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 prod- uct. Write electron-dot structures for both products, and propose mechanisms to account for their formation.

Short answer

Ethyl cyanide major via S\(N_2\); ethyl isocyanide minor via N-attack of CN\(^{-}\).

Step-by-step solution

Analyze the Reactants and Products

Identify the reactants and desired products. Iodoethane (C2H5I) is the starting material. The products are ethyl isocyanide (CH3CH2NC) and ethyl cyanide (CH3CH2CN).

Draw Electron-Dot Structure for Ethyl Cyanide

For ethyl cyanide (CH3CH2CN), the structure involves a carbon triple-bonded to nitrogen: - CH3: Three H atoms bonded to C with a single bond. - CH2: Two H atoms bonded to C, which is then single-bonded to CN. - CN: C atom triple-bonded to N atom, with N having a lone pair of electrons.

Draw Electron-Dot Structure for Ethyl Isocyanide

For ethyl isocyanide (CH3CH2NC), the structure involves: - CH3: Three H atoms bonded to C with a single bond. - CH2: Two H atoms bonded to C, which is then single-bonded to NC. - NC: N atom triple-bonded to C, with N having a lone pair of electrons.

Mechanism for Ethyl Cyanide Formation

The cyanide ion ( CN^{-} ) acts as a nucleophile, attacking the carbon bonded to the iodine in iodoethane (C2H5I), displacing the iodide ion. This is an S_N2 reaction leading directly to the cyanide product: -EtI + CN^{-} → Et-CN + I^{-}

Mechanism for Ethyl Isocyanide Formation

The formation of ethyl isocyanide can involve nitrogen acting as the nucleophile. This minor pathway involves the nitrogen in the CN^{-} ion attacking the carbon in iodoethane, forming the isocyanide: -EtI + CN^{-} (nitrogen) → Et-NC + I^{-}

Key concepts

Electron-dot structures

Electron-dot structures, also known as Lewis structures, are a way to visualize the valence electrons in atoms and molecules. They help us understand how atoms bond and the shape of the molecule.

For ethyl cyanide CH₃CH₂CN, the structure includes:

• CH₃ - A carbon atom single-bonded to three hydrogen atoms.
• CH₂ - A central carbon single-bonded to two hydrogen atoms and linked to the CN group.
• CN - A carbon atom triple-bonded to a nitrogen atom, with the nitrogen holding a lone pair of electrons.

On the other hand, for ethyl isocyanide CH₃CH₂NC, the electron-dot structure includes:

• CH₃ - A carbon single-bonded to hydrogen atoms.
• CH₂ - Similar to ethyl cyanide, with a link to the NC group.
• NC - Here, the nitrogen atom is triple-bonded to a carbon, carrying a lone pair of electrons.

These structures are crucial for predicting chemical behavior.

SN2 reaction

In nucleophilic substitution reactions, the SN2 mechanism is a common pathway. It involves a single step where the nucleophile attacks the substrate, leading to the displacement of the leaving group.

The reaction of iodoethane with cyanide ion (CN^{-}) follows an SN2 mechanism to produce ethyl cyanide. During this reaction:

• The cyanide ion, acting as a nucleophile, attacks the carbon atom that is bonded to the iodine in iodoethane.
• This attack leads to the simultaneous displacement of the iodide ion.
• The entire process occurs in a concerted fashion, meaning the bond-making and bond-breaking occur at the same time.

This reaction typically results in the inversion of configuration at the carbon center, a hallmark of SN2 reactions.

Nitriles

Nitriles are organic compounds that contain a carbon-nitrogen triple bond (C≡N). They are well-known for their strong, stable bonding.

In the reaction between iodoethane and cyanide ion, ethyl cyanide (CH₃CH₂CN) is the major product.

• In ethyl cyanide, the C≡N bond is linear, contributing to the rigidity and stability of the molecule.
• This strong bond influences the boiling point and solubility of the nitrile compound.
• Nitriles are versatile in organic synthesis, often used as intermediates to produce other functional groups.

Understanding nitriles is essential for grasping many complex chemical reactions.

Isocyanides

Isocyanides, while somewhat similar to nitriles, have their unique properties and structural characteristics. They feature a nitrogen-carbon triple bond (N≡C).

In the given reaction, ethyl isocyanide (CH₃CH₂NC) is formed in smaller amounts.

• Here, the N≡C group is linear, similar to the C≡N group in nitriles.
• However, isocyanides are less stable and often have a strong, distinctive odor.
• They are less commonly encountered in reactions due to their propensity to rearrange or react further.

Despite being less prevalent, understanding isocyanides is important for certain advanced chemical processes.