Pyrolytic Elimination | Pyrolytic Syn Elimination | Regioselectivity of E1 E2

The pyrolytic 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.  The pyrolytic 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 syn elimination

 

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.

 

pyrolytic elimination

 

The Cope elimination reaction — pyrolysis of amine oxide

The Cope elimination 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.

 

cope elimination reaction

 

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

 

Tschugauv reaction

 

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.

 

pyrolytic dehydrohalogenation

 

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 the 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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