of the leaving group, resulting in an inversion of stereochemistry at the carbon center. In simpler terms, if a compound is like a door with a knob on the left, the substitution results in the knob facing the right. This back-side attack mechanism means it's vital to choose appropriate substrates and leaving groups to ensure the reaction proceeds smoothly. The SN2 reaction is crucial in organic synthesis, particularly for forming carbon-nitrogen and carbon-carbon bonds.

Reaction with potassium cyanide

Potassium cyanide (KCN) is a salt comprised of potassium and cyanide ions. It acts as a powerful nucleophile due to the presence of the cyanide ion (CN⁻). This ion is particularly skilled in nucleophilic substitution reactions like SN2 due to its ability to donate a lone pair of electrons directly to the carbon center of the substrate. Here's why KCN is effective:

• The cyanide ion is a strong nucleophile due to its high charge density and electron-rich nature.
• The nucleophilic attack often results in the formation of a stable carbon-nitrogen bond, common in the production of nitriles.
• This type of reaction is valuable in synthesizing extended carbon chains or introducing functional groups into organic molecules.

When KCN reacts with an organic compound like ethyl bromide, the cyanide ion attacks the electrophilic carbon of the ethyl group, displacing the bromine. This leads to the formation of ethyl cyanide, a common nitrile, in a direct nucleophilic substitution process. Choosing the right conditions and substrates ensures a high yield of the desired product.

Leaving groups in organic chemistry

In nucleophilic substitution reactions, leaving groups play a pivotal role. They are the atoms or groups that detach from the substrate, allowing the reaction to proceed. Ideal leaving groups stabilize the negative charge or lone pair of electrons they acquire after separation. Here are key points about leaving groups:

• Good leaving groups are usually the conjugate bases of strong acids, making them weak bases themselves. This ensures they can effectively leave the molecule.
• Examples include halide ions like Br⁻ and Cl⁻, which are common in organic reactions.
• A poor leaving group can hinder or stall the reaction entirely, as seen with hydroxide ions (OH⁻) in unmodified alcohols.

The capacity of a leaving group to depart smoothly influences the feasibility and rate of the reaction. In the SN2 reaction with ethyl bromide: the bromide ion (Br⁻) is an excellent leaving group. It assists in making the carbon susceptible to nucleophilic attack, allowing the mechanism to occur efficiently. Understanding the nature of leaving groups is crucial for designing effective synthetic pathways in chemistry, determining the speed and success of nucleophilic substitution reactions.