102 MOLECULAR STRUCTURE 



The substituting group always exercises most influence 

 in the ortho-position (1.2); benzoic acid is strengthened 

 by chlorine and the nitro group to the greatest extent, 

 and the latter too brings out the acid character of phenol 

 and suppresses the basic character of aniline most in the 

 ortho-position. But comparison of the meta- and para- 

 derivatives leads to varying results, since chlorine exercises 

 the greatest influence in the meta-, the nitro group in the 

 para-position. The oxyderivatives appear to behave like 

 those of chlorine, except that here the acid character is 

 weakened in the para-position, while it is strengthened in 

 the ortho- and meta-. 



Velocity of reaction, which often in other ways goes with 

 this dissociation constant, but is not usually so easy to 

 measure, shows the same peculiarities, so the velocity 

 constant k for some reactions 1 may be given here (see 

 Part I, p. 191). 



Combination of Bromallyl with 

 Methylaniline (Toluidine) Chloraniline 



(1.2) .... fc= 54 9 



(1.3) .. . . fc = 4 45 23 



(1.4) . . . . k= 96 34 



( Aniline k = 68) 



Brommeihyl with 

 Methylaniline 



13 

 86 



52 

 (Aniline Jc = 24) 



The case is again simplest for the chlorderivatives : 

 the velocity, 68 for aniline, is reduced, the most by chlorine 

 in the ortho-position to 9, then in the meta- to 23, then in 

 the para- to 34. 



C. Stereochemistry. 



Since constitutional formulae, expressing only the mode 

 of combination of the atoms in the molecule, no longer 

 sufficed to explain the cases of isomerism observed, a new 

 attempt at explanation had to be made. Such a case arose 

 in the discovery by Pasteur in 1853 ^ laevo-tartaric acid, 



COOHCH(OH) CH(OH) COOH ; 

 but in the then state of the study of constitution was not 



1 Menschutkin, BerL Ber. 30. 2966, 31. 1423. 



