Pyrolytic
syn elimination reaction (Ei - 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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