672 Mr. N. P. McCleland on the 



linkage is rendered more stable, i. e., the halogen is less 

 reactive when the carbon to which the halogen is attached is 

 less stable when an adjacent carbon is unsaturated, 



e.g., OH 2 = CHBr, 



CH— CH 2 Br, 



CH 3 = CH-CH 2 Br, 



are in ascending order o£ reactivity. 



Similarly the C — H linkage is in general rendered more 

 reactive, i. e. less stable, by the presence of unsaturation ; and 

 this effect is increased when it is between two unsaturated 

 groups. 



Thus the hydrogens marked with a star in the following are 

 abnormally reactive: — 



CH 3 — CO— CH 2— CO— CH 3 , acetylacetone, 



COOC 2 H 5 — CH = CH— CH % — COOC 2 H 5 ,glutaconic ester, 



C 6 H 5 — CH 2 — COOC 2 H 5 , phenylacetic ester, 



C 6 H 5 — CH 5 —ON, benzyl cyanide, 



/\. 



-OH 



CH 



\ / * 



CH2 



indene, 



HC— CH 



II II 

 HC CH cyclopentadiene. 



CH^ 



It is immaterial to the theory whether the hydrogen atoms 

 react in the positions they occupy in the formulae, or whether 

 an isomeric change first takes place. 



6. As to the stereochemical effects of the ethylene linkage, 

 the experimental results may be summed up in the two 

 " laws " that in a substance 



R2 R4 



in stable equilibrium : 



(1) All the R groups lie in a plane ; 



(2) There are two stable positions in the plane. 



