Question

Write the mechanisms for the acidic and basic hydrolyses of benzonitrile.

Short answer

In acidic hydrolysis of benzonitrile, we observe the formation of an imine intermediate by protonation of the nitrile group and nucleophilic attack of water, followed by imine hydrolysis, producing benzoic acid: \( C_6H_5C \equiv N + H_2O \xrightarrow[\ce{H+}]{=} C_6H_5C(NH)(OH) \) \( C_6H_5C(NH)(OH) + H_2O \xrightarrow[\ce{H+}]{=} C_6H_5C(O)OH + NH_4^+ \) In basic hydrolysis of benzonitrile, a tetrahedral intermediate forms by nucleophilic attack of water, then undergoes proton transfer to produce a carboxylate ion that can optionally form benzoic acid: \( C_6H_5C \equiv N + H_2O \xrightarrow[\ce{OH-}]{=} C_6H_5C(N^--)(OH) \) \( C_6H_5C(N^--)(OH) + OH^- \xrightarrow[]{=} C_6H_5C(O)^{-} + N^-- + H_2O \) \( C_6H_5C(O)^{-} + H^+ \xrightarrow[]{=} C_6H_5C(O)OH \)

Step-by-step solution

Acidic Hydrolysis of Benzonitrile

Acidic hydrolysis of benzonitrile involves the following mechanisms: 1) Formation of imine intermediate Under acidic conditions, benzonitrile reacts with water, and the nitrile group gets protonated, behaving as a nucleophile attacking the oxygen of water. This results in the formation of an intermediate imine structure. \[ C_6H_5C \equiv N + H_2O \xrightarrow[\ce{H+}]{=} C_6H_5C(NH)(OH) \] 2) Imine hydrolysis to form a carboxylic acid The imine intermediate reacts with another water molecule, resulting in hydrolysis. Proton transfer reactions occur to form a stable carboxylic acid group, namely benzoic acid. \[ C_6H_5C(NH)(OH) + H_2O \xrightarrow[\ce{H+}]{=} C_6H_5C(O)OH + NH_4^+ \]

Basic Hydrolysis of Benzonitrile

Basic hydrolysis of benzonitrile involves different mechanisms, as follows: 1) Formation of a tetrahedral intermediate Under basic conditions, the water molecule acts as a nucleophile to attack the carbon of the nitrile group, leading to the formation of a tetrahedral intermediate. \[ C_6H_5C \equiv N + H_2O \xrightarrow[\ce{OH-}]{=} C_6H_5C(N^--)(OH) \] 2) Proton transfer, forming a carboxylate ion The amide ion abstracts a proton from the hydroxyl group, resulting in the formation of a carboxylate ion. \[ C_6H_5C(N^--)(OH) + OH^- \xrightarrow[]{=} C_6H_5C(O)^{-} + N^-- + H_2O \] 3) Formation of the carboxylic acid (optional) The carboxylate ion may then react with an acidic solution, like hydrochloric acid (HCl), to form benzoic acid. This step is optional, as the carboxylate ion itself could be the final product in certain cases. \[ C_6H_5C(O)^{-} + H^+ \xrightarrow[]{=} C_6H_5C(O)OH \] In summary, both acidic and basic hydrolyses of benzonitrile lead to the formation of benzoic acid or benzoate ion, but through different intermediate steps and mechanisms.

Key concepts

Understanding Acidic Hydrolysis

Acidic hydrolysis is the process of breaking down a compound in the presence of an acid. When benzonitrile undergoes acidic hydrolysis, a sequence of chemical transformations occurs. Initially, the nitrile group in benzonitrile gets protonated by the acid. This means a hydrogen ion is added to the nitrogen atom, making it more reactive and ready for the next chemical step. During this stage, water acts as a nucleophile, a molecule that seeks out positive charges, and attacks the nitrile group. Through this interaction, an imine intermediate is formed. The imine intermediate is a crucial transient structure that facilitates further reactions. With additional water molecules and continuous assistance from your trusty hydrogen ions, this intermediate then hydrolyzes, or reacts with water, to eventually form benzoic acid.

How Basic Hydrolysis Works

In basic hydrolysis, a base, such as OH⁻ ions, is used instead of an acid to break down the compound. The basic hydrolysis of benzonitrile starts with the OH⁻ ion acting as a nucleophile. Instead of attacking the nitrogen, OH⁻ targets the carbon atom of the nitrile group, leading to the formation of a negatively charged tetrahedral intermediate. This specific intermediate is like a temporary holding zone where the next reactions occur. A crucial step in the process is the transfer of a proton, which helps to foster the move towards forming a stable product. In this setup, the amide ion is responsible for engaging with the surrounding environment to find a proton to transfer. Finally, the evolved product in basic hydrolysis is often a carboxylate ion, but further steps might convert it into a carboxylic acid if acidic conditions are later applied.

The Role of Nucleophilic Attack

A nucleophilic attack is a fundamental part of both acidic and basic hydrolyses. It is the initial action where a nucleophile, a chemical entity rich in electrons, seeks out positively charged areas to bond with. In the context of benzonitrile hydrolysis, water acts as the nucleophile in the acidic reaction, while hydroxide ions (OH⁻) fulfill this role under basic conditions. - **In Acidic Hydrolysis:** - Water is the nucleophile. - It attacks the protonated nitrile, forming an imine intermediate. - **In Basic Hydrolysis:** - OH⁻ ions are the nucleophiles. - They target the nitrile carbon, aiding in creating a tetrahedral intermediate. This nucleophilic action is what drives the transformation from a nitrile to either a carboxylic acid or a carboxylate ion.

Decoding the Imine Intermediate

In the realm of chemistry, an imine intermediate is a fleeting molecule that emerges during certain reactions, such as the acidic hydrolysis of benzonitrile. Imine intermediates consist of a carbon-nitrogen double bond and are often pivotal in converting one compound into another. During the acidic hydrolysis process, the momentary formation of an imine intermediate signifies a critical halfway point. At this stage, the molecule has transformed just enough to be open to further reaction and progress. Interestingly, imine intermediates are not meant to last; they quickly undergo changes, with water adding and rearranging parts until what started as a benzonitrile turns into a stable entity like a benzoic acid.

Formation and Importance of Carboxylate Ion

A carboxylate ion arises prominently in basic hydrolysis but plays a significant role overall when discussing benzonitrile transformations. The carboxylate ion is essentially a de-protonated form of a carboxylic acid, characterized by a negatively charged oxygen atom attached to a carbon atom. In the basic hydrolysis of benzonitrile, after the nucleophilic attack by OH⁻, the ensuing reactions lead to the creation of this carboxylate ion. It stands as a potential final product unless conditions change: - For instance, if the environment becomes acidic, the carboxylate ion will readily gain a proton to convert into a carboxylic acid. - This dual identity means it can end the reaction or transform further based on external changes. Therefore, understanding carboxylate ions broadens one's knowledge of compound transformations in both acidic and basic environments.