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Related Concept Videos

Electrophilic Aromatic Substitution: Nitration of Benzene01:20

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.
2° Amines to N-Nitrosamines: Reaction with NaNO201:20

2° Amines to N-Nitrosamines: Reaction with NaNO2

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

meta-Directing Deactivators: –NO2, –CN, –CHO, –⁠CO2R, –COR, –CO2H

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 the...
SN1 Reaction: Stereochemistry02:15

SN1 Reaction: Stereochemistry

This lesson provides an in-depth discussion of the stereochemical outcomes in an SN1 reaction.
In the first step of an SN1 reaction, the bond between the electrophilic carbon and the leaving group ionizes to generate the carbocation intermediate. The second step of the mechanism is the nucleophilic attack.
In the formed carbocation, the positively charged carbon is sp2 hybridized with a trigonal planar geometry. As all the three substituents lie on the same plane, a plane of symmetry for the...
SN2 Reaction: Stereochemistry02:23

SN2 Reaction: Stereochemistry

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.
If the substrate is an achiral molecule at the α-carbon, the inversion of configuration is not observed.
SN2 Reaction: Transition State02:26

SN2 Reaction: Transition State

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.
When the nucleophile approaches the electrophilic carbon with its lone pairs, the halide acts as a leaving group and moves away with the electron-pair bonded to the carbon. Dotted partial bonds represent the bonds being formed or broken...

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Related Experiment Video

Updated: Jun 14, 2026

Detection of Nitric Oxide and Superoxide Radical Anion by Electron Paramagnetic Resonance Spectroscopy from Cells using Spin Traps
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Detection of Nitric Oxide and Superoxide Radical Anion by Electron Paramagnetic Resonance Spectroscopy from Cells using Spin Traps

Published on: August 18, 2012

Substrate binding to a nitrite reductase induces a spin transition.

Gabriel Martins1, Luisa Rodrigues, Filipa M Cunha

  • 1Instituto de Tecnologia Química e Biologica, Universidade Nova de Lisboa, Av Republica (EAN), 2780-157 Oeiras, Portugal.

The Journal of Physical Chemistry. B
|April 8, 2010
PubMed
Summary

Nitrite reductase enzyme activity shifts from high-spin to low-spin. This spin transition in the catalytic heme, observed via resonance Raman spectroscopy, favors the heterolytic pathway for nitrite reduction to ammonia.

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Chemiluminescence-based Assays for Detection of Nitric Oxide and its Derivatives from Autoxidation and Nitrosated Compounds

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Detection of Nitric Oxide and Superoxide Radical Anion by Electron Paramagnetic Resonance Spectroscopy from Cells using Spin Traps
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Chemiluminescence-based Assays for Detection of Nitric Oxide and its Derivatives from Autoxidation and Nitrosated Compounds
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Chemiluminescence-based Assays for Detection of Nitric Oxide and its Derivatives from Autoxidation and Nitrosated Compounds

Published on: February 16, 2022

Area of Science:

  • Biochemistry
  • Enzymology
  • Spectroscopy

Background:

  • Nitrite reductase is a multiheme enzyme catalyzing the 6-electron reduction of nitrite to ammonia.
  • Enzyme catalysis involves substrate binding to the heme active site, influencing reaction pathways.

Purpose of the Study:

  • To investigate the spin configuration changes of the catalytic heme upon nitrite binding.
  • To determine the effect of spin state on the N-O bond cleavage mechanism in nitrite reduction.

Main Methods:

  • Resonance Raman spectroscopy was employed to study the enzyme.
  • Experimental evidence was gathered on the catalytic heme of D. vulgaris nitrite reductase.

Main Results:

  • Nitrite binding induces a spin transition from high-spin to low-spin in the catalytic heme.
  • This spin transition favors the heterolytic cleavage of the N-O bond.

Conclusions:

  • The study provides the first experimental evidence for spin transition in nitrite reductase upon substrate binding.
  • The observed spin transition is critical for directing the catalytic cycle towards the heterolytic pathway.