Explanation for Chemical Puzzle Number

In the case of hydrogen chloride addition to a-pinene (5.41), it is likely that the mechanism has a considerable degree of synchronous character for, if the free cation were to be formed, the molecule would rearrange as happens when pinene hydrochloride is allowed to warm up to room temperature. The reason for the instability of pinene hydrochloride 

1- bicycloheptane ring structure is much less strained than the starting 

1- bicycloheptane. The high selectivity for bornyl chloride (5.45) results from the fact that the chloride anion is already on that side of the molecule and adds in an SN2 like fashion relative to the breaking of carbon-carbon bond. As in previous figures, bornyl chloride is drawn in two ways. Structure (5.45) shows its relationship to carbocation (5.44) whereas (5.46) is the more usual representation. 

During the reaction, we saw the formation of a secondary carbocation from a tertiary one. This is, of course, all other things being equal, energetically unfavourable. However, in this case the gain in energy from relief of ring strain more than compensates and drives the reaction forward. 

For bornylene to be formed from bornyl chloride (5.46), it would require loss of one or other of the hydrogen atoms on the carbon adjacent to that carrying the chlorine. One of these hydrogens is eclipsed relative to the chlorine and the other is skew. Neither can adopt a TAP 

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