Allocinnamic acid

Sometimes the prefix iso is used to signify the less stable isomer as in isocrotonic acid which is cis and it is less stable. Similarly the prefix alio is also used to signify the less stable isomer e.g., allocinnamic acid. 

This substance is prepared by adding a hot solution of mercuric acetate to a lK>iling aqueous solution of allocinnamic acid. It crystallises in colourless microscopic crystals, which have neutral properties. Its alkaline solutions are not affected by ammonium sulphide, unless allowed to stand for a considerable time, whea mercuric sulphide separates out. When heated with hydrochloric acid, mereuiic chloride and ordinary ciimamic add are produced. The anhydride reacts with potassium icKlitle according to the equation ... 

The formula of cinnamic acid leads to the view that it should exist in two stereochemical modifications, which bear a relation to each other similar to that shown in the case of fumaric and maleic acids. The stereoisomer melts at 68° and is called allocinnamic acid it has been shown to be the cis modification (140). 

In Chemical Society practice, prefixes are italicized if, and only if, (i) they define the position of named substituents or (ii) they define a stereoisomer. Thus italics, by clause i) continue, for example, to be used for 0-, m-, p-, and for element symbols which denote substitution on that element, as in o-dichlorobenzene, A -methyl-2-napthylamine, etc. By clause (n), italics continue to be used for m-, trans-, JS-, /S-, etc. But italics are no longer used in names such as isobutane, isopropyl, t-butyl, for here, though the prefixes define the substance, they do not define named substituents. Similarly prefixes which have varied use are not italicized when in particular cases they refer to stereoisomers, as in allocinnamic acid, for here they do not define a particular stereoisomer. These rules differ somewhat from American practice. 

Cryst. in three forms. Mp 68°, Mp 58°, Mp 42°. Bp 95° (high vac.). pA 3.85 (25°). The highest-melting (Z)-form has been known as Allocinnamic acid. The other two forms have both been referred to as Isocinnamic acid. 

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