Common Types of Reaction Intermediates in Chemistry
A chemical reaction is a process that results in the chemical transformation of one set of chemical substances into another set of chemical substances. Chemical reactions are typically defined as changes in the positions of electrons in the formation and breaking of chemical bonds between atoms, with no change in the nuclei (no change in the elements present), and can be explained using a chemical equation.
This article will study intermediate chemistry, reaction intermediate and examples of inter chemistry.
What is a Reaction Intermediate?
Any chemical substance formed during the conversion of a reactant to a product is referred to as a chemical intermediate. Most synthetic processes entail a series of steps that turn a readily available and often inexpensive material into a desired product. Intermediates are all the substances generated by one step and used in a subsequent step.
Aside from substances that can be recovered as products if the reaction is stopped at the point where the intermediate, unstable molecules are produced, some chemical substances are known or suspected to be intermediates, even if they have not yet been isolated. Free radicals, carbenes, carbonium ions, and carbanions are some of the more well-studied classes of theoretically unstable intermediates. These intermediates are highly reactive fragments of molecules that are normally uncombined for just a few seconds.
For example, consider this hypothetical stepwise reaction:
A + B → C + D
The reaction includes these elementary steps:
A + B → X*
X* → C + D
The chemical species X* is an intermediate.
Intermediate Compound
An intermediate, according to the IUPAC Gold Book, is a molecular entity (atom, ion, molecule, etc.) that is formed (directly or indirectly) from the reactants and reacts further to give (directly or indirectly) the products of a chemical reaction. The lifetime condition distinguishes actual, chemically distinct intermediates from vibrational states or transition states with lifetimes similar to those of molecular vibration, and thus intermediates correspond to potential energy minima of depth greater than available thermal energy arising from temperature (RT, where R is gas constant and T is temperature).
Since many intermediates have a short half-life and are highly reactive, their concentration in the reaction mixture is low. Definitions like fast/slow, short/long-lived are subjective, and rely on the relative rates of all the reactions involved, as is often the case when discussing chemical kinetics. Species that are unstable in one reaction mechanism may be stable in another, and molecular entities that are intermediates in one reaction mechanism may be stable enough to be detected, classified, isolated, or used as reactants in (or products of) other reactions. Free radicals or unstable ions are often used as reaction intermediates. Since oxidising radicals (OOH and OH) are so reactive in combustion reactions, they must be generated at a high temperature to compensate for their absence, or the combustion reaction will stop.
When the reaction's necessary conditions are no longer met, the intermediates react further and are no longer present in the reaction mixture. In certain processes, several reactions are carried out in the same batch. In the esterification of a diol, for example, a monoester product is formed first, which can be isolated, but the same reactants and conditions facilitate the monoester's conversion to a diester. The lifespan of such a "intermediate" is significantly reduced.
Types of Reaction Intermediate in Organic Chemistry
Given are the Examples of Intermediate Chemistry
Carbanion
A carbanion (also called a carbonium ion in some texts) is a reaction intermediate in organic chemistry that has a negative one charge on a carbon atom. Carbanions are formed when an organic compound is treated with a very strong base. Consider the reaction of butane with a base as an example. A carbanion is formed when the base removes a hydrogen atom from butane.
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Carbanions are highly reactive, and they don't survive long after they've been formed in a chemical reaction. They usually go on to react with a positive species in the reaction to form the reaction's final product. This makes sense because we're forming a negatively charged intermediate, which means it'll be drawn to something with a positive charge.
Free Radical
Free radicals are another common form of reaction intermediate. A single unpaired electron exists in free radicals. When a covalent bond (a bond made up of two electrons) is broken, each atom takes one of the bond's electrons. If a carbon-hydrogen bond in methane is broken, for example, one of the bond's electrons goes to carbon and the other to hydrogen. Notice how we use single dots on the atom where the radical is positioned to reflect free radicals.
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Did You Know?
There are no intermediate products in the elementary reaction, which is the smallest division into which a chemical reaction can be decomposed. The majority of experimentally observed reactions are made up of a series of elementary reactions that happen in a parallel or sequential order. The reaction mechanism is the actual sequence of the individual elementary reactions. Because of the low probability of many molecules meeting at the same time, an elementary reaction only requires a few molecules, typically one or two.
Unimolecular and bimolecular reactions are the most important elementary reactions. In a unimolecular reaction, only one molecule is involved; it is converted into one or more other molecules through isomerization or dissociation. The addition of energy in the form of heat or light is needed for such reactions.
FAQs on Reaction Intermediate: Meaning, Types & Role in Reactions
1. What is a reaction intermediate in chemistry? Explain with a simple example.
A reaction intermediate is a short-lived, highly reactive molecule that is formed in one step of a chemical reaction and consumed in a subsequent step. It does not appear in the overall balanced equation for the reaction. For instance, in the chlorination of methane, the methyl free radical (•CH₃) is a key intermediate formed and then used up to create the final product.
2. What are the main types of reaction intermediates covered in the Class 11 Chemistry syllabus?
In accordance with the CBSE syllabus for Class 11, the primary carbon-based reaction intermediates you need to know are:
- Carbocations: These are species containing a carbon atom with a positive charge and only six valence electrons (e.g., methyl cation, CH₃⁺).
- Carbanions: These are species containing a carbon atom with a negative charge and a lone pair of electrons (e.g., methyl anion, CH₃⁻).
- Free Radicals: These are neutral atoms or groups having an unpaired electron, making them highly reactive (e.g., methyl radical, •CH₃).
3. What is the fundamental difference between a reaction intermediate and a transition state?
The core difference is that a reaction intermediate is a real, albeit unstable, molecule that exists for a finite lifetime in a potential energy well on a reaction diagram. It can potentially be detected. A transition state, however, is a theoretical, maximum-energy point on the reaction pathway that cannot be isolated. It represents the fleeting moment of bond breaking and forming.
4. Why are reaction intermediates generally so unstable?
Reaction intermediates are unstable primarily because they lack a stable electronic configuration. For example, carbocations have an incomplete electron octet, while carbanions have a concentrated negative charge. This electronic instability makes them extremely reactive, as they actively seek to form new bonds to achieve a more stable state, like that of the final products.
5. How is the stability of different carbocations compared?
The stability of carbocations depends on how well the positive charge is dispersed. The general order of stability is tertiary (3°) > secondary (2°) > primary (1°). This is because alkyl groups attached to the positive carbon help stabilise it through:
- Inductive Effect (+I): Alkyl groups donate electron density, reducing the intensity of the positive charge.
- Hyperconjugation: Adjacent C-H bonds share electron density with the empty p-orbital of the carbocation, further delocalising the charge.
6. How do the structure and hybridization of carbocations and carbanions differ?
Carbocations and carbanions have distinct geometries due to their hybridization:
- Carbocation: The positively charged carbon is sp² hybridized, leading to a flat, trigonal planar structure with a vacant p-orbital.
- Carbanion: The negatively charged carbon is typically sp³ hybridized, resulting in a trigonal pyramidal shape, with the lone pair of electrons occupying one of the hybrid orbitals.
7. How are reaction intermediates formed through the fission of a covalent bond?
Intermediates are formed when a covalent bond breaks. This can happen in two ways:
- Homolytic Fission: The bond breaks symmetrically, where each atom gets one electron from the shared pair. This process forms free radicals.
- Heterolytic Fission: The bond breaks asymmetrically, where one atom takes both bonding electrons. This creates a pair of ions, for example, a carbocation and a carbanion.
