Question

Reaction of iodoethane with CN \(^{-}\) yields a small amount of isonitrile, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{~N} \equiv \mathrm{C}\), along with the nitrile \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{N}\) as the major product. Write electron-dot structures for both products, and propose mechanisms to account for their formation.

Short answer

Both electron-dot structures involve triple bonds, with different arrangements and mechanisms involving S_N2 attack for nitriles and rearrangement for isonitriles.

Step-by-step solution

Draw Electron-Dot Structure for the Nitrile

The major product is the nitrile, ethyl cyanide (CH₃CH₂C≡N). The electron-dot structure shows carbon (C) triple-bonded to nitrogen (N), with nitrogen having a lone pair of electrons. The ethyl group (CH₃CH₂) is single-bonded to the cyanide carbon (C). This is depicted as: \[ \text{CH}_3\text{CH}_2\text{-C}\equiv\text{N} \].

Draw Electron-Dot Structure for the Isonitrile

The minor product is the isonitrile, ethyl isocyanide (CH₃CH₂NC). The electron-dot structure shows carbon (C) triply bonded to nitrogen (N), with the nitrogen furthest from the ethyl group. The carbon (C) closest to the ethyl group has a lone pair. The structure is: \[ \text{CH}_3\text{CH}_2\text{-N}\equiv\text{C} \].

Proposed Mechanism of Formation for the Nitrile

The formation of the major product, ethyl cyanide, occurs via an S_N2 mechanism. The nucleophile CN^(-) attacks the electrophilic carbon in iodoethane (CH₃CH₂I) opposite the iodine, leading to the inversion of configuration. Iodine leaves as a leaving group and the CN^(-) forms a new bond with the carbon to yield CH₃CH₂C≡N.

Proposed Mechanism of Formation for the Isonitrile

The minor product, ethyl isocyanide, forms through an alternative reaction pathway. The CN^(-) attacks the electrophilic carbon from a different angle, resulting in the formation of the N-C bond. This less favorable pathway leads to the formation of CH₃CH₂NC, where the nucleophile's lone pair forms a bond with nitrogen attached to carbon in the isonitrile.

Key concepts

SN2 Reaction

In chemistry, an S_N2 reaction represents a "Substitution Nucleophilic Bimolecular" process. Here, the term bimolecular suggests that the reaction rate depends on the concentration of two reactants. An S_N2 reaction typically involves a nucleophile attacking an electrophilic carbon atom.
When iodoethane ( CH₃CH₂I) reacts with a nucleophile like CN^-, the nucleophile approaches the electrophilic carbon that is bonded to iodine. This carbon is partially positive due to iodine's electronegativity pulling electrons away.

• The CN^- ion attacks the carbon from the direction opposite to the iodine, resulting in a backside attack.
• This leads to the inversion of configuration, much like an umbrella flipping inside-out against the wind.
• As CN^- forms a bond with carbon, the iodine acts as a leaving group, detaching from the molecule.

This exact mechanism is what predominately forms the nitrile product, ethyl cyanide ( CH₃CH₂C≡N), in the reaction. The efficient displacement of the iodine by the CN^- ion characterizes the SN₂ mechanism.

Nitrile Formation

Nitriles are organic compounds characterized by a carbon-nitrogen triple bond. In this reaction, the primary and desirable product is ethyl cyanide ( CH₃CH₂C≡N). This compound is the result of the S_N2 reaction between iodoethane and CN^-.

• As the CN^- ion successfully performs a nucleophilic attack on the iodoethane carbon, the iodine departs, leading to the formation of the nitrile bond.
• This process involves the creation of a new C-CN bond, where the lone pair on the CN^- ion becomes part of the bond with the newly bonded carbon.

The strong triple bond between carbon and nitrogen in the nitrile is stable, making ethyl cyanide the major product. Its formation aligns perfectly with the favorable thermodynamic and kinetic conditions of the SN₂ mechanism. This gives the nitrile priority over the formation of isonitrile products.

Isonitrile Structure

Isonitriles, also known as isocyanides, are unique organic compounds that contain the N≡C functional group. They are isomers of nitriles but differ in structure and properties.

• In the case of ethyl isocyanide ( CH₃CH₂NC), the CN^- nucleophile attacks from a different angle than in the nitrile-forming pathway.
• The difference arises from CN^- forming a bond with nitrogen instead of with carbon.

This reaction pathway is less favorable both kinetically and thermodynamically compared to nitrile formation. As a result, isonitriles form as minor products. The structure of ethyl isocyanide shows the nitrogen bonded to carbon with a triple bond, making nitrogen the connecting atom to the carbon chain. This bonding results in a significant structural variation that influences the properties and reactivity of isonitriles. Despite being minor, understanding their formation underscores the importance of stereochemistry and reaction conditions in organic synthesis.