Isobomyl chloride

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 ... 

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... 

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. 

In 1939 Hevell, Salas and Wilson assumed an intermediate, bridge ion 2 to be formed when camphene hydrochloride 1 is rearranged into isobomyl chloride 3. This happened 17 years after Meerwein first postulated the intermediate formation of carbonium ions in chemical reactions. 

Carbon-to-carbon migration of alkyl, aryl or hydride ions. The original example is the acid-catalyzed rearrangement of camphene hydrochloride to isobomyl chloride ... 

What followed were decades of investigation to determine the mechanism through which such a rearrangement would occur. Notably, the work of Meerwein and van Emster demonstrated the presence of a cationic intermediate that preceded the 1,2-alkyl migration. Through his studies of equilibrium isomerism of bomyl chloride, camphene hydrochloride (3), and isobomyl chloride (6), Meerwein established that the rearrangement mechanism relied on ionization, and exhibited a heavily solvent-dependant kinetic profile. These studies, which laid the foundation for modem... 

Scheme 7.46. The formation of camphene hydrochloride (ex< -2-chloro-233-trimethylbicyclo[2.2.1]heptane and its rearrangement to isobomyl chloride (ex<7-2-chloro-l,7,7-trimethylbicyclo[2.2.1]heptane).The addition of a proton to the carbon-carbon double bond of camphene 3,3-dimethyl-2-methyienebicyclo[2.2.1]heptane is shown as accompanied by o-bond migration to produce a singie ion with partial bonding to two sites (called, variously, nonclassical or bridged ) or a pair of rapidly equilibrating ions. The classical ions are shown, leading to the observed products. Debate raged over a period of years about the nature of the ion or ions lying between the starting materials and products. Additional discussion is provided in Chapter 8.

Isobomeol to Camphor. Nitric add has been widely used in the production of synthetic camphor from turpentine. The commonly accepted general practice for this manufacture (the one adapted by Gubehnann fw use in this country) involves the following steps (1) distillation of turpentine to obtain pinene, (2) saturation with HCl gas to obtain bomyl chloride, (3) hydrolyzing this to obtain camphene, (4) esterifying camphene to isobomyl acetate, (5) saponification to isobomeol, and (6) oxidation to camphor. 

Kok, M., Demirelli, K., and Aydogdu, Y. (2008) Thermophysical properties of blend of poly(vinyl chloride) with poly(isobomyl acrylate). Int. J. Sci. Technol., 3, 37-42. 

---