Question

At one time, a raw material for the production of hexamethylenediamine was the pentose-based polysaccharides of agricultural wastes, such as oat hulls. Treatment of these wastes with sulfuric acid or hydrochloric acid gives furfural. Decarbonylation of furfural over a zinc-chromium-molybdenum catalyst gives furan. Propose reagents and experimental conditions for the conversion of furan to hexamethylenediamine.

Short answer

Answer: To convert furan to hexamethylenediamine, we first hydrogenate the furan ring using H2 as a reducing agent along with a metal catalyst (Pd, Pt, or Raney nickel) under high pressure (10 bar) and at elevated temperature (100-150 °C). Then, aminate the cyclic structure using NH3 in the presence of a dehydrating agent like P4O10 at high temperature (250-350 °C) and under high pressure.

Step-by-step solution

Understanding the structure of furan and hexamethylenediamine

Furan is a five-membered ring heterocyclic compound containing an oxygen atom, and its structure can be represented as follows: Furan: C_4H_4O Hexamethylenediamine, on the other hand, is a linear, aliphatic compound with two amine groups at the terminal positions and is represented as: Hexamethylenediamine: H_2N(CH_2)_6NH_2

Identifying possible reactions for the transformation

The transformation of furan to hexamethylenediamine can be achieved in two main steps: 1. Hydrogenation: Reduction of the aromatic ring to form a cyclic hexane structure. 2. Amination: Conversion of the six carbon cyclic structure to hexamethylenediamine by introducing amine groups at the terminal positions.

Proposing reagents and experimental conditions

For each of the two reactions identified in step 2, we need to propose suitable reagents and experimental conditions. 1. Hydrogenation: Furan can be hydrogenated using a strong reducing agent like H_2 in the presence of a metal catalyst, such as palladium (Pd) or platinum (Pt) supported on carbon, or Raney nickel, under high pressure (e.g. 10 bar) and at elevated temperature (e.g. 100-150 °C). Reaction: C_4H_4O + 2H_2 ⟶ C_4H_8O 2. Amination: The hydrogenation product, tetrahydrofuran (C_4H_8O), can be aminated using a strong Brønsted-Lowry base like ammonia (NH_3) in the presence of a dehydrating agent such as phosphorus pentoxide (P_4O_10). The reaction can occur at high temperature (e.g. 250-350 °C) and under high pressure. Reaction: C_4H_8O + 2NH_3 + 2H_2 ⟶ H_2N(CH_2)_6NH_2 In summary, furan can be converted to hexamethylenediamine by hydrogenating the aromatic ring followed by aminating the obtained cyclic structure, using H_2 (in the presence of a metal catalyst) and NH_3 (in the presence of a dehydrating agent), under high temperature and pressure.

Key concepts

Reactant Transformation

Reactant transformation is a crucial aspect in organic synthesis, where a starting compound, or reactant, is chemically converted into a more complex product. In the context of converting furan to hexamethylenediamine, the process begins with altering the basic structure of furan. Transformations often involve multiple steps to reach the desired end product, utilizing various chemical reactions.

• In the initial step, furan, a heterocyclic compound with a five-membered ring, undergoes a transformation to prepare it for subsequent reactions.
• This involves breaking and forming bonds to yield new functional groups or carbon frameworks.
• The transformation process is guided by the selection of appropriate reagents and conditions planned for specific reactions, such as hydrogenation and amination.

Chemical reactions used, such as those described in the exercise, highlight the importance of carefully choosing conditions that facilitate the desired transformations efficiently and selectively.

Hydrogenation Reactions

Hydrogenation reactions are fundamental in organic chemistry for adding hydrogen across double bonds, usually in unsaturated compounds, effectively "saturating" them. In the proposed exercise, the hydrogenation of furan is a key step to converting it into tetrahydrofuran.

• Achieving hydrogenation often requires a hydrogen gas (H₂) in the presence of a catalyst like palladium on carbon (Pd/C) or Raney nickel.
• The process requires controlled conditions, such as elevated temperatures of 100-150 °C and pressures around 10 bar, to facilitate effective hydrogenation of furan.
• The goal is to saturate the double bonds of the furan ring, transforming it into tetrahydrofuran, which is a more flexible cyclic ether.

This transformation is critical as it sets the stage for further functionalization, specifically the introduction of amine groups during the amination step.

Aminations

Amination is the process of introducing amine groups (NH₂) into a molecule, a vital step in synthesizing many organic compounds, including pharmaceuticals. In the conversion of furan to hexamethylenediamine, amination is crucial for incorporating the amine functionalities necessary for the desired end product.

• Following hydrogenation, tetrahydrofuran can be aminated using ammonia as a source of the amine group, often requiring a dehydrating agent such as phosphorus pentoxide (P₄O₁₀) for effectiveness.
• Reactions are carried out under high temperatures (e.g., 250-350 °C) and pressures to facilitate the formation of the amine groups.
• Accurate control over the reaction conditions ensures high yields and purity of hexamethylenediamine.

Understanding the mechanics of amination helps in designing efficient pathways for synthesizing complex amine-containing compounds.

Heterocyclic Compounds

Heterocyclic compounds like furan form the backbone of many organic chemical reactions due to their unique properties. They contain rings with atoms of two or more elements, typically carbon and another element like oxygen or nitrogen in this case.

• Furan, which starts the organic synthesis process in our exercise, is a five-membered ring containing an oxygen atom, giving it specific aromatic properties.
• The behavior of heterocyclic compounds under various reactions can differ significantly from all-carbon rings because of the electronegativity and hybridization of the heteroatom.
• Knowledge of these differences is crucial for correctly predicting reaction outcomes and designing synthesis routes, such as the conversion route from furan to hexamethylenediamine.

These compounds are highly valuable in synthetic chemistry for constructing more complex molecular architectures, often employed as intermediates in pharmaceutical syntheses.