Question

Explain why in the Ritter reaction with hydrogen cyanide and t-butyl alcohol, the product is t-butylamine and not t-buty1 cyanide (or trimethylacetic acid).

Short answer

The formation of t-butylamine in the Ritter reaction between hydrogen cyanide and t-butyl alcohol is due to the nucleophilic addition of hydrogen cyanide to the carbocation intermediate and the subsequent hydrolysis of the N-alkylated nitrile intermediate. This process does not favor the formation of t-butyl cyanide or trimethylacetic acid, resulting in t-butylamine as the main product.

Step-by-step solution

Understand the Ritter reaction mechanism

The Ritter reaction is a nucleophilic addition reaction that involves the formation of a carbocation from a tertiary alcohol, followed by the addition of a nitrile to generate a secondary amine. The overall mechanism can be described as follows: 1. The tertiary alcohol reacts with a strong acid, such as H2SO4, to form a carbocation. 2. The carbocation reacts with the nitrile to form a new carbon-nitrogen bond, generating an N-alkylated nitrile intermediate. 3. The intermediate undergoes hydrolysis (treatment with water) to yield a secondary amine and an alkylated carboxylic acid. In our case, the reaction between hydrogen cyanide and t-butyl alcohol will follow this mechanism.

Formation of carbocation

The first step of the reaction involves the generation of a carbocation from t-butyl alcohol. This is achieved through protonation of the alcohol by a strong acid, typically H2SO4 or HCl. The protonated alcohol then loses a water molecule to form a carbocation: \[t-BuOH + H^+ \rightarrow t-BuOH_2^+ \] \[t-BuOH_2^+ \rightarrow t-Bu^+ + H_2O \]

Nucleophilic attack by hydrogen cyanide

Next, the carbocation reacts with hydrogen cyanide, which acts as a nucleophile due to the presence of a highly polarized carbon-nitrogen triple bond. The nucleophilic attack occurs at the carbon of the carbocation, forming a new carbon-nitrogen bond in the process: \[t-Bu^+ + HCN \rightarrow t-BuCNH_2^+ \]

Hydrolysis

Finally, the N-alkylated nitrile intermediate undergoes hydrolysis. This step takes place under aqueous conditions, where the nitrile group is attacked by a water molecule, resulting in the formation of a secondary amine (t-butylamine) and a carboxylic acid (acetic acid): \[t-BuCNH_2^+ + H_2O \rightarrow t-BuNH_2 + CH_3COOH \]

Explaining why t-butylamine forms instead of t-butyl cyanide or trimethylacetic acid

The Ritter reaction forms t-butylamine and acetic acid instead of other products because of the carbocation intermediate formed from t-butyl alcohol and the nucleophilic addition of hydrogen cyanide. T-butyl cyanide or trimethylacetic acid would not form under these conditions, as the reaction specifically directs the formation of a secondary amine due to the nucleophilic attack and hydrolysis steps in the mechanism. In conclusion, the formation of t-butylamine in the Ritter reaction between hydrogen cyanide and t-butyl alcohol is due to the nucleophilic addition of hydrogen cyanide to the carbocation intermediate and the subsequent hydrolysis of the N-alkylated nitrile intermediate. This process does not favor the formation of t-butyl cyanide or trimethylacetic acid, resulting in t-butylamine as the main product.

Key concepts

Carbocation Formation

The Ritter reaction begins with carbocation formation, and understanding this step is crucial to understanding the whole process. When t-butyl alcohol is subjected to a strong acid such as sulfuric acid (H\(_2\)SO\(_4\)), it undergoes protonation. This protonation makes the alcohol group a better leaving group, eventually leading to the loss of a water molecule. As water leaves, it generates a highly unstable carbocation:

• t-butyl alcohol + H⁺ → t-butyl oxonium ion (t-BuOH₂⁺)
• t-BuOH₂⁺ → t-Bu⁺ + H₂O

Carbocations are reactive intermediates that prefer to stabilize quickly. The t-butyl carbocation is tertiary, making it relatively more stable due to hyperconjugation and inductive effects. However, its inherent instability drives the reaction forward as it eagerly seeks a stabilizing partner.

Nucleophilic Addition

At this point in the Ritter reaction, the carbocation is primed for nucleophilic addition. A crucial feature of hydrogen cyanide (HCN) is its polarized C-N triple bond, which allows it to act as a nucleophile. As such, when introduced to the reaction environment, HCN attacks the positively charged carbon atom of the t-butyl carbocation. This attack forms a new carbon-nitrogen bond:

• t-Bu⁺ + HCN → t-butyl N-cyanide (t-BuCNH₂⁺)

In essence, the nucleophilic addition is driven by the carbocation's instability and HCN’s strong nucleophilic character. This step arranges the crucial framework of the molecule that will eventually lead to the formation of t-butylamine.

Hydrolysis

Following the nucleophilic addition, the newly formed N-alkylated nitrile intermediate must undergo hydrolysis to complete the transformation into a secondary amine. Hydrolysis is the reaction of the compound with water and is facilitated under acidic aqueous conditions. Here, water molecules attack the nitrile group, breaking the triple bond and replacing it with equivalent bonds to form a carboxylic acid and a secondary amine:

• t-BuCNH₂⁺ + H₂O → t-butylamine (t-BuNH₂) + acetic acid (CH₃COOH)

This part of the reaction successfully converts the N-cyanide structure into a stable t-butylamine, aided by the additional water molecules in the reaction, concluding the Ritter reaction's mechanism.

Secondary Amine Formation

Finally, we explore why the Ritter reaction ends in the formation of a secondary amine like t-butylamine, rather than remaining as an N-cyanide or transforming fully into a carboxylic acid. The specific pathway of the Ritter reaction is governed by the intermediate structures and their propensity towards stabilization through predictable mechanisms.

• The initial carbocation facilitates a high-energy landscape, seeking stable resolution through nucleophilic addition.
• Upon nucleophilic attack by HCN, the formation of a secondary amine becomes favored during subsequent hydrolysis.

The secondary amine formation is an aspect of the final step, where the reaction's conditions specifically support hydrolysis of the intermediate, directing the breakdown of the bound nitrogen in the nitrile group and forming an amine instead of continuing towards a complete carboxylic acid transformation. This neatly explains why we observe t-butylamine as the end product.