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Structure of Benzene: Kekulé Model

Structure of Benzene: Kekulé Model

In 1865, August Kekule suggested the structure of benzene according to the structural theory of organic chemistry based on the three assertions—formula of benzene is C6H6, all the hydrogens of benzene are equivalent, and each carbon must have four bonds due to its tetravalency.
He proposed that benzene has a cyclic structure of six carbon atoms attached to one hydrogen atom each, with three alternating pi bonds.

Basicity of Heterocyclic Aromatic Amines

Basicity of Heterocyclic Aromatic Amines

Heterocyclic amines, where the N atom is a part of an alicyclic system, are similar in basicity to alkylamines. Interestingly, the heterocyclic amine having a nitrogen atom as part of an aromatic ring has much less basicity than its corresponding alicyclic counterpart. For this reason, as presented in Figure 1, piperidine (pKb = 2.8) is significantly more basic than pyridine (pKb = 8.8).

Structure of Amines

Structure of Amines

The hybridized nitrogen atom in amines possesses a lone pair of electrons and is bound to three substituents with a bond angle of around 108°, which is less than the tetrahedral angle of 109.5°. However, the C–N–H bond angle is slightly larger at 112°, with a carbon–nitrogen bond length of 147 pm. This carbon–nitrogen bond length of of amines is longer than the carbon–oxygen bond of alcohols (143 pm) but shorter than alkanes’ carbon–carbon bond (154 pm). These aspects are illustrated in Figure...

Electrophilic Aromatic Substitution: Nitration of Benzene

Electrophilic Aromatic Substitution: Nitration of Benzene

The nitration of benzene is an example of an electrophilic aromatic substitution reaction. It involves the formation of a very powerful electrophile, the nitronium ion, which is linear in shape. The reaction occurs through the interaction of two strong acids, sulfuric and nitric acid.

α-Alkylation of Ketones via Enolate Ions

α-Alkylation of Ketones via Enolate Ions

Ketones with α protons are deprotonated by strong bases like lithium diisopropylamide (LDA) to form enolate ions. The anion is stabilized by resonance, and its hybrid structure exhibits negative charges on the carbonyl oxygen and the α carbon. This ambident nucleophile can attack an electrophile via two possible sites: the carbonyl oxygen, known as O-attack, or the α carbon, known as C-attack. The nucleophilic attack via the carbanionic site is preferred. This is due to the strong interaction...

Aldehydes and Ketones with Amines: Imine and Enamine Formation Overview

Aldehydes and Ketones with Amines: Imine and Enamine Formation Overview

Primary amines react with carbonyl compounds—aldehydes and ketones—to generate imines. Imines consist of a C=N double bond and are named Schiff bases after its discoverer—the German chemist Hugo Schiff. On the other hand, secondary amines react with carbonyl compounds to give enamines. In enamines, the presence of a C=C double bond adjacent to the nitrogen atom leads to the delocalization of the lone pair.

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Updated: Jul 17, 2026

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

Published on: August 22, 2018

Amino substituted bisketenes: generation, structure, and reactivity.

Nanyan Fu¹, Annette D Allen, Shinjiro Kobayashi

¹Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.

The Journal of Organic Chemistry

|January 25, 2007

Summary

New diamino-substituted bisketenes were synthesized and studied. The bis(dimethylamino) bisketene exhibits the highest reactivity in ring closure, surpassing previously reported compounds.

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Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions

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Preparation of Stable Bicyclic Aziridinium Ions and Their Ring-Opening for the Synthesis of Azaheterocycles

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Published on: August 22, 2018

Preparation of Enantiopure Non-Activated Aziridines and Synthesis of Biemamide B, D, and epiallo-Isomuscarine

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Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions

Preparation of Contiguous Bisaziridines for Regioselective Ring-Opening Reactions

Published on: July 28, 2022

Area of Science:

Organic Chemistry
Photochemistry
Spectroscopy

Background:

Bisketenes are reactive intermediates crucial in organic synthesis.
Understanding their reactivity is key to controlling chemical transformations.

Purpose of the Study:

To synthesize and characterize novel diamino-substituted bisketenes.
To investigate the kinetics of their ring-closure reactions.
To explore the influence of amino substituents on bisketene reactivity.

Main Methods:

Laser flash photolysis for generating bisketenes from cyclobutenediones.
Time-resolved IR and UV spectroscopy for kinetic measurements.
Density Functional Theory (DFT) calculations for computational analysis.

Main Results:

Hitherto unknown free and tethered diamino-substituted bisketenes were generated.
First-order rate constants for ring closure were determined, showing significant variation.
The bis(dimethylamino) bisketene (14) demonstrated exceptionally high reactivity.
Tethered dialkylamino bisketenes (16) were less reactive than compound 14.
DFT calculations indicated angle strain in tethered precursors inhibits cyclization.

Conclusions:

The study successfully synthesized and characterized novel diamino-substituted bisketenes.
Reactivity in ring closure is highly dependent on the nature and positioning of amino substituents.
The bis(dimethylamino) bisketene represents a new benchmark for reactivity in this class of compounds.