Question

Butanitrile is formed by reaction of \(\mathrm{KCN}\) with (a) butyl chloride (b) propyl chloride (c) butyl alcohol (d) propyl alcohol

Short answer

(a) butyl chloride.

Step-by-step solution

Identify the Reaction Type

The formation of butanitrile from another compound indicates a substitution reaction, where potassium cyanide (KCN) acts as a nucleophile to replace a leaving group in an organic molecule. KCN provides the cyanide ion, CN⁻, which is key in forming the nitrile group (-C≡N).

Analyze Chemical Structures

Butanitrile has the chemical formula C\(_4\)H\(_7\)CN. To form butanitrile, we need to replace a leaving group with the CN⁻ ion. Recognize that leaving groups are typically halogens or other structures that can be easily replaced by better nucleophiles.

Assess Potential Reactants

Consider the reactants given: (a) butyl chloride - C\(_4\)H\(_9\)Cl (b) propyl chloride - C\(_3\)H\(_7\)Cl (c) butyl alcohol - C\(_4\)H\(_9\)OH (d) propyl alcohol - C\(_3\)H\(_7\)OH. Halides are good leaving groups compared to hydroxyl groups, so chlorides are more suitable than alcohols for this substitution.

Match Reactant to Product

Butyl chloride has the molecular formula C\(_4\)H\(_9\)Cl, similar to butanitrile once the chlorine atom is replaced by the CN⁻ ion from KCN. This makes it the best candidate among the given options.

Conclude the Best Choice

The reaction of KCN with butyl chloride will substitute the chlorine using the CN⁻ ion, forming butanitrile. This is a classic nucleophilic substitution reaction (SN2), suitable for primary alkyl halides like butyl chloride.

Key concepts

The Formation of Butanitrile

Butanitrile is an organic compound with the formula C\(_4\)H\(_7\)CN. It is formed through a substitution reaction where a specific atom or group in a molecule is replaced by another atom or group. In this case, the formation of butanitrile involves replacing a leaving group in a suitable precursor molecule with a cyanide ion (CN\(^{-}\)). This process transforms the molecule into butanitrile, which contains a nitrile group (-C≡N). Among the options provided, butyl chloride (C\(_4\)H\(_9\)Cl) acts as a suitable precursor. When it reacts with potassium cyanide (KCN), the chlorine atom in butyl chloride is substituted by the cyanide ion, resulting in the formation of butanitrile.

Potassium Cyanide as a Nucleophile

Potassium cyanide (KCN) plays a critical role in nucleophilic substitution reactions. KCN dissociates in solution to release cyanide ions (CN\(^{-}\)), which act as nucleophiles. A nucleophile is a chemical species that donates an electron pair to form a chemical bond in a reaction. The cyanide ion is a strong nucleophile because of its negative charge and the ability of carbon to form a highly stable triple bond with nitrogen. This makes it highly reactive and efficient in replacing weaker leaving groups like a halogen atom in an organic molecule. In the context of butanitrile formation, KCN uses its nucleophilic property to replace the chlorine in butyl chloride, effectively converting it to butanitrile.

Understanding the SN2 Mechanism

The SN2 mechanism is a type of nucleophilic substitution reaction. The abbreviation stands for "Substitution Nucleophilic Bimolecular." This indicates that two species are involved in the rate-determining step - one is the nucleophile, and the other is the substrate (the organic molecule containing the leaving group). In the SN2 reaction, the nucleophile attacks the carbon atom from the opposite side of the leaving group. This simultaneous action causes an inversion of the carbon's stereochemistry. For the formation of butanitrile, the cyanide ion approaches from the opposite side of the leaving chlorine atom in butyl chloride, successfully replacing it with the nitrile group. SN2 reactions are particularly efficient with primary substrates like butyl chloride, where steric hindrance is minimal, allowing the reaction to proceed smoothly.

Organic Synthesis: The Bigger Picture

Organic synthesis refers to the construction of complex organic molecules through chemical processes. It involves transforming starting materials into target molecules through a series of chemical reactions. In the case of butanitrile formation, the process is a clear example of how simple substitution reactions can be employed to synthesize more complex organic compounds. Using substitution reactions like the SN2 mechanism is common in organic synthesis because they are reliable, predictable, and can be tailored to produce a wide range of products. In industry and research, creating compounds like butanitrile is essential for developing pharmaceuticals, plastics, and many other types of chemicals. Understanding the basics of reactions like SN2 provides a foundation for advancing in organic chemistry and creating innovative solutions in chemical synthesis.