INTERCONVERSIONS OF PROJECTION FORMULA

It is important to have a clear idea of the inter-relationship between various projections of a given organic molecule to understand the stereochemical implications of the reactions. For this, we should be well conversant with the methods of translating one projection into another. The methods used for converting one projection into another without changing the configuration are given in the following description.

Conversion of Fischer Projection into Sawhorse Projection and vice-versa:

(i) Fischer Projection to Sawhorse Projection:

Fischer projection of a compound can be converted into Sawhorse projection; first in the eclipsed form (in Fischer projection the groups on neighbouring carbons are considered to be eclipsing each other), by holding the model in horizontal plane in such a way that the groups on the vertical line point above, and the last numbered chiral carbon faces the viewer. Then, one of the two carbons is rotated by an angle of 180⁰  to get the staggered form (more stable or relaxed form).

For example, Fischer projection of an optically active tartaric acid is converted into staggered Sawhorse projection as shown.

(ii) Sawhorse Projection to Fischer Projection:

First, the staggered Sawhorse projection is converted to an eclipsed projection. It is then held in the vertical plane in such a manner that the two groups pointing upwards are away from the viewer, i.e. both these groups are shown on the vertical line. Such a conversion for 2,3-dibromobutane is shown.

Conversion of Sawhorse Projection to Fischer Projection via Newman Projection and vice-versa:

(i) Sawhorse Projection to Newman Projection And then Fischer Projection:

Conversion of Sawhorse projection to Newman projection is quite easy. The molecule is viewed from front carbon (the central C-C bond being invisible) to get the staggered Newman projection. The rear carbon is rotated by 180^o to get eclipsed Newman projection. Then, the molecule is held in the vertical plane, i.e. central bond is visible in the vertical plane in such a manner that front carbon is the lowest carbon.

(ii) Fischer Projection to Newman Projection and then Sawhorse Projection:

The molecule is viewed through the lowest chiral carbon, which becomes the front carbon, and thus eclipsed Newman projection is drawn. It is then converted into staggered conformation. Finally, the molecule is viewed through the bond connecting the front carbon with rear carbon. Such a conversion of D-erythrose is illustrated in the following scheme.

Conversion of Fischer Projection into Flying Wedge Projection and vice-versa:

(i) Fischer Projection to Flying Wedge Projection:

The vertical bonds in the Fischer projection are drawn in the plane of the paper using simple lines(—). Consequently, horizontal bonds will project above and below the plane (‘a’ and ‘b’ in the fig.). Conversion of Fischer projection of one of the enantiomers of α-bromopropanoic acid into five flying wedge formulae (without changing the configuration) is illustrated in the fig.

(ii) Flying Wedge Projection to Fischer Projection:

The molecule is rotated (in the vertical plane) in such a way that the bonds shown in the plane of the paper go away from the viewer, and are vertical.    

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Hybridization

Hybridization may be defined as the mixing of two or more than two atomic orbitals of an atom, having comparable energy, to give an equal number of identical orbitals hπaving same energy and shape. All hybrid orbitals are oriented symmetrically to have a maximum distance from each other. Thus, the molecule of methane can be represented by the overlap of a hydrogen 1s orbital with each of the four sp³  orbitals of carbon. Hybrid orbitals of other molecules may similarly be represented. Linear combination of a 2s and two of the 2p orbitals, for example in ethylene, gives rise to three trigonal sp² orbitals directed towards the corners of an equilateral triangle. The plane defined by the two original 2p orbitals leaves the remaining 2p orbitals, perpendicular to the plane of the triangle.

The planar structure of methyl radical can be represented by the overlap of each of the three sp² orbitals of carbon with an s orbital of hydrogen, forming three C-H bonds, leaving the odd electron on the third unhybridized 2p orbital free.

Likewise, hybridization of an s and p orbital gives two diagonal (sp) orbitals, directed towards the opposite ends of the line defined by the p orbital. Methane, ethylene and acetylene are the classic examples of sp³, sp² and sp hybridized carbon atom, respectively. The pictorial representation of hybridized orbitals of methane is given. Ethylene is represented by two carbon atoms combining through two sp² orbitals, and overlapping of the remaining two sp²  orbitals on each carbon atom with 1s orbitals of two hydrogen atoms. The unhybridized parallel 2p orbitals, one on each of the trigonal carbon atoms overlap each other sideways to form a π bond. The electrons involved in such a bonding are called π-electrons. The π-electron cloud (pi bond) is distributed above and below the plane of the molecule, which is the nodal plane of the pi cloud. The bond energy of the carbon-carbon pi bond is about 60 kcal or 250.8 kJ, and, is, therefore, weaker than a C-C sigma bond which is 83 kcal or 346.9 kJ of energy. As the carbon atoms are held more tightly, the carbon-carbon bond distance in ethylene is shorter (1.34 Angstrom) than the C-C sigma bond length in ethane (1.54 Angstrom). The angle between the bonds is 120^o, and the molecule is planar.

Likewise, in acetylene, as represented in the fig., each carbon atom is bonded diagonally to two other atoms, a carbon and hydrogen, through the overlap of two sp-hybridised orbitals of the carbon atoms, and of the remaining two sp orbitals of carbon atoms with two 1s orbitals of hydrogen. This leaves two p orbitals on each carbon atom, perpendicular to each other, as also to the sp hybrid orbital. The sideways overlap of the two parallel pairs of p orbitals leads to the formation of two π bonds, which merge into something like a cylindrical π electron cloud.

Hybridization and Bond Properties:

Bond properties, such as bond length and bond energy, are greatly influenced by the state of hybridization in which the atom exists. An s orbital is at a lower level than a p atomic orbital (AO), which is at a lower level than a d AO. Therefore, the greater the s contribution in the hybrid AOs of the valence state, the greater is the electronegativity of the atom relative to a second atom is determined by the electronegativity of the hybridized AO with which they enter into bonding. The dissociation energy of a bond increases with the difference in electronegativities of the bonded atoms. Therefore, it depends on the state of hybridization of the bonded atoms. The electronegativity of carbon is greatest in sp hybridized state and least in sp³ state. As a result, the C-H bond formed with a carbon orbital of high p-character. The change in hybridization of AOs in carbon, thus, produces a change in the size of covalent atomic radius, decreasing from the tetrahedral (sp³) to the diagonal type (sp). In fact, the state of hybridization in which the bonded atoms exist is the most important factor in determining bond length.

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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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