Bicyclic and Polycyclic Alkanes

Many of the molecules that we encounter in our study of organic chemistry contain more than one ring (Section 4.4B). One of the most important bicyclic systems is bicyclo [4.4.0]decane, a compound that is usually called by its common name, decalin  

Chemical Abstracts Service (CAS) determines the number of rings by the formula S - A + 1 = N, where S is the number of single bonds in the ring system, A is the number of atoms in the ring system, and N is the calculated number of rings (see Problem 4.30). 

In cis-dtcdXm the two hydrogen atoms attached to the bridgehead atoms lie on the same side of the ring in trans-dtcdXin they are on opposite sides. We often indicate this by writing their structures in the following way  

Simple rotations of groups about carbon-carbon bonds do not interconvert cis- and trans-decalins. They are stereoisomers and they have different physical properties. 

Adamantane is a tricyclic system that contains a three-dimensional array of cyclohexane rings, all of which are in the chair form. 

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The CNMR data for selected substituted bicyclic and polycyclic alkanes are listed in Table 9. Noteworthy is the upheld shift for the quaternary carbon atom of the tert-buiy group in 24, which can be attributed to the special bonding character in the tetrahedrane... 

Bicyclic and higher polycyclic alkanes exhibit much more complex MS fragmentation. Budzikiewic and coworkers caution that great prudence, even in the use of published spectra, must be exercised for comparison and identification purposes of polycyclic alkanes. As examples they show the mass spectrum of cis-hydrindane, in which m/e 95,96,81 are major fragments which can be assigned only in retrospect to the structure once known. However, given only the MS it will be impossible to reconstruct the structure of the molecule. 

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