Pyrolytic elimination reactions involve the application of heat to induce an elimination reaction in an organic substrate without the need for an external base or solvent. This type of elimination proceeds through a concerted, syn-elimination via a cyclic transition state that allows for an intramolecular proton transfer and the formation of a new carbon-carbon double bond. Specific examples of pyrolytic eliminations discussed in the document include the conversion of esters to carboxylic acids and alkenes, eliminations in alicyclic systems, Cope eliminations, sulfoxide eliminations, xanthate pyrolysis, and selenoxide eliminations.

1. Pyrolytic Elimination Reactions
ORGANIC CHEMISTRY – II (I-MSc)
Subject Code: 33CT21 (UNIT-I)
By
Dr. G. Balakrishnan
Assistant Professor
Department of Chemistry
Vivekananda College
Madurai, Tamil Nadu

2. Pyrolytic elimination
 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. (pyrolytic Ei (Elimination internal)
reactions)
(430-480 0C)

4. The term pyrolytic elimination literally means an elimination
reaction occurring in the organic substrate due to the application of
heat ( absence of solvent/reagent etc.,), and mostly carried out in
the gaseous phase though can be performed in inert solvents.
These type of eliminations are different from other types of
eliminations (E1,E2 or E1cB) as all other types of elimination
reactions require an added base (external base or solvent) to
proceed.

5. Ester Pyrolysis

6.  Ester pyrolysis reaction converting esters containing a β-
hydrogen atom into the corresponding carboxylic acid and the
alkene. The reaction is an Ei elimination and operates in a syn
fashion.
 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.

7. pyrolytic Ei eliminations in alicyclic systems
1,3 and 1,n- pyrolytic eliminations may also take place.

8. Cope Elimination

9.  The Cope proceeds through a concerted syn-elimination
mechanism. The oxygen from the N-oxide acts a base, forming
an O-H bond, while the C-H and C-N bonds break to form the
new C-C pi bond.

10. Sulfoxide Elimination
https://en.wikipedia.org/wiki/Ei_mechanism#Sulfoxide_Elimination

11.  β-hydroxy phenylsulfoxides were found to undergo thermal
elimination through a 5-membered cyclic transition state, yielding
β-keto esters and methyl ketones after tautomerization.
 Allylic alcohols can be formed from β-hydroxy phenylsulfoxides
that contain a β’-hydrogen through an Ei mechanism, tending to
give the β,γ-unsaturation.
 1,3-Dienes were found to be formed upon the treatment of an
allylic alcohol with an aryl sulfide in the presence
of triethylamine. Initially, a sulfenate ester is formed followed by
a [2,3]-sigmatropic rearrangement to afford an allylic sulfoxide
which undergoes thermal syn elimination to yield the 1,3-diene.

12. Xanthate pyrolysis
(Chugaev elimination)

13. Selenoxide Elimination

14.  The selenoxide elimination has been used in converting ketones, esters,
and aldehydes to their α,β-unsaturated derivatives.
 The mechanism for this reaction is analogous to the sulfoxide elimination,
which is a thermal syn elimination through a 5-membered cyclic transition
state. Selenoxides are preferred for this type of transformation over
sulfoxides due to their increased reactivity toward β-elimination, in some
cases allowing the elimination to take place at room temperature.
 The areneselenic acid generated after the elimination step is in
equilibrium with the diphenyl diselenide which can react with olefins to
yield β-hydroxy selenides under acidic or neutral conditions. Under basic
conditions, this side reaction is suppressed.

15. Thank you