50 SECTIONAL ADDRESSES. 



thereby that we shall gain the most information regarding the chemistry of 

 carbon structures in the near future. The course of a reaction in 

 organic chemistry which involves an equation such as the following 



^,^^000 Et. 

 CHa.CHa.CHjBr + CHNa 



-^ COO Et. 



->- NaBr + etc. 



is determined by the tendency to form NaBr — the organic residues have 

 to make the best they can of the situation, and the manner in which they 

 will combine with themselves or react each with the solvent is dependent 

 on the influence of many factors. Undoubtedly there will be a tendency 

 to produce the most stable system and the one whose formation involves 

 the greatest loss of free energy, but there must be a possible mechanism, 

 and this involves the polar factors. Even these cannot force a group 

 into a position in which there is no room for it, and therefore the effect of 

 polarity must always be dependent on steric conditions. No doubt polar 

 conditions determine the order of priority of a number of possible arrange- 

 ments, but it is the steric condition that determines which of these arrange- 

 ments shall be followed. 



Strainless Systems. 



It is reasonable to assume that the organic substances that occur in 

 nature as such are produced by means which involve the least expenditure 

 of energy, and that they are, therefore, strainless. Among such natural 

 products there are many containing carbon rings belonging to ring systems 

 which cannot normally be produced without distorting the carbon tetra- 

 hedral angles of the component carbon atoms, and thus imparting intra- 

 molecular strain to the compounds formed. Nevertheless it is inter- 

 esting to note the means adopted by nature to relieve this strain and thus 

 to confer equilibrium and stability on quite unlikely ring systems. Ring 

 systems stabilised in this way are found frequently among terpenes ; 

 two, namely camphor and pinene, need only be mentioned to illustrate 

 the general method. In camphor the bridged ring is stabilised by the 

 presence of two dimethyl groups, and in pinene, where the junction of 

 the inner ring has to take effect in the position 3, the presence of a double 



C . CHs C . CH, 



/ \ //\ 



CHs CO CH CH 



QCHs)., 



CHs 

 \ 



CH, CHa 



/ \ 



C(CH3) 



! 

 CH., 



/ 



CH CH 



(Camphor.) (Pmene.) 



bond on the shoulder of the external ring is necessary. Still more remark- 

 able examples are afforded by more complex natural ring systems. For 

 instance, there is a substance named civetone, which is extracted from 



