Carboxylic adds ionisation

For analytes that are ionisable, the other factor to consider in solvent extraction is pH. Using a compound with a carboxylic add fimction (RCOOH) or a base (B) as an example, then the compoimd can exist in its ionised or its unionised form as shown below. 

For example, any compound containing the carboxyl functional group, -CXDOH, bdongs to the carboxylic acid homolo- us series and addic properties are always assodated with the carboxylic adds due to the presence of an ionisable hydrogen atom.. 

The determination of the tertiary structure of the hens egg white lysozyme, and the subsequent proposal that the side-chain carboxylic add group of Glu 35 act as a general acid, and the ionised side chain of Asp 52 acted to electrostatically stabilise an oxocarbonium ion intermediate, led to a search for a manifestation of these phenomena in model systems [6). In the scramble electrostatic stabiUsation was invoked where there was no compelling reason to distinguish it from ordinary nucleophilic participation. Thus, 3,5-dichlorophenolate departs from compound XXXII 100 times faster than from its p-isomer (94) a similar 22-fold acceleration of the ortho compared to the para isomer was observed with compound XXXIII. The rate acceleration in both cases was attributed to electrostatic stabiUsation, but since the 3-methoxyphthahde was in both cases the product, it is not clear why humdrum ndghbouring group participation was not invoked. 

However, the penidllins require a free carboxylic add moiety at C-3 and therefore ester groups used in suppressing ionisation must be readily degradable to fadlitate... 

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. 

Ethanoic acid on the left hand of the equation is called the free add, whereas the carboxylate ion formed on the right hand side is called its conjugate base. The extent of ionisation or dissociation is given by the equilibrium constant (K ) ... 

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