Ester pyrolysis

What is ester pyrolysis?

According to reports, the reaction under discussion was first observed by either Smith in 1842 or Heintz in 1854.
Ester pyrolysis involves the thermal cleavage of esters that have β-hydrogen(s) on the alcoholic moiety via cis-elimination. The result is the formation of olefins without carbon skeleton isomerization or double bond shift, and carboxylic acids are produced as by-products.

Ester pyrolysis - general reaction scheme
Ester pyrolysis

The ester pyrolysis is particularly suitable for esters that contain volatile components of either acid or alcohol.

It is believed that the primary controlling factor in ester pyrolysis is the difficulty in forming p-π conjugation between the carbonyl oxygen and β-hydrogen in the ground state of the ester. The stronger the conjugation, such as in the case of semipolar six-membered cyclic rings, the easier the pyrolysis occurs. This explains why γ-lactones are stable at 600 ºC while larger lactones that can form a transient six-membered ring decompose at 500 ºC.

Initially, it was thought that this reaction occurred via a concerted mechanism, but recent experimental evidence indicates that it occurs via a semipolar six-membered transition state, with a partial negative charge on the departing oxygen atom. The Hofmann rule is followed in pyrolysis of esters with aliphatic alcohols, where the olefinic mixtures show a similar ratio of available β-hydrogens in the alcoholic component. As a result, the pyrolysis is easier with more primary protons. Even tert-butyl acetate pyrolyzes at 360 ºC, indicating that the pyrolysis rate is linear to the pKa of the corresponding acids for the case of tert-butyl esters. However, for isopropyl α-haloacetates, the pyrolysis rate is in the order:

I > Br > Cl > F

When the acyl portion is varied within a series of esters, the pyrolysis temperature decreases in a zigzag pattern, with esters from acids with an even number of carbon atoms being slightly more stable than their neighboring analogues. The ester pyrolysis is significant in the thermal modification, recycling, and degradation of polyesters (such as poly ε-caprolactone) and the synthesis of difficult-to-obtain olefins, such as highly unsaturated compounds (e.g., 2-vinylbutadiene).

Examples of special products obtained from ester pyrolysis include methylenecyclobutenone from the flash vacuum pyrolysis of furfuryl benzoate, alkyl or aryl phenyl 2-methylene-1,3-diones from 3-phenylpropargyl esters, and cyclic ethers containing three- to six-membered rings from cyclic carbonates with one or two hydroxyl groups.

References

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