Camphene hydrochloride

Camphene hydrochloride, Cj(,H gHCl, is prepared by passing dry hydrochloric acid into an alcoholic solution of camphene. When re-ctystallised from an alcoholic solution containing excess of hydrochloric acid, it melts at 155° (or possibly a few degrees lower).- Melting-points from 149° to 165° have been recorded for this compound, but the products examined were probably not in a state of purity. 

It is possible, however, that camphene hydrochloride is not a uniform body, but that some of the terpene suffers some rearrangement in the molecule by the action of hydrochloric acid, and that the hydrochloride consists of a mixture of a-camphene hydrochloride and /8-camphene hydrochloride there is, however, no evidence to suggest that camphene itwlf is a mixture of two terpenes, so that the two camphenes are not known to exist. Aschan obtained an alcohol, camphene hydrate, by acting on camphene hydrochloride with milk of lime, a reagent which does not produce molecular rearrangement in the terpene nucleus. 

Reactions. Part. I. The Mechanism of the Wagner-Meerwein Rearrangement. Exchange of Radioactive Chlorine and of Deuterium between Camphene Hydrochloride and Hydrogen Chloride. J. chem. Soc. [London] 1939, 1188. 

The relative rates of solvolysis of camphene hydrochloride and t-butyl chloride indicate that the free energy of activation is 5 kcal mole higher with the latter compound. This might be attributed either to non-classical stabilization of the camphenyl transition state... 

Various unsubstituted solid alkenes are able to quantitatively add gaseous halogenohydrides. Prominent examples are the cholesterol esters 128 that give stereospecifically the bromides 129 at -30 °C [75,75al and camphene (130) that gives stereospecifically the rearranged bromide 131 or the elusive camphene hydrochloride (132) with 100% yield [11] (Scheme 15). The quantitative solid-... 

The rearrangement of camphene hydrochloride is called the Nametkin Rearrangement... 

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 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 can see, in a general way, how this particular rearrangement could take place. Camphene hydrochloride loses chloride ion to form cation I, which rearranges by a 1,2-alkyl shift to form cation II. Using models, and keeping careful... 

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. 

To test these alternative hypotheses, a tremendous amount of work has been done, by Brown and by others. For example, camphene hydrochloride is known to undergo ethanolysis 6000 times as fast as rer/-butyl chloride, and this had been attributed to anchimeric assistance with formation of a bridged ion. Brown pointed out that the wrong standard for comparison had been chosen. He showed that a number of substituted (3°) cyclopentyl chlorides (examine the structure of camphene hydrochloride closely) also react much faster than rerr-butyl chloride. He attributed these fast reactions—including that of camphene hydrochloride—to relief of steric strain. On ionization, chloride ion is lost and the methyl group on the ap hybridized carbon moves into the plane of the ring four non-bonded interactions thus disappear, two for chlorine and two for methyl. For certain systems at least, it became clear that one need not invoke a nonclassical ion to account for the facts. 

ProUem 28.19 (a) Show how a nonclassical ion intermediate could account for both the stereospecificity and the unusually fast rate (if it is unusually fast) of rearrangement of camphene hydrochloride into isobomyl chloride, (b) How do you account for the fact that optically active product is formed here, in contrast to what is obtained from solvolysis of norbomyl compounds ... 

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

Since the initial studies by Wagner at the turn of the century, the Wagner-Meerwein rearrangement has been the subject of almost continuous investigation. The reaction was initially examined in terms of the rearrangement of a-pinene (1) to bomyl chloride (2 Scheme 1), and of camphene (3) via camphene hydrochloride (4) to isobomyl chloride (5). 

It was Meerwein, who, in 1922, first recognized that the facile rearrangement of camphene hydrochloride (/) into isobomyl chloride (2) (1) involves a prior conversion into the corresponding ions or ion-pairs1 (2). This appears to be the first... 

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... 

In such comparisons, it is important to use suitable models. Possibly ten-butyl chloride is not an appropriate model for camphene hydrochloride. A more suitable model would doubtless be the pentamethylcyclopentyl chloride 6 realized by opening the 5,6-ethano bridge of I. 

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