Acetate esters, pyrolysis

C is a primary alcohol and is quite prone to undergo Carbon skeleton rearrangement when treated with hoi acid. To obviate the carbon skeleton rearrangement and effect the dehydration so that B is the major product, it is suggested that the acetate ester of C be formed and pyrolyzed as shown below. The acetate ester pyrolysis is known to yield olefins in which there is no carbon skeleton rearrangement. 

Methyl Substitution in Acetate Ester Pyrolysis (Relative Rates)... 

Acetic anhydtide is a mature commodity chemical ia the United States and its growth rate in the 1970s and 1980s was negative until 1988 when foreign demand neatly doubled the exports of 1986. This increase in exports was almost certainly attributable to the decline in the value of the U.S. doUar. Over four-fifths of all anhydtide production is utilized in cellulose acetate [9004-35-7] manufacture (see Cellulose esters). Many anhydtide plants are integrated with cellulose acetate production and thus employ the acetic acid pyrolysis route. About 1.25 kg acetic acid is pyrolyzed to produce 1.0 kg anhydtide. 

Rearrangements and other side-reactions are rare. The ester pyrolysis is therefore of some synthetic value, and is used instead of the dehydration of the corresponding alcohol. The experimental procedure is simple, and yields are generally high. Numerous alkenes have been prepared by this route for the first time. For the preparation of higher alkenes (> Cio), the pyrolysis of the corresponding alcohol in the presence of acetic anhydride may be the preferable method." The pyrolysis of lactones 9 leads to unsaturated carboxylic acids 10 ... 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C and is usually a vapor phase reaction. In the laboratory this is done by using a glass tube in the heating zone of a small furnace. The vapors of the reactant are swept through the hot chamber by an inert gas and into a cold trap. Similar reactions occur with esters derived from long-chain acids. If the boiling point of the ester is above the decomposition temperature, the reaction can be carried out in the liquid phase, with distillation of the pyrolysis product. 

A third category of syn eliminations involves pyrolytic decomposition of esters with elimination of a carboxylic acid. The pyrolysis of acetate esters normally requires temperatures above 400° C. The pyrolysis is usually a vapor-phase reaction. In the... 

The pyrolysis of acetate esters yields olefins by a concerted syn elimination of acetic acid. In open-chain systems where rotations are facile, it is possible for the... 

In general, electron impact-induced elimination reactions formally proceed via five-, six-, or seven-membered ring transition states. The pyrolysis of acetate esters to give olefins is thought to proceed via a six-membered ring transition state (Banthorpe, 1963a). The formally similar electron impact-induced elimination of acetic acid from an ester results in both 1,4- and 1,5-elimination (Benz and Biemann, 1964) and this does not closely parallel the pyrolysis (see equation 119). 

Smith and Wetzel (1951) have shown qualitatively that the strength of the acid and the rate of ester pyrolysis increased in the same direction. This conclusion is borne out by the work of Emovon (1963) who found for the series t-butyl acetate, t-butyl chloroacetate and t-butyl dichloro-acetate that the rates at 250°C were in the ratio 1 4-4 18-6. 

The utility of the Chugaev elimination for the formation of olefins without rearrangement of the carbon skeleton can be observed upon pyrolysis of the xanthate of alcohol 14.17 The desired vinyl cyclopropane (IS) was isolated in reasonable yield (42%) along with a small amount of the rearranged xanthate. Alternatively, acid-mediated dehydration with sulphuric acid yielded a variety of rearranged products in low yield. Ester pyrolysis (of the acetate of 14) also furnished a variety of compounds, with the major product being cyclopentene 16. 

The importance of the syn relationship of hydrogen and the leaving group is seen in hydrindanes 227 and 228, which produced 229 upon pyrolysis. The cis-derivative (227) required temperatures at least 200°C higher than the trans-acetate (228).209b synthetic example that involves ester pyrolysis is taken from Fallis synthesis of (-t-)-longifolene (230), formed in 56% yield by pyrolysis of 231. 10... 

C.iv. Xanthate Ester Pyrolysis.2H 209a,b temperatures required for acetate pyrolysis... 

The pyrolysis of esters has usually been done with acetate esters. The thermal requirement for the reaction is very high, with temperatures above 500° usually required. The pyrolysis is thus a vapor-phase reaction. In the laboratory, this is usually accomplished in a packed glass tube heated with a small furnace. The reacting vapors and product are swept through the hot chamber at an appropriate rate by an inert gas such as nitrogen, and into a cold trap or other system for condensation. 

The pyrolysis of acetic esters to give an alkene and acetic acid is thought to involve a planar transition state and cyclic redistribution of (4 + 2) electrons. Propose a mechanism for pyrolysis of the following ester. 

The acetyl content of cellulose acetate may be calculated by difference from the hydroxyl content, which is usually determined by carbanilation of the ester hydroxy groups in pyridine solvent with phenyl isocyanate [103-71-9J, followed by measurement of uv absorption of the combined carbanilate. Methods for determining cellulose ester hydroxyl content by near-infrared spectroscopy (111) and acid content by nmr spectroscopy (112) and pyrolysis gas chromatography (113) have been reported. 

In the case of the cyclohexane derivative 7 however, that bears an equatorial acetate group, two axial cis-/3-hydrogens are available, and elimination in both directions is possible. The pyrolysis of 7 yields the two elimination products 8 and 6. Formation of product 8 is strongly favored, because the new double bond is in conjugation to the ester carbonyl group. ... 

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