In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R-(C:)-R’ or R=C: where the R represent substituents or hydrogen atoms. Methylene is the simplest carbene. The term “carbene” may also refer to the specific compound H2C:, also called methylene, the parent hydride from which all other carbene compounds are formally derived.Carbenes are classified as either singlets or triplets, depending upon their electronic structure. Most carbenes are very short lived, although persistent carbenes are known. One well-studied carbene is dichlorocarbene Cl2C:, which can be generated in situ from chloroform and a strong base. The two classes of carbenes are singlet and triplet carbenes. Singlet carbenes are spin-paired. In the language of valence bond theory, the molecule adopts an sp2 hybrid structure. Triplet carbenes have two unpaired electrons. Most carbenes have a nonlinear triplet ground state, except for those with nitrogen, oxygen, or sulfur, and halides substituents bonded to the divalent carbon. Substituents that can donate electron pairs may stabilize the singlet state by delocalizing the pair into an empty p orbital. If the energy of the singlet state is sufficiently reduced it will actually become the ground state.
Organic chemistry (GOC#14) #Comparative study of Electronic Effects.
This is comparative study of all major electronic effects. These major electronic effects are Inductive effect, Hyperconjugation effect and Resonance or Mesomeric effect.
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 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.
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.
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.
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 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.
A nucleophile is a chemical species that donates an electron pair to form a chemical bond in relation to a reaction. All molecules or ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are by definition Lewis bases. Nucleophilic describes the affinity of a nucleophile for positively charged atomic nuclei. Nucleophilicity, sometimes referred to as nucleophile strength, refers to a substance’s nucleophilic character and is often used to compare the affinity of atoms. Neutral nucleophilic reactions with solvents such as alcohols and water are named solvolysis. Nucleophiles may take part in nucleophilic substitution, whereby a nucleophile becomes attracted to a full or partial positive charge. Nucleophilicity is closely related to basicity. The difference between the two is: basicity is a thermodynamic property, while nucleophilicity is a kinetic property.
Resonance structures are sets of Lewis structures that describe the delocalization of electrons in a polyatomic ion or a molecule. In many cases, a single Lewis structure fails to explain the bonding in a molecule/polyatomic ion due to the presence of partial charges and fractional bonds in it. In such cases, resonance structures are used to describe chemical bonding.
Hybridization: How to calculate
This video dedicated to free education to all, Atomic orbital hybridisation (or hybridization) is the concept of mixing atomic orbitals into new hybrid orbitals (with different energies, shapes, etc., than the component atomic orbitals) suitable for the pairing of electrons to form chemical bonds in valence bond theory. For example, in a carbon atom which forms four single bonds the valence-shell s orbital combines with three valence-shell p orbitals to form four equivalent sp3 mixtures which are arranged in a tetrahedral arrangement around the carbon to bond to four different atoms. Hybrid orbitals are useful in the explanation of molecular geometry and atomic bonding properties and are symmetrically disposed in space. Usually hybrid orbitals are formed by mixing atomic orbitals of comparable energies.
Hybridization of Atomic orbitals
Hybridization is defined as the concept of mixing two atomic orbitals with the same energy levels to give a degenerated new type of orbitals. This intermixing is based on quantum mechanics. The atomic orbitals of the same energy level can only take part in hybridization and both full filled and half-filled orbitals can also take part in this process, provided they have equal energy.
Molecular Orbital Theory (M.O.T.-2)
The region of an electron is likely to be found in a molecule. A MO is defined as the combination of atomic orbitals. This is to used for calculation of chemical and physical properties such as the probability of finding an electron in any specific region. It is the mathematical function describes the wave-like behavior of an electron in a molecule. The molecular orbitals are obtained by combining the atomic orbitals on the atoms in the molecule. For example consider the H2 molecule. One of the molecular orbitals in this molecule is constructed by adding the mathematical functions for the two 1s atomic orbitals that come together to form this molecule, it is also call additive combination or bonding. Another orbital is formed by subtracting one of these functions from the other called the subtractive combination or antibonding, as showing in video
Molecular Orbital Theory (M.O.T.-1)
The Molecular Orbital Theory (often abbreviated to MOT) is a theory on chemical bonding developed at the beginning of the twentieth century by F. Hund and R. S. Mulliken to describe the structure and properties of different molecules. The valence-bond theory failed to adequately explain how certain molecules contain two or more equivalent bonds whose bond orders lie between that of a single bond and that of a double bond, such as the bonds in resonance-stabilized molecules. This is where the molecular orbital theory proved to be more powerful than the valence-bond theory (since the orbitals described by the MOT reflect the geometries of the molecules to which it is applied).
Atom and Atomic orbitals
Atomic orbitals are mathematical functions that provide insight into the wave nature of electrons (or pairs of electrons) that exist around the nuclei of atoms. In the fields of quantum mechanics and atomic theory, these mathematical functions are often employed in order to determine the probability of finding an electron (belonging to an atom) in a specific region around the nucleus of the atom.
Respiratory and circulatory system
Respiratory system is not complete without circulatory system, respiratory and circulatory system are connected with each other. Our body have two purification house, one is our kidney and other is lungs. Kidney purifying the blood from liquid wastes and Lungs purifying blood from gaseous waste (Carbon dioxide).
Reaction intermediate: Free Radicals
In chemistry, a radical is an atom, molecule, or ion that has an unpaired valence electron.With some exceptions, these unpaired electrons make radicals highly chemically reactive. Many radicals spontaneously dimerize. Most organic radicals have short lifetimes.
Reaction intermediate: Carbocation
A carbocation is a molecule in which a carbon atom has a positive charge and three bonds. We can basically say that they are carbon cations. Formerly, it was known as carbonium ion. Carbocation today is defined as any even-electron cation that possesses a significant positive charge on the carbon atom. Talking about some general characteristics, the carbon cations are very reactive and unstable due to an incomplete octet. In simple words, carbocations do not have eight electrons, therefore they do not satisfy the octet rule.
Rearrangement of Carbocations
Carbocation rearrangements can be defined “as the movement of the carbocation from an unstable state to a more stable state by making use of different structural reorganizational shifts within the molecule”. Alkyl carbocation is a carbocation comprising an alkyl group. They are the most common carbocation. Carbocation Rearrangement occurs whenever the alcohols are converted into several carbocations and this phenomenon is termed as carbocation rearrangement. In simple carbocation comprises +ve charge in a molecule that is connected to 3 more groups and holds a sextet. Carbocation rearrangement can be carried out to a reaction that does not involve alcohol. There are 3 types of carbocation rearrangements namely Alkyl and Hybrid Shift and phenyl shift.
In chemistry, the inductive effect is an effect regarding the transmission of unequal sharing of the bonding electron through a chain of atoms in a molecule, leading to a permanent dipole in a bond. It is present in a σ (sigma) bond, but not present in a π (pi) bond.
Fajan’s’ rule
Fajan’s’ rule predicts whether a chemical bond will be covalent or ionic. Few ionic bonds have partial covalent characteristics which were first discussed by Kazimierz Fajan’s in 1923. In the time with the help of X-ray crystallography, he was able to predict ionic or covalent bonding with the attributes like ionic and atomic radius
Hydrogen bonding
Hydrogen bonding refers to the formation of Hydrogen bonds, which are a special class of attractive intermolecular forces that arise due to the dipole-dipole interaction between a hydrogen atom that is bonded to a highly electronegative atom and another highly electronegative atom while lies in the vicinity of the hydrogen atom. For example, in water molecules (H2O), hydrogen is covalently bonded to the more electronegative oxygen atom. Therefore, hydrogen bonding arises in water molecules due to the dipole-dipole interactions between the hydrogen atom of one water molecule and the oxygen atom of another H2O molecule.
Chemical Bonding : Types of Bond
Chemical Bonding refers to the formation of a chemical bond between two or more atoms, molecules, or ions to give rise to a chemical compound. These chemical bonds are what keep the atoms together in the resulting compound.
VSEPR Theory
VSEPR Theory is used to predict the shape of the molecules from the electron pairs that surround the central atoms of the molecule. The theory was first presented by Sidgwick and Powell in 1940. VSEPR theory is based on the assumption that the molecule will take a shape such that electronic repulsion in the valence shell of that atom is minimized.
Stability of resonating structure
stability of resonating structure, means which resonating structure is more stable and which is less and what are the orders of stability.
Mesomeric effect:
In 1938, scientist Ingold developed the concepts of mesomeric effect, mesomerism and mesomer. Interestingly, mesomerism is synonymous to resonance which was introduced by Scientist Pauling. Up until 1950, the word mesomerism was widely used in French and German language. However, in the English language, the term “resonance” has become very popular and is widely used today. On the whole, they refer to the same concept. The polarity developed between atoms of a conjugated system by the electron transfer or pi–bond electron transfer is known as the Mesomeric effect. In simple terms, we can describe mesomeric effect occurs when π electrons move away from or towards a substituent group in a conjugated orbital system.
Mesomeric effect: Applications.
Application of mesomeric effect predicts the stability of derivatives of Benzene.
Hyperconjugation :
Hyperconjugation effect is a permanent effect in which localization of σ electrons of C-H bond of an alkyl group directly attached to an atom of the unsaturated system or to an atom with an unshared p orbital takes place. We also observe that the hyperconjugation stabilizes the carbocation as it helps in the dispersal of positive charges. Thus, we can say that the greater the number of alkyl groups attached to a positively charged carbon atom, the greater is the hyperconjugation interaction and stabilization of the carbonation.
Ortho effect :
Ortho effect refers mainly to the set of steric effects and some bonding interactions along with polar effects caused by the various substituents which are in a given molecule altering its chemical properties and physical properties. In a general sense the ortho effect is associated with substituted benzene compounds.
Carbene :
In chemistry, a carbene is a molecule containing a neutral carbon atom with a valence of two and two unshared valence electrons. The general formula is R-(C:)-R’ or R=C: where the R represent substituents or hydrogen atoms.
Comparative Study of Electronic effects:
This is comparative study of all major electronic effects. These major electronic effects are Inductive effect, Hyperconjugation effect and Resonance or Mesomeric effect.
Reaction Intermediate: Carboanion or Carbanion
Carboanion also called Carbanion. A carbanion can be defined as a negatively charged ion in which a carbon atom exhibits trivalence (implying it forms a total of three bonds) and holds a formal negative charge whose magnitude is at least -1. When pi delocalization does not occur in the organic molecule (as it does in the case of aromatic compounds), carbanions typically assume a bent, linear, or a trigonal pyramidal molecular geometry. It is important to note that all carbanions are conjugate bases of some carbon acids.
Aromatic/Non-Aromatic/Anti-Aromatic
This differs from aromaticity only in the fourth criterion: aromatic molecules have 4n +2 π-electrons in the conjugated π system and therefore follow Hückel’s rule. Non-aromatic molecules are either noncyclic, nonplanar, or do not have a complete conjugated π system within the ring. An antiaromatic compound may demonstrate its antiaromaticity both kinetically and thermodynamically. As will be discussed later, antiaromatic compounds experience exceptionally high chemical reactivity (being highly reactive is not “indicative” of an antiaromatic compound, it merely suggests that the compound could be antiaromatic). An antiaromatic compound may also be recognized thermodynamically by measuring the energy of the cyclic conjugated π electron system. In an antiaromatic compound, the amount of conjugation energy in the molecule will be significantly higher than in an appropriate reference compound
Reaction Mechanism: An Introduction
In chemistry, a reaction mechanism is the step by step sequence of elementry reaction by which overall chemical change occurs. A reaction mechanism must also account for the order in which molecules react.
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