Effect of Chain - Length on the acid strength in the p-n-Alkylbenzoic Acid Series

Abstract

p-n-i.llkyl'benzoic acids were prepared so that tilOY could be used in the study of the effects of oha.in=Leng-th on acid strength. The acids which were used in Ghjf~ investigation w-ere the p-n-alkyl benzoic acids f'r-om p­ toluic to p-n-decylbenzoic acid. The aoids p-ethylbenzoic to p-n-hexyl benzoic ~ p-n-pentyl benzoic excepbed , 1'[e:r:G obtained by car-bonat i cn of the appropriate Grigmu'd reagent wi th finely p owd.re-ed solid carbon dioxide. The acids above p-n-hexyl benzoic together 'Hi th p-n-pentyl benzoic HEl:L'e obtained by the oxidation of the appropriate :p-n-' alkylacetopheno118 under suitable conditions. At the time of carrying out the investigation four methods were ava iLab Le for the determination of the dissociation constants and these methods were the conductometric) pot~ntiometric~ electromotive force determination ancl the spectrophotometric. However~ in recent yeal',,)~ aeve ra.I novel methods have been cleveloped for the determination of rate constants. These methods are lumped togethel' under the term _I disturbance of equilibrium I. In these mabhods , the starting point is the new a+t o.i ned equili br Lum, o.,g. for acid d i aaoc i.a t i.on s

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By sudden change) either once or periodically, in one of the parameters controlling eq_uilibrium, such as pr-e ssur-e, temperature, concentration or the energy of state of the acid molecule the initial equilibrium is momentarily disturbed. The relaxation time for the establishment of a new state of equilibrium can then be measured or the rate of reaction necessary for its maintenance. One method of determination based on this

disturbance of equilibrium is high level faradic rectification that was recently developed at Harwell. (10,105).

Owing to the sparing solubility of the acids in water the method of choice was the spectrophQtometric. This method is sensitive to concentrations of as lO•H as 10-5 molar and i;3applicable to compounds that absorb

light. In spite of the high senaitivity of the

apect r-opho'otmeur-Lc met hod , the acids from p-n-hexylbenzoic acid upwaro,s were not sufficiently soluble in water to be investigate4. In order to make an investigation of the whole seri~s, from p-toluic to p':"n-decylbenzoicacid? use was made of- 50;;-aqu8ou8 et nano l and subsequently

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50~~aqueous methanol.

All doterminations were carried out at a constant

o t emperrrt uz-e of 25. For the ac i.ds soluble enough in

water two otiler sets of determination were carried out for etiCh acid at two other constant temperatures namely 5° ffild 45°, and the two sets of data used for the

de t c'rrni.n.rt Lon of activation enorgy 9 entropy and the free energy of ionisation. Dissociation constants and absorption data in various buffer solutions ranging from 0 % to 100 )b dissoci~tion of the acids were recorded. The experiment 801 data obtained in this work were used in interpreting tho effed of chain-length on acid strength.