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Reaction Mechanisms

Reaction Mechanisms

Chemical reactions often occur in a stepwise fashion, involving two or more distinct reactions taking place in a sequence. A balanced equation indicates the reacting species and the product species, but it reveals no details about how the reaction occurs at the molecular level. The reaction mechanism (or reaction path) provides details regarding the precise, step-by-step process by which a reaction occurs.
For instance, the decomposition of ozone appears to follow a mechanism with two steps:

SN2 Reaction: Mechanism

S_N2 Reaction: Mechanism

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...

SN1 Reaction: Mechanism

S_N1 Reaction: Mechanism

Kinetic studies of ionization of a tertiary halide in a protic solvent suggest that only the substrate participates in the rate-determining step (slow step). The nucleophile is involved only after the slowest step. The SN1 reaction takes place in a multiple-step mechanism.
Firstly, the haloalkane ionizes to generate a carbocation intermediate and a halide ion. This heterolytic cleavage is highly endothermic with large activation energy. The ionization of the substrate, facilitated by a...

E2 Reaction: Kinetics and Mechanism

E2 Reaction: Kinetics and Mechanism

SN2 substitutions and E2 eliminations of alkyl halides proceed via a concerted pathway. While the nucleophile attacks the alpha carbon in SN2 reactions, it functions as a strong base and abstracts a beta hydrogen in the E2 mechanism. The rate-limiting transition state in E2 elimination reactions is characterized by partially broken carbon–hydrogen and carbon–halogen bonds and a partially formed pi bond between the alpha and beta carbons. The beta hydrogen and halide are eliminated...

E1 Reaction: Kinetics and Mechanism

E1 Reaction: Kinetics and Mechanism

Here, in contrast to the E2 reaction mechanism, we delve into the aspects of the E1 reaction mechanism, which has two steps: rate-limiting loss of the leaving group and abstraction of the beta hydrogen by a weak base. Typically, the experimental proof for the E1 mechanism is via kinetic studies or isotope studies. While the former demonstrates the first-order kinetics—the dependence of the reaction solely on substrate concentration—the latter proves the abstraction of hydrogen only...

Automatic Processing and Automatic Social Behavior

Automatic Processing and Automatic Social Behavior

Automatic processing refers to the cognitive operations that occur without conscious intent or awareness, playing a fundamental role in shaping social cognition and behavior. These processes enable individuals to navigate complex social environments efficiently by relying on mental shortcuts and pre-existing knowledge structures known as schemas. One of the most influential mechanisms underlying automatic processing is priming, which subtly activates mental representations through exposure to...

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Reconstituting and Characterizing Actin-Microtubule Composites with Tunable Motor-Driven Dynamics and Mechanics

Reconstituting and Characterizing Actin-Microtubule Composites with Tunable Motor-Driven Dynamics and Mechanics

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Automatic Proposal of Multistep Reaction Mechanisms using a Graph-Driven Search.

Idil Ismail¹, Holly B V A Stuttaford-Fowler¹, Curtis Ochan Ashok¹

¹Department of Chemistry and Centre for Scientific Computing , University of Warwick , Coventry CV4 7AL , United Kingdom.

The Journal of Physical Chemistry. A

|March 23, 2019

Summary

This study introduces a novel random-walk algorithm for automated chemical reaction mechanism generation. It bypasses the need for prior chemical knowledge, enabling broader mechanistic exploration in computational chemistry.

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Light-driven Enzymatic Decarboxylation

Light-driven Enzymatic Decarboxylation

Published on: May 22, 2016

Area of Science:

Computational Chemistry
Chemical Reaction Mechanisms
Catalysis

Background:

Computational chemistry often relies on user-defined reaction mechanisms based on existing knowledge.
This reliance on prior expertise limits automated mechanistic analysis and exploration of novel reaction pathways.

Purpose of the Study:

To develop an automated algorithm for generating elementary chemical reaction steps.
To overcome the limitations of user-defined mechanisms in computational chemistry.
To enable exploration beyond standard reaction pathways.

Main Methods:

A random-walk algorithm operating in the space of molecular connectivity matrices.
Identification of chemically sensible bonding changes between reactant and product structures.
Generation of atomic coordinates using a graph-restraining potential for quantum chemical calculations.

Main Results:

Successfully demonstrated the algorithm for carbon monoxide oxidation, the water-gas shift reaction, and n-hexane aromatization.
Validated the approach for reactions catalyzed by Platinum (Pt) nanoparticles.
Showcased the algorithm's ability to propose reaction pathways without prior chemical knowledge.

Conclusions:

The proposed random-walk algorithm offers a powerful tool for automated mechanistic hypothesis generation.
This method expands the scope of computational mechanistic studies by removing the need for pre-existing chemical intuition.
The approach facilitates the discovery of novel reaction mechanisms in catalysis.