Carboxylic acids ionisation

Carboxylic acids ionise when in water, some more easily than others, but all of those discussed above are weak acids. The equilibrium in the... 

Weakly acidic cation exchangers (e.g. polymethylacrylic acid resins). These resins (Zerolit 226, Amberlite 50, etc.) are usually supplied in the hydrogen form. They are readily changed into the sodium form by treatment with 1M sodium hydroxide an increase in volume of 80-100 per cent may be expected. The swelling is reversible and does not appear to cause any damage to the bead structure. Below a pH of about 3.5, the hydrogen form exists almost entirely in the little ionised carboxylic acid form. Exchange with metal ions will occur in solution only when these are associated in solution with anions of weak acids, i.e. pH values above about 4. 

The reaction is less selective than the related benzoylation reaction (/pMe = 30.2, cf. 626), thereby indicating a greater charge on the electrophile this is in complete agreement with the greater ease of nuclophilic substitution of sulphonic acids and derivatives compared to carboxylic acids and derivatives and may be rationalized from a consideration of resonance structures. The effect of substituents on the reactivity of the sulphonyl chloride follows from the effect of stabilizing the aryl-sulphonium ion formed in the ionisation step (81) or from the effect on the preequilibrium step (79). 

The heats of ionisation and neutralisation of amino and hydroxylic bis and tris phosphonic acids have been investigated.253 Calorimetry in combination with u.v. and n.m.r. spectroscopy uas used to study the adducts of fluoroalkyl carboxylic acids with diethyl phosphonate.254 The heats of formation of the t-butoxytriphenylphosphoranyl radical uas consistent with the phosphonium structure (92).255 There has been a thermal analysis of the adducts of phosphonic and phosphoric acids with... 

Another early example of Hammett s is shown in Fig. 13.2, which represents a plot of log k for base-catalysed hydrolysis of a group of ethyl esters (3) against log K for ionisation in water of the corresponding carboxylic acids (2). Judged... 

Shi et al.71 have assigned the backbone and side-chain chemical shifts for 103 of 238 residues of proteorhodopsin using solid state NMR spectroscopy. Analysis of the chemical shifts has allowed determination of protonation states of several carboxylic acids as well as boundaries and distortions of trans-membrane a-helices and secondary structure elements in the loops. It has been shown that internal Asp227, making a part of the counterion, is ionised, while Glul42 located close to the extracellular surface is neutral. 

The main fluorescent pH indicator probes are based on fluorescein and therefore it is important to understand the pH-dependent ionic equilibria of it and its derivatives, hi aqueous solutions above pH 9 the phenolic and carboxylic acid functional groups in the molecule are almost totally ionised (Figure 3.14). Upon acidification of the dianion, firstly, protonation of the phenolic group occurs (pK 6.4) to yield the monoanion followed by the carboxylic acid (pA < 5), giving the neutral species of fluorescein. On further acidification the fluorescein cation pK 2.1) is generated. In strongly acidic environments fluorescein is non-fluorescent, only the mono-anion and di-anions are fluorescent, with quantum yields of 0.37 and 0.93, respectively. The pH-dependent absorption spectrum of fluorescein exhibits a blue-shift and... 

Structures III and IV assist ionisation of the C-X bond, whereas structure II facilitates nucleophilic addition and consequently a bimolecular displacement of X. The various derivatives of carboxylic acids form a series with varying degrees of resonance stabilisation decreasing in the following order ... 

Ion suppression is a technique used to suppress the ionisation of compounds (such as carboxylic acids) so they will be retained exclusively by the reversed phase retention mechanism and chromatographed as the neutral species. Column packings with an extended pH range are needed for this application as strong acids or alkalis are used to suppress ionisation. In addition to carboxylic acids, the ionisation of amines can be suppressed by the addition of a base to the mobile phase, thus allowing chromatography of the neutral amine. 

Higher carboxylic acids Tertbutyl methyl ether — Flame ionisation Carboxylic acids derivitivised with diazomethane 5 fig L-1 [628]... 

Higher carboxylic acids Capillary column Electron capture flame ionisation Carboxylic acids derivitivised to p-bromophenacyl esters [629]... 

In both cases, the alcohol must first act as a nucleophile with values of most phenols is in the order of 11, compared to 18 for alcohols and 4.74 for acetic acid. This means the phenols can be ionised with weaker bases than those needed to ionise alcohols, but need stronger bases than those needed to ionise carboxylic acids. For example, phenols are ionised by sodium hydroxide but not by the weaker base sodium hydrogen carbonate. 

Alcohols beings less acidic are not ionised by either base but carboxylic acids are ionised by both sodium hydroxide and sodium hydrogen carbonate solutions. 

Ionic bonding occurs between molecules which have opposite charges and it involves an electrostatic interaction between the two opposite charges, the functional groups that most easily ionise are amines and carboxylic acids ... 

Ethanoic acid (acetic acid) partially dissociates in water and an equilibrium is set up between the carboxylic acid (called the free acid) and the carboxylate ion (Following fig.). An acid that only partially ionises in this way is called a weak add. 

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