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A topic from the subject of Physical Chemistry in Chemistry.

  • 10 months ago
  • 6 min read
Introduction to Reactivity and and Mechanism in Organic Chemistry

Overview:
Organic reaction mechanism is the step-by-step process by which an initial organic reagent (reactant) is chemically rearranged to yield a final product.
Reactivity of organic molecule is its chemical stability or instability against slight change of its structures or other reaction parameters.
Mechanism Study is an important part of organic reaction research and has important guide to the development of organic synthsis.
This guide provides a detailed explanation of the basic principles, techniques, and applications of reactivity and mechanistic organic chemistry.


Basic Concepts


1. Nucleophiles and electrophiles:
The central concept in organic reaction is the interaction of two functional groups, called a nucleophiles (electron-pair donor) and electrophiles (electron-pair acceptors).
Many nucleophiles are negatively charged species, while electrophiles are positively charged or have a low electron density on a particular atom and are capable of accepting electron from nucleophiles.
2. Reaction rates and equilibria:
Rates of organic reaction are determined by the difference in energy between the starting materials (reactants) and the final product, and the presence of a catayst.
Two kind of organic reaction: A step-wise reaction mechanism has a number of discrete steps between starting material (reactant) and final product and each step has its own transition state. In contrast, a coner-state reaction has a single transition state and one step mechanism.
Organic reaction can reach a state of dynamic equilibrium where the rate of the forward reaction becomes equal to the rate of the reverse reaction.
3. Transition states:
The transition state is a hypothetical, high- energy species that results from the bond-bond making(breaking) change that occurs on the reaction path.
4. Activation energy:
The energy difference between the energy of the transition state and the energy of the reactant.
The rate of a reaction can be increased by lowering the transition state energy with the introduction of a catalyst.


Equipment and Techniques
1. Spectroscopy:
Spectroscopic techiniques, such as IR, Uv-Vis, and mass spectrometry, are used to identify and characterize organic reaction products and characterize starting material by functional group analysis.
2. Chromatography:
Chromatographic techniques, such as gas chromatography (GC) and high-performance liquid chromatography (HPLC), are used to separate and purify organic reaction products and determine their composition.
3. Calorimetry:
Calorimetric techniques, such as titration calorimetry and reaction calorimetry, are used to measure the energy change in organic reaction and determine the mechanism of the reaction.
Calorimetric measurements provide important information on the thermodynamics of the reaction and allow the enthalpy change, and thus the equilibrium constant, to be calculated.
4. Computational methods:
Computational methods, such as density functional theory (DFT) and ab initio methods, are used to model and study organic reaction with the same accuracy as high-level ab initio quantum chemical methods, but orders of magnitudes faster.
The accuracy of DFT methods depends on the choice of the functional (an approximation to the true electron exchange-correlation potential energy), and the quality of the basis set (a set of basis functions used to represent the wave function of the electron).
Types of Experiments
1. Product analysis:
Reaction products are observed by almost any of the spectroscopic instrument mentioned above.
Identification and characterization of product helps to determine the stoichiometry of the reaction, understand the reaction pathway, and design new experimental design to have a better yield of desired product.
2.Kinetic studies:
Kinetics studies are carried out to elucidate mechanism of the reaction, determine the rate-determining step, and measure the rate of the individual reaction step.
The rate of reaction is measured by monitoring the concentration of the reactants or products as a function of time, either by continuous or discontinuous measurements, and monitored by spectroscopy or chromatography. The rates of the reaction steps are determined from the relative product yield of the different steps in the mechanism.
The kinetic isotopic effect is a useful method for elucidating the mechanism of reaction by examining how much the rate of reaction is changed when a non-reactive istope of an element is substituted for a reactive one.
3. Isotope labeling:
Isotope labeling is used to determine the mechanism, the reaction pathway, and the rate-determining step of the reaction by labeling a specific atom with radioisotopes or a stable isotope.
In addition to labeling, isotopes are also used to calculate the reaction yield.
Reactivity and Mechanistic Organic Chemistry
Introduction
Reactivity and mechanistic organic chemistry explores the chemical reactions of organic molecules, focusing on the mechanisms by which these reactions occur.
Key Concepts
Functional Groups:Organic molecules contain functional groups, which are specific atoms or groups of atoms that determine their reactivity. Reaction Mechanisms:
The mechanisms of organic reactions describe the sequence of steps and intermediates involved in a given reaction.
Reactivity:The reactivity of an organic molecule depends on its structure, functional groups, and the surrounding environment. Stereochemistry:
Reactions can occur with different stereochemical outcomes, affecting the spatial arrangement of atoms in the products.
Thermodynamics and Kinetics:Thermodynamics predicts the direction of reactions based on energy changes, while kinetics studies the rates of reactions.Types of Reactions Nucleophilic Reactions:
Involve an electron-rich species attacking an electron-deficient species.
Electrophilic Reactions:Involve an electron-deficient species attacking an electron-rich species. Free Radical Reactions:
Involve species with unpaired electrons that can react by chain reactions.
Pericyclic Reactions:Concerted reactions involving cyclic transition states.ApplicationsReactivity and mechanistic organic chemistry has applications in various fields, including: Drug Design:
Understanding mechanisms helps develop drugs with specific targets and reduced side effects.
Polymer Chemistry:Predicting reaction mechanisms aids in designing polymers with desired properties. Environmental Chemistry:
Understanding reaction mechanisms helps predict the fate of pollutants and develop remediation strategies.
Conclusion
Reactivity and mechanistic organic chemistry is a fundamental aspect of chemistry, providing insights into the chemical behavior of organic molecules and laying the foundation for practical applications in various scientific disciplines.
Experiment: Sn2 Reaction of Benzyl Chloride with Potassium Iodide
Objective:
To demonstrate a nucleophilic substitution reaction (Sn2) and measure the rate of the reaction.
Materials:

  • Benzyl chloride (0.1 M solution)
  • Potassium iodide (0.1 M solution)
  • Ethanol
  • Sodium thiosulfate solution
  • Starch solution
  • Clock

Procedure:

  1. In a 100 mL beaker, combine 25 mL of benzyl chloride solution and 25 mL of potassium iodide solution.
  2. Start the clock.
  3. Swirl the beaker gently to mix the solutions.
  4. At regular intervals (e.g., 30 seconds), remove a drop of the reaction mixture and place it on a white tile.
  5. Add a drop of sodium thiosulfate solution to the tile, followed by a drop of starch solution.
  6. Observe the color change. A blue color indicates the presence of iodine, which is formed when the benzyl chloride reacts with potassium iodide.
  7. Record the time it takes for the blue color to appear.
  8. Repeat steps 4-7 until the blue color appears immediately.

Observations:
The blue color will appear more quickly as the reaction progresses. The time it takes for the blue color to appear will decrease as the concentration of potassium iodide increases.
Data:
Collect data for the time it takes for the blue color to appear at different concentrations of potassium iodide.
Analysis:
The rate of the reaction can be calculated using the following equation:

Rate = 1/(time)

A graph of the rate versus the concentration of potassium iodide will show a linear relationship. This indicates that the reaction is second-order overall, with first-order dependence on both benzyl chloride and potassium iodide.
Significance:
This experiment demonstrates a classic Sn2 reaction. Sn2 reactions are important in organic chemistry because they are one of the most common ways to form carbon-carbon bonds. The experiment also illustrates the effect of concentration on reaction rate.

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