Question

Reaction of 4-octyne with n-propylamine, \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\),

Short answer

The reaction between 4-octyne and n-propylamine proceeds as follows: the nucleophilic amine donates its lone pair of electrons to the electrophilic carbon of the alkyne, forming an azomethine ylide. Subsequently, a 1,3-H shift and an isomerization take place, leading to the formation of an enamine. The final product of this reaction is N-propylated 5-octenimine: \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{CHCH}_{2} \mathrm{C} ( \mathrm{NH} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} ) = \mathrm{CH} \mathrm{CH}_{2} \mathrm{CH}_{3}\).

Step-by-step solution

Title: Identify the reactants

For this reaction, we have two reactants, 4-octyne and n-propylamine. 4-octyne is an alkyne with the chemical formula \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{C} \equiv \mathrm{C} \mathrm{CH}_{2} \mathrm{CH}_{3}\), and n-propylamine is an amine with the chemical formula \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{NH}_{2}\).

Title: Understand the reactive moieties

In this reaction, the reactive moieties are the alkyne (C≡C triple bond) in 4-octyne, and the amino group (–NH₂) in n-propylamine. The alkyne can act as an electrophile because the triple bond is electron-rich, whereas the amino group can act as a nucleophile since nitrogen has a lone pair of electrons.

Title: Form an iminium ion intermediate

When the nucleophilic amine donates its lone pair of electrons to the electrophilic carbon of the alkyne, a nitrogen-carbon σ bond is formed. This gives rise to an azomethine ylide: $$\mathrm{RCH} \equiv \mathrm{CHR^{'}} + \mathrm{HNR_{2}^{'} \rightarrow \mathrm{RCH} = \mathrm{C(NR_{2}^{'})} \equiv \mathrm{CHR^{'}}$$. Here, R represents the rest of the 4-octyne molecule and R' represents the n-propylamine molecule.

Title: Rearrangement to form the product

A 1,3-H shift and an isomerization take place subsequently. The hydrogen atom from the carbon next to the nitrogen shifts to the carbon attached to the nitrogen, forming a double bond between the nitrogen and carbon. The double bond next to this carbon moves down to form a new double bond, and the positive charge is transferred to the other carbon. The final product is an enamine: $$\mathrm{RCH} = \mathrm{C(NR_{2}^{'})} \equiv \mathrm{CHR^{'}} \rightarrow \mathrm{RCH} = \mathrm{CHR^{'}C(NR_{2}^{'})} = \mathrm{CHR^{''}}$$. Here, R'' represents the remaining part of the n-propylamine molecule.

Title: Write the final product

The final product of the reaction between 4-octyne and n-propylamine is N-propylated 5-octenimine: $$\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH} = \mathrm{CHCH}_{2} \mathrm{C} ( \mathrm{NH} \mathrm{CH}_{2} \mathrm{CH}_{2} \mathrm{CH}_{3} ) = \mathrm{CH} \mathrm{CH}_{2} \mathrm{CH}_{3}$$.

Key concepts

Nucleophilic Addition

Nucleophilic addition is a cornerstone in the study of organic chemistry, forming the basis for synthesizing a plethora of organic compounds. It involves the addition of a nucleophile, which is an electron-rich species, to an electrophilic carbon of a molecule, typically one that's part of a multiple bond, such as a carbon-carbon double or triple bond. In the case of an alkyne, like 4-octyne, the triple bond is highly electron-rich and serves as a site for reaction with nucleophiles, such as n-propylamine.

Nucleophilic addition to alkynes is particularly significant because it can lead to a variety of functional groups. During the reaction, the nucleophile attacks the electrophilic carbon of the alkyne, forming a new sigma bond. This is oftentimes followed by subsequent steps that modify the initial adduct, eventually leading to a diverse range of products including imines, amines, and enamines as seen with the reaction of 4-octyne and n-propylamine.

Imine and Enamine Formation

The formation of imines and enamines is typical in reactions involving amines and carbonyl compounds; however, similar reactions involving alkynes result in related structures. When an alkyne reacts with an amine, an iminium ion intermediary is often part of the process.

During the reaction between 4-octyne and n-propylamine, the initial press of the reaction forms an iminium ion intermediate, which is a type of imine where the nitrogen has a positive charge. In a form of isomerization, the iminium ion transforms into an enamine, which features a double bond adjacent to the nitrogen atom.

Understanding imine and enamine formation is crucial for organic synthesis, as these functional groups often serve as building blocks for more complex molecules. They are also key intermediates in many biosynthetic pathways, highlighting the importance of these reactions in both chemistry and biochemistry.

Organic Reaction Mechanisms

Grasping organic reaction mechanisms is fundamental for mastering organic chemistry. Reactions like the one between 4-octyne and n-propylamine are described through a series of steps that detail how reactants transform into products. The steps include electron flow, formation of intermediates, and the movement of atoms or groups of atoms within the molecules.

The step-by-step solution provided shows the electron-pair donates from the nitrogen to the electrophilic carbon, the rearrangement that follows to stabilize the structure, and finally the transformation into the product. These mechanisms provide a blueprint that helps understand how and why certain products form, and they predict the outcomes of reactions, which is essential for the synthesis of chemical compounds.

By studying and learning various organic mechanisms, aspiring chemists can design novel syntheses and improve existing ones, which has broad implications in pharmaceuticals, materials science, and synthetic biology.

Alkyne Reactivity

Alkynes are among the most reactive organic molecules due to their carbon-carbon triple bonds. This bond is composed of one sigma bond and two pi bonds, making alkynes an easy target for nucleophiles. In the reaction of 4-octyne with n-propylamine, the triple bond's reactivity is the key driver for the nucleophilic addition that initiates the rest of the reaction.

Alkynes do not only partake in nucleophilic addition reactions but are also known for undergoing other types of chemical transformations, including hydrohalogenation, hydration, hydrogenation, and polymerization, to list a few. Their versatility in forming a wide range of chemical products makes them indispensable in the synthesis of complex organic molecules.

Understanding alkyne reactivity equips students with the knowledge to manipulate these compounds in a controlled manner, enabling the synthesis of desired compounds with high precision.