538. Dissociation constants of some methylnaphthoic and acenaphthoic acids

Abstract

pK values have been determined, in 20% (by weight) dioxan, for aromatic monocarboxylic acids derived from naphthalene, l-methylnaphthalene, 1,s-dimethylnaphthalene, and acenaphthene. Both the pK values and the ultraviolet spectral data are in accord with the order of increasing steric effect: ethylene bridge < 1-methyl < 1,8-dimethyl.ACENAPHTHENE, l-methylnaphthalene, and 1,8-dimethylnaphthalene form a series in which the steric and electronic effects of the alkyl substituents are expected to fall into different sequences. Information on these substituent effects should be given by the dissociation constants of the derived carboxylic acids and we report thermodynamic pK values for the three acenaphthoic acids, the three 1,8-dirnethylnaphthoic acids, and l-methyl-2-, 4-methyl-2-, 4-methyl-l-, and 5-methyl-l-naphthoic acid. Relevant ultraviolet spectral data are also reported. EXPERIMENTALMicroanalyses were carried out by the Microanalytical Laboratory (Dr. A. D. Campbell) of the University of Otago.Methylnaphthoic Acids.-(a) 1-Methyl-2-naphthoic acid. To a stirred suspension of 2-dimethylaminomethyl-l-methylnaphthalene (5.0 g. ; b. p. 118-122"/1 mm.) in water (150 ml.) was added (3 hr.) finely powdered potassium permanganate (10.0 g.). The mixture was stirred for a further 2 hr. and the manganese dioxide formed was filtered off. Acidification of the filtrate afforded the required acid (2-1 g., 45%), m. p. 179-5-180".was used for the oxidation, lower yields of the acid were

peri Interaction in Naphthalene Derivatives

“…The higher acid strength of 1-naphthoic acid is attributed to the steric effect of peri-hydrogen which presumably twists the 1-carboxyl group out of the ring plane, thereby inhibiting the resonance stabilization of the free acid and thus making it less stable vis-á-vis the anion ("steric inhibition of resonance"). This has been pointed out by other workers also (82,213,279). 4 As judged from the figures of Table XIX, the steric effect of peri-hydrogen appears to be intermediate between the effects for o-ethyl and o-isopropyl groups.…”

“…They are in general 1:1 complexes. Considering the suggested nonplanarity of pen-dimethylnaphthalene system (170,213,300), the apparent stability of these complexes is remarkable. In the extreme case of octamethylnaphthalene, a severe deviation of the molecule from planarity has been clearly established from crystal structure studies (170).…”

“…The p.K» values of a number of alkylnaphthoic and acenaphthoic acids (Table XXI) have been measured in 70% dioxane (213). The introduction of an o- methyl group in position 1 lowers the p of 2-naphthoic acid by 0.47 unit as compared to 0.30 unit in o-toluic acid.…”

Relative reactivities of polynuclear aromatic systems. V. Kinetics, isomer distribution, and additivity in the bromination of the dimethylnaphthalenes

“…Strain in this molecule has been discussed in connection with the pAl's of various related acids, chemical shifts, and the X-ray crystallographic study of its 3-bromo derivative. 34 1,5-dimethylnaphthalene,16 the interaction factors derived from its reaction were also discarded.…”

Acid-Base Properties of Substituted Naphthoic Acids in Nonaqueous Media

“…The reports on the acid-base properties of major sets of substituted naphthoic acids involve the potentiometric determination of dissociation constants of five substituted 1-naphthoic acids in 78% aqueous ethanol 2 , thirteen nitronaphthoic acids in 50% aqueous 2-butoxyethanol 3 , and the dissociation constants of 1-and 3-halo-2-naphthoic acids in 71% aqueous ethanol determined by conductometry 4 . Spectrophotometry was adopted to study the dissociation of a series of four methylnaphthoic acids in 20% aqueous dioxane 5 , conductometry was applied to a series of five methylnaphthoic acids in 50% aqueous ethanol 6 , and again spectrophotometry was used to study a series of 6-substituted-2-naphthoic acids in 50% aqueous ethanol 7 . (Methylsulfonyl)naphthoic acids were investigated potentiometrically in 50% aqueous ethanol 8 , 50% aqueous 2-butoxyethanol 9 , and 80% aqueous 2-methoxyethanol 9 .…”