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Related Concept Videos

meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H01:13

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All meta-directing substituents are deactivating groups. These substituents withdraw electrons from the aromatic ring, making the ring less reactive toward electrophilic substitution. For example, the nitration of nitrobenzene is 100,000 times slower than that of benzene because of the deactivating effect of the nitro group. The first step in an electrophilic aromatic substitution is the addition of an electrophile to form a resonance-stabilized carbocation. The energy diagrams for...
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Secondary amines react with nitrous acid to form N-nitrosamines, as depicted in Figure 1. Nitrous acid, a weak and unstable acid, is formed in situ from an aqueous solution of sodium nitrite and strong acids, such as hydrochloric acid or sulfuric acid, in cold conditions. In the presence of an acid, the nitrous acid gets protonated. The subsequent loss of water results in the formation of the electrophile known as nitrosonium ion.
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SN2 Reaction: Kinetics02:14

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Kinetic Studies and Significance
In a chemical reaction, a relationship exists between the concentration of reactants and the rate at which the reaction proceeds. The study to measure this relationship is known as the kinetics of a chemical reaction. Kinetic studies are used to deduce the rate law of a chemical reaction, which provides information about the species involved during the transition state of the rate-determining step. Thus, kinetic studies help to derive the mechanism of a...
10.2K
SN2 Reaction: Mechanism02:27

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17.2K
The kinetic studies of SN2 reactions suggest an essential feature of its mechanism: it is a single-step process without intermediates. Here, both the nucleophile and the substrate participate in the rate-determining step.
The presence of the more electronegative halogen in the substrate creates a polarized carbon-halide bond. The halide pulls the electron cloud generating an electrophilic center at the carbon atom. Thus, the carbon atom carries a partial positive charge while the halide has a...
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An SN2 reaction of an alkyl halide is a single-step process in which bond formation between the nucleophile and the substrate and bond breaking between the substrate and the halide occurs simultaneously through a transition state without forming an intermediate.
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11.7K
SN2 Reaction: Stereochemistry02:23

SN2 Reaction: Stereochemistry

11.6K
In an SN2 reaction, the nucleophilic attack on the substrate and departure of the leaving group occurs simultaneously through a transition state. As the nucleophile approaches the substrate from the back-side, the configuration of the substrate carbon changes from tetrahedral to trigonal bipyramidal and then back to tetrahedral, leading to an inversion in the configuration of the product.
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Quantitative SERS Detection of Uric Acid via Formation of Precise Plasmonic Nanojunctions within Aggregates of Gold Nanoparticles and Cucurbit[n]uril
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A Cucurbit[8]uril 2:2 Complex with a Negative pKa Shift.

Hang Yin1, Qian Cheng1, Roselyne Rosas2

  • 1State Key Laboratory of Quality Research in Chinese Medicine, Institute of Chinese Medical Sciences, University of Macau, Avenida da Universidade, Taipa, Macau, P. R. China.

Chemistry (Weinheim an Der Bergstrasse, Germany)
|July 10, 2019
PubMed
Summary

A novel viologen-phenylene-imidazole (V-P-I) derivative forms a 2:2 complex with cucurbit[8]uril (CB[8]) in water, causing a significant negative shift in its acidity (pKa). This supramolecular complexation triggers proton release, mimicking biological processes and enabling new bioinspired catalysts.

Keywords:
2:2 complexescucurbiturilhost-guest systemsimidazolesviologen

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Area of Science:

  • Supramolecular Chemistry
  • Host-Guest Chemistry
  • Bioinspired Catalysis

Background:

  • Viologen derivatives are known for their redox and photophysical properties.
  • Cucurbiturils (CBs) are macrocyclic hosts that form inclusion complexes with various guests.
  • Typically, CB complexation leads to a positive shift in guest pKa due to the electron-withdrawing nature of the host.

Purpose of the Study:

  • To investigate the supramolecular complexation of a viologen-phenylene-imidazole (V-P-I) derivative with cucurbit[8]uril (CB[8]).
  • To characterize the impact of 2:2 complex formation on the guest molecule's acidity (pKa).
  • To explore the potential of this system for triggering proton release and developing bioinspired catalysts.

Main Methods:

  • Synthesis of a viologen derivative functionalized with a benzimidazole group (V-P-I).
  • Formation of 2:2 supramolecular complexes between V-P-I and cucurbit[8]uril (CB[8]) in aqueous solution.
  • Spectroscopic analysis (NMR) and titration methods (pH-metry) to determine pKa shifts and proton release.
  • Probing guest accessibility using silver cations.

Main Results:

  • The 2:2 complex of V-P-I with CB[8] induced a negative pKa shift of over 1 pH unit, contrary to typical CB complexation.
  • Proton release was observed upon CB[8] addition to V-P-I solutions, confirmed by pH-metry.
  • The system demonstrated host/guest swapping and proton transfer capabilities, suggesting a unique binding interaction.
  • The negative pKa shift is attributed to an optimal charge state and spatial arrangement of guest fragments within the CB[8] cavity.

Conclusions:

  • CB[8] complexation with V-P-I can trigger proton release, a phenomenon with significant implications for molecular recognition.
  • This proton release mechanism, driven by supramolecular assembly, offers a novel approach to bioinspired catalysis.
  • The findings open new avenues for designing artificial enzyme mimics that catalyze proton transfer and other chemical reactions.