Abstract
A prototypical reaction between ammonia and formaldehyde has been investigated at the DFT(M06)/6-311++G(d,p) computational level using the Bonding Evolution Theory (BET). BET is a very useful tool for studying reaction mechanisms as it combines topological analysis of electron localisation function with the catastrophe theory. Each of two studied reactions: H2C=O + NH3 ↔ HO–C(H2)–NH2 (hemiaminal) and HO–C(H2)–NH2 ↔ HN = CH2 (Schiff base) + H2O consists of six steps. Formation of hemiaminal starts from a nucleophillic attack of nitrogen lone pair in NH3 on the carbon atom in H2C=O and is subsequently followed by hydrogen transfer within the N–H..O bridge. A Schiff base is formed via the dehydration reaction of the hemiaminal, where the C–O bond is broken first, followed by hydrogen transfer towards the [HO]δ− moiety, resulting in water and methanimine. The present paper focuses on differences in reaction mechanisms for the processes described above. The results have been compared to the reaction mechanism for stable hemiaminal synthesis from benzaldehyde and 4-amine-4H-1,2,4-triazole studied previously using the BET theory.
Highlights
The reaction of ammonia with formaldehyde was investigated in 1835 by Liebig [1]
The calculations have been performed for isolated molecules in the gas phase at temperature of 0 K, using DFT method
Application of the Bonding Evolution Theory enabled a clearcut description of the reaction mechanism between NH3 and H2CO, a useful prototype reaction for the formation of hemiaminal and Schiff base
Summary
The reaction of ammonia with formaldehyde was investigated in 1835 by Liebig [1]. Among its products were hemiaminals— molecular compounds containing amino and hydroxyl groups, bound to the same carbon. Study of electronic structure, performed by means of topological analysis of electron localisation function (ELF) [6, 7], shows that after the O–H bond formation, two localisation basins, representing non-bonding electron density (formal lone pairs) on the nitrogen, V1(N) and V2(N), are present. It is worth emphasising that for the reaction with triazole, the N–C bond is formed by electron density sharing while the formal mechanism suggests the nucleophillic attack of the amine lone pair on the carbon atom. Scheme 3 Simplified representation of the reaction mechanism between ammonia and formaldehyde on the basis of the BET results the Bonding Evolution Theory [5] as it combines topological analysis of electron localisation function (ELF) [6, 7, 13,14,15,16] with catastrophe theory [17].
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