ISOMERS OF 2-PENTENE. II. PENTENE-2 FROM 2-BROMOPENTANE AND FROM 2-PENTANOL¹

Preparation of 2-bromopentane

“…Since the rate of reaction of the cations with bromide ion is comparable to that of 1,2-hydride migration (6), short reaction times should favor the predominant formation of product corresponding to the structure of the particular starting material, either 2or 3-pentanol. Again this is supported by experimental observation (7)(8)(9)(10)(11).…”

“…Historically, reaction of 2-pentanol with hydrogen bromide gas (7)(8)(9)(10), aqueous mixtures of sulfuric acid and hydrobromic acid (7, 8), and phosphorous tribromide (7,10), yields mixtures of the 2-bromoand 3-bromopentane ranging from 70/30 to 94/6 depending upon reagents and conditions. alkyl cations of this kind is small; 5-6 kcal/mole (12)(13)(14)(15).…”

Rearrangement in Alkyl Groups during Substitution Reactions

“…Rearrangement of 2-and 3-pentanol, however, occurs under normal preparative conditions (26,135,156,157) with phosphorus tribromide or pentabromide, to afford mixtures of 2-and 3-bromopentanes (containing 5-30y0 rearranged isomer according to procedure), and with phosphorus pentachloride, secisoamyl alcohol gives t-amyl chloride (180,193). Heated under reflux with phosphorus tribromide, neopentyl alcohol gives rearranged bromides, but if sufficient quinoline to remove free hydrogen bromide is added, then pure neopentyl bromide is said tobeobtained (168,169).…”

“…HBr Secondary alcohols, both straight-and branchedchain, give rearranged halides even at low temperatures (see Table II). The 2and 3-pentyl structures are thus readily interconvertible, octan-2-ol affords the 2-, 3-, and traces of 4-octyl compounds, and in the example of optically active octan-2-ol some loss in rotatory power also occurs (69,70,156,157,182). Eearrangement to a primary alkyl group is not observed and a carbonium-ion mechanism (in part at least) will explain the results.…”

Isomerization of Hydrocarbons.

“…They found no evidence of cis-trans isomerization and no shift of the double bond, and suggested that the halide derivatives may be a racemic or meso form of cis-trans isomers. Sherrill, Baldwin, and Haas (109) showed that pentene-2 made from 3-bromopentane was transformed by ultra-violet light but not that made from 2-bromopentane. Absorption spectrum evidence favored the existence of electromers and not cis-trans isomers, according to Carr (11).…”