Resonance or Mesomerism

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ELIMINATION REACTION - E2- ELIMINATION REACTION - REACTION MECHANISM (ORGANIC CHEMISTRY)

Elimination reaction involves the loss of two groups are atoms from a Molecule. The reactions are classified under 2 general headings β- elimination  (1, 2 elimination),  the most common elimination reaction,  in which groups on adjacent atoms are eliminated with the formation of an unsaturated bond.  β- elimination includes acid catalysed dehydration of alcohol,  solvolytic and base- induced elimination reaction from sulphonates, alkyl halides and the Hofmann elimination from quaternary ammonium salts.

The 2nd mode of elimination involves two groups departing from the same atom. 1,1- or α-elimination are used for generating the reactive intermediate called carbenes. β- elimination reaction occurs through  3 mechanistic pathway out of which E1 and  E2  are the most common.  These processes are closely related to the SN2 and SN1 mechanism of substitution. The 3rd mechanism is designated as E1cB (elimination, Unimolecular of the conjugate base)  which involves a carbanion intermediate and the substrate must contain substituents which stabilize it.  The substrate undergoing E1cB elimination has a leaving group which is β placed to the carbanion stabilizing group.  A good example is of Knoevenagel reaction.  If a substrate undergoing elimination as a poor leaving group, the transition state of otherwise E2 elimination gains E1cB character. 

The Bimolecular mechanism for elimination  E2 process:

E2 mechanism:

The formation of ethylene on the treatment of ethyl Bromide with sodium ethoxide is an example of this type.  The rate of alkene formation is proportional to the concentration of ethyl Bromide as well as that of sodium ethoxide. 

In this process substrate and the base participate in a single step transition state from which the removal of a Proton β to the leaving group is concerted with the leaving group.  

The structure of the organic compound undergoing elimination and the strength of the base used for the E2 elimination reflex on the extent of these three Bond changes at the transition state.  This structure gains individual importance which is based on several factors.

Structure (iii)  would gain importance when  L^- is the for leaving group or if the negatively charged carbanion is adjacent to the group when -I effect. 

Structure (iv) gains importance when on the other hand  L^- is a good leaving group while B: base is a weak base.

Structure (ii), i.e., the alkene-like character of the transition state becomes significant when L^- is a good leaving group and B:  is a strong base.

The transition state  (iii) will have considerable carbanion character when the leaving group is poor, example NMe₃ in Hoffman elimination.

The rate-determining step involves the breaking of the C-H bond has been shown by Kinetic isotope effect.  Changing H to D can affect the rate of the reaction only if that H (or D)  is involved in the rate-determining step.

It is known that the C-D bond is stronger than the C-H bond and thus requires more energy to be broken.  The rate of elimination in one should be much faster than 1a  which has indeed been found to be the case. 

The direction of elimination in E2 reaction:

With several substrates, the elimination can take place in more than one way.  Generally, the more substituted alkene is formed as the major product. This generalisation is known as the Saytzeff rule.  When 2 bromobutane reacts with a base  2 elimination products are expected since in the transition state both the C-H as well as the C-Br bond is breaking.  The transition state has alkene like structure and the factors which stabilize an alkene also stabilizes the transition state.  Thus  2-butane is formed as the major product.

The relative reactivity of alkyl halide in an E2 mechanism  follows the order:

Tertiary alkyl halide > secondary alkyl halide > primary alkyl halide

This is due to the predominant-formation of a more substituted alkene. 

An exception to the saytzeff rule is observed from base-induced elimination from quaternary ammonium salts and from sulphonium salts which gives predominantly the less substituted alkene (Hofmann rule).

It can be understood that this difference between the elimination from an alkyl Bromide (saytzeff rule)  and from a quaternary ammonium ion (Hofmann rule)  on the basis of the poor leaving group tendency of amine compared to Br^-.  

Thus in the E2 elimination of a quarterly ammonium ion, the C-H  bond is almost fully broken in the transition state and consequently, the structures 1 and 2  may be considered as important contributors to the transition state. The structure 1  is much more significant since and alkyl group destabilizes an adjacent negative charge, therefore, the less substituted alkene predominates.  In case of an alkyl Bromide, the transition state will be more alkene-like.  

Further, it is seen that with an alkyl Bromide itself, the Hoffman orientation predominates in case the proton removed the saytzeff orientation is in a  sterically hindered environment. In such a case the use of sterically hindered base may lead to Hoffman orientation.

The transition state is the hybrid of structures that help in explaining the orientation observed in the four  2-halohexane.  With X = F  the orientation is largely Hoffman while with  X=I  the orientation is predominantly saytzeff.  Two factors may be considered, firstly the bond strengths  lie in the order  C-I < C-Br<  C-Cl <C -F  and secondly  the electron withdrawing effect  of  X  follows the order F > Cl > Br > I. In the fluorine case,  the primary hydrogen is preferentially extracted by base  since it allows the negative charge to develop on a primary carbon which can best accommodate it to give Hoffman orientation. 

The Rate of E2 reactions  and other aspects:

The rate of a reaction increases with the increasing strength of the base.  It also increases with a good leaving group and the leaving group ability parallels the stability of the anion.  Ethers and alcohols do not undergo E2 elimination reaction since alkoxide and Hydroxide ions are relatively high energy species while a sulphonate displays this reaction since sulphonate anion is very stable (the conjugate base of a strong acid). 

The stabilization energy associated with conjugation in the product formed is partly developed at the transition state. Thus CH₂=CH-CH_2-CH2-Br eliminates HBr to give butadiene more readily when compared with  2-bromobutane.

It has been seen that during E2 elimination saytzeff rule predict the formation of a more substituted alkene and the exceptions are when the leaving group is poor. In this case, negative charge will build upon the Carbon from which the proton is lost and therefore the carbanion stability determines the major alkene product. Moreover, in several eliminations, the less stable alkene predominates.  Example: when the base is bulky and sterically hindered, the conjugate alkene is more stable even though it may not be the most substituted alkene. It is more stable than a non-conjugate product due to resonance.  The transition state in such cases has a partial development of conjugation which provides it with enough stabilization.  Thus the elimination gives the conjugated product as the major alkene though it is not highly substituted.

Dehalogenation:

An elimination reaction is not only confined only to those reactions in which one of the leaving group is hydrogen. Reactive metals such as zinc and magnesium are capable of removing halogens from  1,2-dihalides to yield alkenes. Like other E2 reactions, these reaction also proceed by a concerted trans elimination process. Thus dl-and meso-2,3-dibromobutane yield cis and trans-2-butenes, respectively on treatment with zinc.

A similar treatment of Trans-2,3-dibromo-2-butene with zinc results in the formation of dimethylacetylene. 

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ELIMINATION REACTION- E1 MECHANISM; E1cB MECHANISM (ORGANIC CHEMISTRY)

E1- Mechanism:

This elimination takes place (without the participation of a base) in 2 steps,  unimolecular ionization,  being rate-determining step. 

The main feature of this mechanism is that under the influence of solvation forces the electron attracting group (leaving group) breaks away along with the bonding electrons. The resulting Carbocation subsequently loses a proton to the solvent or to some other proton acceptor.  

The reaction has 2 stages,  of which the first is the rate determining step and as a result, the reaction rate depends only on the concentration of the first reactant. In E1 reactions,  a Proton is eliminated from the carbon adjacent to the positive, electron deficient carbon and the pair of electrons formerly shared by this hydrogen is available for the formation of a π-Bond. 

The Carbocations are planar species and their formation at the bridgehead position will be a difficult process.  Thus, bicyclic structures prevent the bridgehead carbon becoming planar even though the cation would be tertiary.  It is not formed due to very high energy. Such compounds will not undergo E1 reactions. 

The direction of elimination: 

2-Bromo-2-methylbutane  on reaction with water in ethanol gives substitution as the major product when both water and ethanol can act as a nucleophile to give an alcohol or an ether.  The major alkene formed is more highly substituted. 

 Thus, from among alkyl halides with the same alkyl group the alkyl fluorides display the least reactivity in E1 reaction.

                               RF < RCl < RBr < RI

E1 elimination from cyclic compound: 

Since a carbon is formed in the first step of the E1 reaction, the relative stereochemistry of the leaving groups  (anti coplanarity)  is not important. When menthyl chloride undergoes E2 reaction only one alkene is formed in hundred percent yield due to the need for the departed group to attend diaxial positions. When menthyl chloride is subjected to E1 reaction conditions 2 alkenes are formed, the major product is in accord with the saytzeff rule. 

Curtin-Hammett principle:

That Curtin Hammett principle applies to a conformationally heterogeneous reactant where the products must be non-equilibrating.  The Curtin Hammett principle implies that in a chemical reaction which gives one product from one conformer and a different product from another conformer.  The product composition is not determined by the relative population of the ground state conformer but largely depends on the relative energies of the corresponding transition state involved.

E1cB Mechanism:

In the E1cB mechanism,  the base rapidly removes the proton from the β carbon resulting in the formation of carbanion, which loses the leaving group in the rate-determining step.  Since the conditions of base catalysed elimination reaction does not allow the formation of an unstabilized carbanion,  it is reasonable to presume that if formed,  they must be either rapidly reconverted to the substrate is converted to the alkene. In this mechanism, the overall rate is limited to that of the slower state 2nd stage,  which depends only on the concentration of the conjugate base of the reactant.  This mechanism is called as E1cB as the leaving group is lost from the conjugate base of the starting material and the reaction is Unimolecular. 

The first step of is reversible, and hence, when the reaction is carried out in C₂H₅OD instead of C₂H₅OH, the intermediate carbanion should pick up deuterium.

If the E1cB mechanism is correct,  we recover  2-phenyl-ethyl bromide after a partial transformation to styrene.  On the other hand, there should be no incorporation of deuterium if the E2 mechanism is operative.  Actual experiments have shown that there is no deuterium incorporation and hence the E1cB mechanism does not operate in this case.  

However, this mechanism does operate under special circumstances.  1,1,1-trifluoro-2,2-dichloroethane (3), for instance, undergoes a base-catalyzed exchange of β hydrogen atom with the solvent deuterium faster than dehydrofluorination. 

A strong carbon-fluorine bond (and the consequent poor leaving ability of fluoride ion) coupled with the electron-withdrawing effect of halogens explains the formation of carbanion before elimination.

The removal of a proton and loss of the leaving group occurs simultaneously in E2 mechanism whereas removal of a Proton is the first step in the E1cB reaction.  In the subsequent rate-determining step of the E1cB reaction, the leaving group departs from the conjugate base of the substrate. 

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PYROLYTIC SYN OR THERMAL ELIMINATION; REGIOSELECTIVITY OF E1 & E2 REACTION (ORGANIC CHEMISTRY)

Pyrolytic syn elimination reaction (E_i - elimination internal):

This thermal elimination occurs in a family of a compound like an acetate Esters, methyl xanthate ester, tertiary amine oxide, sulphoxides and selenoxides which contain at least one β hydrogen atom with the formation of olefins.  This elimination has a common mechanistic feature: a concerted reaction via a cyclic transition state within which an intramolecular proton transfer is accompanied by syn-elimination to form a new carbon-carbon double bond.  If more than 1 β hydrogen is present then mixtures of alkanes are generally formed.  Since this reaction involved cyclic transition states, conformational effects play an important role in determining the composition of the alkene product. 

Pyrolytic elimination undergoes Unimolecularly through a cyclic mechanism.  These reactions are carried out in the gas phase and proceed in a concerted fashion yielding the product of cis elimination.  A common example is the pyrolysis of acetate esters resulting in the formation of alkenes. 

The Cope elimination reaction -- pyrolysis of amine oxide-

Cope reaction involves the pyrolysis of amine oxide having a hydrogen atom β to the amine group.  The syn elimination affords an alkene and dialkylhydroxylamine. 

There is complete retention of deuterium in the alkene obtained from 22 whereas no deuterium was found in the product obtained from the pyrolysis of 23.

The synthetically important Tschugauv reaction involves pyrolysis of xanthate at relativity low temperature.   

Unsymmetrical Acetate and xanthate esters yield a mixture of all the possible alkenes, but there is usually a predominance of the more highly substituted alkene. Another analogous reaction of comparative value is the pyrolysis of amine oxide to yield alkene. This reaction is also a cyclic process resulting in cis elimination.

Alkyl halides also undergo pyrolytic dehydrohalogenation via 4 membered cyclic transition state.

Stereospecificity and Stereoselectivity:

A stereoselective reaction is one in which a single starting material can give two or more stereoisomeric products but one of these in a greater amount (or even to the exclusion of the other).  Thus, 2 bromobutane undergoes a stereoselective base-induced elimination of hydrogen bromide.

In a stereospecific reaction, stereoisomer starting material yields product which are stereoisomers of each other. The dehalogenation of meso and (+-)-2,3- dibromobutane is thus a stereospecific reaction.

 Regioselectivity of E2 and E1 mechanism:

2-bromobutane has to structurally different β-carbon from which a Proton can be removed. So, when 2-bromobutane  reacts with a base, 2 elimination products are formed:  2-butene and 1-butene.  Thus, an E2 reaction is regioselective because of the preferred formation of 2-butene.

Regioselectivity of an E2 reaction is determined by the alkene which is formed more easily or which is formed faster.  The reaction coordinate diagram for E2 reaction of 2-bromobutane is shown.

It may be noted that in this transition state,  the  C-H  and C-Br bond are partially broken to generate an alkene-like structure.  It is reasonable to assume that factors stabilizing an alkene will also stabilize the transition state leading to its formation.  The greater number of alkyl subsequent bonded to the sp² carbon of an alkene increases its stability.  This explains the greatest ability of 2-butene  as compared to 1-butene. Thus,  the most stable of the two alkenes is found to be a major product of the reaction.  Preferential formation of the more highly substituted alkene in an E1  reaction is based on the assumption that the entropies of the product-determining transition states parallel those of the isomeric alkenes.

Because the first step is the rate determining step,  the rate of an E1 reaction depends on the ease with which the carbocation is formed.  The more stable the Carbocation the easier it is formed because more stable Carbocations have more stable transitions states leading to their formation.  The tertiary benzylic halide is the most reactive alkyl halide because of a tertiary benzylic cation, being a very stable carbocation is the easiest to form.

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HYDROGEN BONDING

The hydrogen atom which is bonded to an electronegative atom can form a hydrogen bond to a second electronegative atom.  It is thus a force of attraction between opposite partial charges δ⁺ charge on H in the OH group and δ^- charge on the O of another group. Hydrogen bond requires a hydrogen bond donor and hydrogen bond acceptor as in the alcohol molecule.

Ammonia, water,  and HF show abnormally high melting and boiling points due to the formation of a link between two electronegative atoms,  one of which is a joined to it by a polar covalent bond and the other by electrostatic attraction. The electrostatic attraction comes into play due to uneven sharing of the pair of bonding electrons between hydrogen and highly electronegative atom. Thus, a partial positive charge develops on hydrogen which attracts the negative end of a group of the same or other molecules forming a relatively weak bond.  This type of bond is called the hydrogen bond and is the most powerful kind of dipole-dipole attraction.  Being electrostatic in nature, it is much weaker than a covalent bond.  The hydrogen bonds are usually represented by dotted lines.

An Ether has no O-H  proton, therefore, the ether group cannot donate hydrogen bonds and thus cannot form a hydrogen bond with another ether molecule. Since ether molecules are not held together by hydrogen bonds, they are more volatile than alcohols of the same molecular weight.  The oxygen of the ether group can, however, form hydrogen bonds with an alcohol or other hydrogen bond donor.  So ethers are most soluble in water than in alkanes.

Due to hydrogen bonding, there is an increase in intermolecular aggregation forces which is reflected in the boiling point and solubility of the organic compound.  There is an increase in the boiling point since energy is required to separate the hydrogen bonded molecule in the translation to the gaseous state.  Compounds which form strong hydrogen bonds may be associated even with the gas phase. Thus Acetic Acid exists as a dimer in the gas phase.

There are two types of hydrogen bonds- intermolecular and intramolecular.

Intermolecular hydrogen bond exists between atoms of two or more molecules resulting in their association. For instance, water and alcohols are associated as polymeric aggregates in the solid and liquid state whereas carboxylic acid and amide exist as dimer due to intermolecular hydrogen bonding.

An intramolecular hydrogen bond is formed between two atoms of the same molecule. When the resulting ring is Five or Six-membered then the phenomena is called chelation and the five or six-membered ring is called the chelate ring. An example of chelation is for the enolic form of acetylacetone. Since on chelation intermolecular aggregation forces are not operative,  chelated compounds have normal boiling points. Thus, Ortho nitrophenol is much more volatile than its para isomers since only the letter can form intermolecular hydrogen bonds.  Some other examples of compounds exhibiting this type of hydrogen bonds are salicylaldehyde and Ortho-chlorophenol.

For forming intramolecular hydrogen bond  the molecule must satisfy the following conditions:

(i)  The molecules must have two groups in such a way that one group contains hydrogen atom linked to a highly electronegative atom and the other group also contains an electronegative atom.  (ii) The molecule must be planar (iii) The hydrogen bond must lead to the formation of a 5 or 6 membered ring.

The distinction can be made between inter and intramolecular hydrogen bonding on the basis of infrared spectroscopy and NMR spectroscopy.

Hydrogen bonding effects structure and molecular shape of molecules.  The role of intramolecular hydrogen bonding is reflected in a large amount of enol present in some tautomeric equilibrium.  The 6 membered heterocycles of oxygen closely resemble the chair conformation of cyclohexane. In the heterocyclic ring, the steric repulsion for axial substituents is reduced due to the replacement of a methylene group of cyclohexane by Oxygen or nitrogen.  Since the divalent oxygen has no substituents, therefore, the 1,3-diaxial interactions which are the main unfavourable interaction for axial substituents in cyclohexanes are absent.  Thus, the preferred conformation of 5-hydroxy-1,3-dioxane has the hydroxyl group in the axial position. This conformation is favoured due to hydrogen bonding of the hydroxyl group with the ring oxygen which is possible only with the axial hydroxyl group to serve as a stabilizing force for this conformation.

Effect of hydrogen bond on the physical property:

As some extra energy  is needed to break the hydrogen bond  intermolecular hydrogen bonding the melting and boiling point  of organic substances  effect of intramolecular hydrogen bonding in transition temperature is the rivers of death of intermolecular hydrogen bonding  Ortho nitrophenol for instance  meals meals  at 44 degree Celsius while the melting point of para-nitrophenol is 114 degree Celsius  similarly salicylic acid melts at 158 degree Celsius as compared to para-hydroxybenzoate is it  para-hydroxybenzoic  acid  which melts at 214 degree Celsius.  

Hydrogen bonding shifts the position of bands in infrared and NMR spectra of organic compounds.

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