ग्रेड 11 → कार्बनिक रसायन विज्ञान - कुछ मूलभूत सिद्धांत और तकनीकें ↓
Reaction mechanism and intermediate species
Introduction to reaction mechanisms
In organic chemistry, it is important to understand how reactions occur. This involves studying reaction mechanisms. The reaction mechanism is a step-by-step description of the process by which reactants are converted into products, providing details about the breaking and formation of bonds and changes in atomic valency.
Example: Consider the reaction between ethene and bromine to form 1,2-dibromoethane:
C2H4 + Br2 → C2H4Br2This mechanism involves several steps, such as the formation of a bromonium ion intermediate and its subsequent reaction with the bromide ion.
Role of intermediate species
Intermediate species are transient species within a reaction mechanism that are not present at the beginning or end of the reaction but are important in the transformation process. They often participate in the initial stages as reactive entities that quickly convert into more stable species.
Example: In the same example as ethene and bromine:
An intermediate, the bromonium ion, is formed:
C2H4 + Br2 → C2H4Br+This intermediate reacts with the bromide ion (Br-) to give the final product:
C2H4Br+ + Br- → C2H4Br2Steps in the reaction mechanism
Understanding a mechanism involves identifying the individual steps a reaction goes through. Each step within the mechanism is typically marked by a distinct series of bond-breaking and bond-forming events.
Early stages
An elementary step is a simple reaction that describes one of these steps. It involves molecular processes that cannot be broken down further. Elementary steps are simple and usually involve only a few molecules.
Each step in a mechanism can be classified as monomolecular, bimolecular, or intermolecular based on the molecularity, which refers to the number of reactant species involved in the reaction step.
Types of preparatory stages
- Unimolecular: It involves a single molecule that undergoes transformation.
- Bimolecular: In this there is collision and reaction between two molecules.
- Thermomolecular: Rare because of the low probability that three molecules collide and react.
Determination of the reaction mechanism
Determining the mechanism of a chemical reaction involves a combination of experimental evidence and theoretical insight. Clues from a variety of sources can help chemists propose plausible mechanisms.
Rate laws and mechanisms
The rate law for a reaction, which relates the rate of the reaction to the concentrations of the reactants, can provide insight into the mechanism. The rate law is derived based on the slowest step in the mechanism, often called the rate-determining step or rate-limiting step.
Example: For a reaction where the velocity law is:
Rate = k[A][B]This reveals a bimolecular interaction between species A and B in the rate determining step.
Evidence from intermediate investigation
The detection of intermediate elements could strongly support the proposed mechanism. Techniques such as spectrometry, magnetic resonance or even trapping experiments could provide evidence of these transient species.
Illustrative system example: SN1 reaction
The SN1 (unimolecular nucleophilic substitution) mechanism is a classic example in organic chemistry that illustrates the concepts of reaction mechanisms and intermediates.
SN1 mechanism phase
- Formation of carbocation: The reaction begins with the dissociation of the leaving group, forming a carbocation intermediate.
- Nucleophile attack: The nucleophile attacks the carbocation, forming the final substitution product.
The SN1 mechanism is characterized by the formation of a carbocation intermediate, which plays an important role in determining the rate and stereochemistry of the reaction.
Visualization example:
R3C—X → R3C+ + X- R3C+ + Nucleophile → R3C—NucleophileHere, X is the leaving group, and the nucleophile can be any molecule that is capable of donating an electron pair.
Illustrative mechanism example: E2 reaction
E2 (binomolecular elimination) mechanism is another example, in which a hydrogen and a replacing group are removed from adjacent carbon atoms to form a double bond.
E2 mechanism stage
- Proton abstraction: The base removes a proton adjacent to the carbon with the leaving group.
- Leaving group departure: The leaving group departs simultaneously as the double bond is formed.
Visualization example:
Base + RCH2—CH2X → RCH=CH2 + BaseH + X-This coordinated mechanism results in the formation of alkanes.
Types of reaction intermediates
Understanding intermediates is important in organic chemistry, as they often determine the pathway a reaction will take to completion.
Carbocations
Carbocations are positively charged carbon species that are often sp2 hybridized and planar. They are common intermediates in mechanisms such as SN1 and E1 reactions.
Example of carbocation:
CH3—CH+—CH3It is a secondary carbocation, known for its relative stability and ability to rearrange.
Radicals
Radicals are species that have an unpaired electron, which makes them highly reactive. Radicals are key intermediates in many halogenation reactions.
Example of radical:
•CH3The methyl radical is often involved in radical chain reactions.
Carbanions
Carbanions are negatively charged species on the carbon atom. They act as strong nucleophiles and bases in many organic reactions.
Example of carbanion:
CH3C-H2Carbanions can undergo a variety of nucleophilic substitution reactions.
Visualization of the system
When studying mechanisms, visual tools and formulas help to gain a deeper understanding of the transformation process. Representing the motion of the electron using curved arrows helps to understand the mechanism at a glance.
Curved arrow explanation:
In reaction mechanisms, curved arrows show the movement of electron pairs. They start at the source of the electrons, such as a lone pair or a bond, and point toward the atom or bond where the electrons are moving.
टिप्पणियाँ