Isobornyl chloride

Present in citronella and valerian oils, tur penline, ginger, rosemary and spike oils. It is produced artificially by the elimination of hydrogen chloride from bornyl chloride (artifi cial camphor) or from isobornyl chloride, by the dehydrogenation of borneol and isobor-neol and by the action of elhanoic anhydride on bornylamine. Chiral. 

In the same way bornyl and isobornyl chlorides react with milk of lime. But, whereas bornyl chloride gives an almost quantitative yield of... 

These effects are very much smaller than those found for the first-order solvolyses of bornyl and isobornyl chlorides, which differ in relative rate by several orders of magnitude. However, the authors argue that this does not necessarily disprove participation by a carbonium ion in the mechanism which they proposed for the oxidation of straight-chain alkanes (see above). 

Nevell, de Salas and Wilson (1939) that the rearrangement of cam-phene hydrochloride [1] into isobornyl chloride [2] involved the... 

WAGNER-MEERWIN REARRANGEMENT. Carbon-to-carbon migration of alkyl, aryl, or hydnde ions. The original example is the acid-catalyzed rearrangement of camphene hydrochloride lo isobornyl chloride. 

In the conversion of a-pinene (2.8) into bornyl chloride (2.9) endo isomer), the rearrangement to a 2° carbocation is favoured by relief of small-ring strain (Scheme 2.9). In a similar manner the conversion of camphene hydrochloride (2.10) into isobornyl chloride (2.11) involves rearrangement known as the Wagner-Meerwein rearrangement (Scheme 2.10). 

The same products, but in different ratios, occur in the gas phase eliminations of HCl from bornyl and isobornyl chlorides and from the pyrolysis reaction of bornyl and isobornyl benzoates and methyl xanthates . The isobornyl reaction is appreciably faster than that of the bornyl ester ((iso-B/B) = 6.8 at 345 °C), but proceeds at a slightly slower rate than that of cyclohexyl acetate, (CH/iso-B) = 2.1 at 600 °K. By analogy with the nonclassical carbonium ion interpretation of the solvolysis rates of bornyl and isobornyl chlorides, the participation of nonclassical carbonium ion intimate ion-pairs, e.g. 

The corner-protonated cyclopropane configuration 45 is analogous to the non-classical norbornyl ion at the center of controversy for the past three decades. The non-classical cation concept had its origin with Wilson and coworkers in 1939 who depicted structure 58 as a possible intermediate in the camphene hydrochloride-isobornyl chloride... 

The rearrangement of carbonium ions was first postulated, by Meerwein (p. 160) in 1922, to account for the conversion of camphene hydrochloride into isobornyl chloride. Oddly enough, this chemical landmark is the most poorly... 

We have accounted for the observed change in carbon skeleton, but we have not answered two questions that have plagued the organic chemist for a generation. Why is only the exo chloride, isobornyl chloride, obtained, and none of its endo isomer, bornyl chloride Why does camphene hydrochloride undergo solvolysis thousands of times as fast as, say, fert-butyl chloride To sec the kind of answers that have been given, let us turn to a simpler but basically similar system. 

Meerwein, H., Van Emster, K., Joussen, J. The equilibrium isomerism between bomyl chloride, isobornyl chloride and camphene hydrochloride. Ber. 1922, 55B, 2500-2528. 

Probably the best-known examples of Wagner-Meerwein rearrangements are the conversions of camphene to isobornyl chloride and pinene to bomyl chloride by hydrochloric acid. In terms of carbonium ion theory these reactions may be interpreted as shown on pages 57 and 58. 

In 1939, in the course of a discussion of the camphene hydrochloride-isobornyl chloride rearrangement, it was suggested by C. L. Wilson that such a rapidly equilibrating pair of cations (3 4) might exist instead as the mesomeric species 52. Previously 5 would have been considered to be the transition state separating 3 and 4. In effect, the proposal was that this transition state might be sufficiently stable so... 

It was Meerwein and van Emster who, in 1922, while studying the kinetics of the rearrangement of camphene hydrochloride (1) to isobomyl chloride (2) [Eq. (5.1)], noticed that the reaction rate increased in a general way with the dielectric constant of the solvent. Further, they found that metalhc chlorides such as SbCh, SnCb, FeCb, AlCb, and SbCf (but not BCh or SiCh), as well as dry HCl (all of which promote ionization of triphenylmethyl chloride by the formation of ionized complexes), also considerably accelerate the rearrangement of camphene hydrochloride. Meerwein concluded that the conversion of camphene hydrochloride to isobornyl chloride actually does not proceed by way of migration of the chlorine atom, but by a rearrangement of a cationic intermediate. Thus, the modern concept of carbocation intermediates was born. 

Formation of isobornyl chloride from camphene hydrochloride corresponds to that of pinacolin from pinacone. 

Wagner-Meerwein rearrangements occur extremely frequently among branched-chain aliphatic and alkylaryl compounds, and are particularly important in the terpene and camphor series. An example of rearrangement of type a) is that of camphene hydrochloride (1) into isobornyl chloride (2), for which Meerwein and van Emster141 give the following directions ... 

A solution of camphene (300 g) in ethyl bromide (150 g) is saturated with hydrogen chloride at 10-20° and heated under reflux for 6 days at 55°. Most of the ethyl bromide is then removed in a current of dry air, and the isobornyl chloride is filtered off and dried in a vacuum over potassium hydroxide (yield 271 g m.p. 145°). Recrystallization from pentyl alcohol gives material of m.p. 161.5°. 

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