3756
VOl. 77
DARLH. MCDANIEL AND HERBERT C. BROWN [CONTRIBUTIOh' FROM THE
DEPARTMEXT OF
CHEMISTRY O F P U R D U E UNIVERSITY]
A Quantitative Approach to the Ortho Effects of Halogen Substituents in Aromatic S ysterns',' B Y DARLH. ~ I C D A N I E ALN"D. ~HERBERT C. BROWN RECEIVED SEPTEMBER 27, 1954 The ortho effects of alkyl groups are n o ~ vfairly \vel1 uiiderstood and they may he interpreted largely in teriiis of I'-straiti interactions and steric inhibition of resoriance. The Iiighly polar nature of the halogen substituents has macle the attairim n t of a similar understanding of the orthu effects of these substituents more difficult. I t is suggested that in thcir r tion with a proton the substituted pyridine bases pro\-ide a system which is largely free of such ortho effects as F-strain, st inhibition of resonance and h~-drogenbonding. Consequently, the substituted pyridine bnses provide a nearly ideal reference system for the estimation of the purelJ- polar contribution of orfizo substituents, In this w a y it has beeii possible to arrive a t a quantitative estimate of the direction and magnitude of the ortho effects of halogen substituents, as well as other subsituents, in a number of aromatic systems. This quantitative estimate of the ortho effects permits an assessment of the relative contributions of such phenomena as F-strain, steric inhibition of resonance, and hydrogen bonding to the total ortho effect of substituents in a number of aromatic sJ-stems.
-1lmost as soon as quantitative data on the dissociation of aromatic acids and bases were available it was recognized that the effect of ortho substituents on the acid strength often differed greatly from the expected behavior. I t has been common to attribute any peculiar effect of a substituent in the ortho position to an ortho or proximity effect,j these terms remaining free of any physical implications as to the precise nature of the interaction. That the ortho effect is a combination of many different types of interaction is now well recognized.6 The ortho effects of alkyl groups provide a relatively simple area for study. The absence of important resonance interactions and relatively small polar contributions of alkyl groups makes it possible to estimate the electrical contribution of such groups with considerable precision. Deviations from the predicted behavior can then be interpreted in terms of such concepts as steric inhibition of resonance, F-strain and, possibly, steric hindrance to solvation. However, it has not been possible previously to estimate accurately the relative importance of electrical effects a ~ i dspecific ortho effects for strongly polar substituents. Consequently, it is usually not possible to state from the experimental data whether specific ortho effects are present and are playing any significant role in determining the behavior of the substituted compounds. Branch and Calvin7 have pointed out that the existence of ortho effects may be observed by the deviation from linearity of a plot of the logarithm of the dissociation constants of a substituted reference acid (such as the phenylboric acids) against other 1 Rased illion
a thesi-. \ii!irnittrd h y Darl H. h i c D a n ~ r l111 .111gii,t
correspondingly substituted aromatic acids ( s ~ c l;is l phenols, benzoic acids, etc.). They point out, h o w ever, that this method shows only that an or!ho e [ fect exists, but not in which acid it exists nor whether it is acid strengthening or acid weakening. Only in the event that a reference system could be found free of ortho effects would it be possible to utilize such plots for the estimation of the magnitude ant1 direction of or,ilzo interactions. I n the pyridine system ortho substitution should be relatively free of the type of specific ortho interactioris under discussion, Steric inhibition of resonaric-e is impossible. The formation of ;L hydrogen bond is highly improbable with 311 hut ;i few substituents. F-strain should be negligibk with all but the most bulky substituents.' Filially, there is evidence that solvation effects and steric hindrance to solvation are relatively unimportant.','' I t follows that the pyridine bases should providi. : I satisfactory referrtice system. Deviations froiii lilie,trity betwaeri the p K , values of substitutr~tl i)yritiine bases and those of the corresponding substituted aromatic acids and bases should provide iiieasure oi the ordho effect of the substituent in thcs particular organic acid or base under considera tiori."' With a quantitative estimate of the effect available, it should be possible to make reasonable interpretations regarding the particular type of i t i teraction involved. This approach will be applied ( 8 ) FI. ($11
C .Brown a n d
11. P. Craig, J
1'.
('lie>ii
a n d H K a n n e r , .THIS JOT.R
(111. I t has been Iwiiited o u t b y one of t h e Referees t h a t t h e takinx f r o m t h e linear lree energy relationships as q u a n t i t a t i v e me.iaurc5 of a t r r i c e f f e c t s assumes t h a t t h e ratio of t h e inductivc ztiil r?- : i t i u i i , i t u t h e t o t a l polar effect of t h e oi.lli~, ~ i i l , ~ L i t a i r n l I I tiir i.iine 111 Iioth t h e pyridine ~ r i c lt h e uther .Iruniatic
o r ileviatiuni
nt-. I t has I,cen ioiinrl
puration F e l l o w a t Pnrdur llniversity, lLI53-1954. 4) D e p a r t m e n t u i C h e m i t t r y . Univeriity oi Pittsiiuryh. Pit1.lhiirgh, Pa G ) I. P. H a r n m e t t , "Phyiical Organic Chemistrl-," \Ic(:rnn 11111 Book C o , I n c , S e w York. 1..\- , l U 4 0 , pp. 204--20i. id thrur\ i r r I 1 C I
