In organic chemistry, free-radical halogenation is a type of halogenation. This chemical reaction is typical of alkanes and alkyl-substituted aromatics under application of UV light. The reaction is used for the industrial synthesis of chloroform (CHCl3), dichloromethane (CH2Cl2), and hexachlorobutadiene. It proceeds by a free-radical chain mechanism. The chain mechanism is as follows, using the chlorination of methane as a typical example: 1. Initiation: Splitting or homolysis of a chlorine molecule to form two chlorine atoms, initiated by ultraviolet radiation or sunlight. A chlorine atom has an unpaired electron and acts as a free radical. 2. chain propagation (two steps): a hydrogen atom is pulled off from methane leaving a primary methyl radical. The methyl radical then pulls a Cl• from Cl2. 3. chain termination: recombination of two free radicals: My other popular videos of chemistry.
Reaction mechanism #5| Reactivity and Selectivity in Free Radical reaction
This video is all about the selectivity of free-radical halogenation: what does “selectivity” mean, anyway? And how do we calculate it? It’s often said that chlorination is less “selective” than bromination. The main point is higher the Reactivity, lower the selectivity.
Reaction mechanism #6|Allylic substitution #mechanism of NBS.
NBS will react with alkenes 1 in aqueous solvents to give bromohydrins 2. The preferred conditions are the portionwise addition of NBS to a solution of the alkene in 50% aqueous DMSO, DME, THF, or tert-butanol at 0 °C. Formation of a bromonium ion and immediate attack by water gives strong Markovnikov addition and anti stereochemical selectivities. Side reactions include the formation of α-bromoketones and dibromo compounds. These can be minimized by the use of freshly recrystallized NBS. With the addition of nucleophiles, instead of water, various bifunctional alkanes can be synthesized. Allylic and benzylic bromination. Standard conditions for using NBS in allylic and/or benzylic bromination involves refluxing a solution of NBS in anhydrous CCl4 with a radical initiator—usually azobisisobutyronitrile (AIBN) or benzoyl peroxide, irradiation, or both to effect radical initiation. The allylic and benzylic radical intermediates formed during this reaction are more stable than other carbon radicals and the major products are allylic and benzylic bromides. This is also called the Wohl–Ziegler reaction.
Reaction mechanism#7 |Electrophilic substitution
Electrophilic substitution reactions are chemical reactions in which an electrophile displaces a functional group in a compound, which is typically, but not always, a hydrogen atom. Electrophilic aromatic substitution reactions are characteristic of aromatic compounds and are common ways of introducing functional groups into benzene rings. Some aliphatic compounds can undergo electrophilic substitution as well.
Friedel Craft Reaction |Reaction Mechanism#8
The Friedel–Crafts reactions are a set of reactions developed by Charles Friedel and James Crafts in 1877 to attach substituents to an aromatic ring. Friedel–Crafts reactions are of two main types: alkylation reactions and acylation reactions. Both proceed by electrophilic aromatic substitution.
Group affecting Electrophilic substitution on Benzene |Reaction mechanism 09| Ortho, Para & Meta.
The ortho-substituted benzoic acids are considerably stronger acids than benzoic acid, no matter whether the substituent is electron-releasing or electron-withdrawing. This effect is known as the ortho effect.
Nucleophilic Substitution Reaction |Reaction Mechanism 10.
Nucleophilic substitution reaction is a class of organic reactions where one nucleophile replaces another. It is very similar to the normal displacement reactions which we see in chemistry, where, a more reactive element replaces a less reactive element from its salt solution. The group which takes electron pair and displaced from the carbon is known as “leaving group” and the molecule on which substitution takes place known as “substrate”. The leaving group leaves as a neutral molecule or anion. In nucleophilic substitution reactions, the reactivity or strength of nucleophile is called as its nucleophilicity. So, in a nucleophilic substitution reaction, a stronger nucleophile replaces a weaker nucleophile from its compound. Nucleophilicity It is defined as the ability of the nucleophiles to denoted their lone pairs to a positive centre. It is a kinetic term which relates to the rate at which the nucleophile attacks the substrates (R – LG). The nucleophilicity of different nucleophiles can be compared by using the following factors.
Nucleophilic Substitution (SN1) Reaction |Reaction Mechanism 11.
The SN1 reaction is a substitution reaction in organic chemistry, the name of which refers to the Hughes-Ingold symbol of the mechanism. “SN” stands for “nucleophilic substitution”, and the “1” says that the rate-determining step is unimolecular. Thus, the rate equation is often shown as having first-order dependence on electrophile and zero-order dependence on nucleophile. This relationship holds for situations where the amount of nucleophile is much greater than that of the intermediate. Instead, the rate equation may be more accurately described using steady-state kinetics. The reaction involves a carbocation intermediate and is commonly seen in reactions of secondary or tertiary alkyl halides under strongly basic conditions or, under strongly acidic conditions, with secondary or tertiary alcohols. With primary and secondary alkyl halides, the alternative SN2 reaction occurs. In inorganic chemistry, the SN1 reaction is often known as the dissociative mechanism. This dissociation pathway is well-described by the cis effect. A reaction mechanism was first proposed by Christopher Ingold et al. in 1940.[3] This reaction does not depend much on the strength of the nucleophile unlike the SN2 mechanism. This type of mechanism involves two steps. The first step is the ionization of alkyl halide in the presence of aqueous acetone or ethyl alcohol. This step provides a carbocation as an intermediate. In the first step of SN1 mechanism a carbonation is formed which is planer and hence attack of nucleophile (second step) may occur from either side to give a racemic product but actually complete racemization doesn’t take place. This is because the nucleophilic species attacks the carbonation even before the departing halides ion has moved sufficiently away from the carbonation.The negatively charged halide ion shields the carbonation from being attacked on the front side and backside attack which leads to inversion of configuration is preferred. Thus the actual product no doubt, consist of a mixture of enantiomers but the enantiomers with inverted configuration would predominate and a complete racemization does not occurs.
Nucleophilic Substitution (SN2)|Reaction Mechanism 12.
The SN2 reaction is a type of reaction mechanism that is common in organic chemistry. In this mechanism, one bond is broken and one bond is formed synchronously, i.e., in one step. SN2 is a kind of nucleophilic substitution reaction mechanism, the name referring to the Hughes-Ingold symbol of the mechanism. Since two reacting species are involved in the slow (rate-determining) step, this leads to the term substitution nucleophilic (bi-molecular) or SN2; the other major kind is SN1 Many other more specialized mechanisms describe substitution reactions.
Addition Reaction: An introduction || Electrophilic, Nucleophilic and free radical addition |concept.
An addition reaction, in organic chemistry, is in its simplest terms an organic reaction where two or more molecules combine to form a larger one (the adduct). Addition reactions are limited to chemical compounds that have multiple bonds, such as molecules with carbon–carbon double bonds (alkenes), or with triple bonds (alkynes), and compounds that have rings, which are also considered points of unsaturation. Molecules containing carbon—hetero double bonds like carbonyl (C=O) groups, or imine (C=N) groups, can undergo addition, as they too have double-bond character. An addition reaction is the reverse of an elimination reaction. For instance, the hydration of an alkene to an alcohol is reversed by dehydration. There are two main types of polar addition reactions: electrophilic addition and nucleophilic addition. Two non-polar addition reactions exist as well, called free-radical addition and cycloadditions. Addition reactions are also encountered in polymerizations and called addition polymerization.
Electrophilic addition reaction||Reaction mechanism 15||Markovnikov’s rule .
In organic chemistry, an electrophilic addition reaction is an addition reaction where a chemical compound containing a double or triple bond has a π bond broken, with the formation of two new σ bonds. The overall reaction for electrophilic addition to ethylene. The driving force for this reaction is the formation of an electrophile X+ that forms a covalent bond with an electron-rich, unsaturated C=C bond. The positive charge on X is transferred to the carbon-carbon bond, forming a carbocation during the formation of the C-X bond. Electrophilic addition mechanism In the second step of an electrophilic addition, the positively charge on the intermediate combines with an electron-rich species to form the second covalent bond. The second step is the same nucleophilic attack process found in an SN1 reaction. The exact nature of the electrophile and the nature of the positively charged intermediate are not always clear and depend on reactants and reaction conditions. In all asymmetric addition reactions to carbon, regioselectivity is important and often determined by Markovnikov’s rule. Organoborane compounds give anti-Markovnikov additions. Electrophilic attack to an aromatic system results in electrophilic aromatic substitution rather than an addition reaction. My other popular videos of chemistry.
Free Radical addition reaction || Reaction Mechanism 16 || Anti-Markovnikov’s Rule|| Peroxide effect.
Free-radical addition is an addition reaction in organic chemistry which involves free radicals. The addition may occur between a radical and a non-radical, or between two radicals. The basic steps with examples of the free-radical addition (also known as radical chain mechanism) are: Initiation by a radical initiator: A radical is created from a non-radical precursor. Chain propagation: A radical reacts with a non-radical to produce a new radical species. Chain termination: Two radicals react with each other to create a non-radical species. Free-radical reactions depend on a reagent having a (relatively) weak bond, allowing it to homolyse to form radicals (often with heat or light). Reagents without such a weak bond would likely proceed via a different mechanism. An example of an addition reaction involving aryl radicals is the Meerwein arylation. Addition of mineral acid to an alkene Edit To illustrate, consider the alkoxy radical-catalyzed, anti-Markovnikov reaction of hydrogen bromide to an alkene. In this reaction, a catalytic amount of organic peroxide is needed to abstract the acidic proton from HBr and generate the bromine radical, however a full molar equivalent of alkene and acid is required for completion. Note that the radical will be on the more substituted carbon. Free-radical addition does not occur with the molecules HCl or HI. Both reactions are extremely endothermic and are not chemically favoured. My other popular videos of chemistry.
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