Ionisation ascorbic acid

Ascorbic acid. Dissolve 20 g of the solid in 100 mL of de-ionised water. This reagent must be freshly prepared. 

In order to concentrate the lead extract, remove the lead from the organic solvent by shaking this with three successive 10 mL portions of the dilute hydrochloric acid solution, collecting the aqueous extracts in a 250 mL beaker. To the combined extracts add 5 mL of 20 per cent ascorbic acid solution and adjust to pH 4 by the addition of concentrated ammonia solution. Place the beaker in a fume cupboard, add 3 mL of the 50 per cent potassium cyanide solution and immediately adjust the pH to 9-10 with concentrated ammonia solution. Transfer the solution to a 250 mL separatory funnel with the aid of a little de-ionised water, add 5 mL of the 2 per cent NaDDC reagent, allow to stand for one minute and then add 10 mL of methyl iso butyl ketone. Shake for one minute and then separate and collect the organic phase, filtering it through a fluted filter paper. This solution now contains the lead and is ready for the absorption measurement. 

It is important to avoid confusion between the processes of ionisation and oxidation. When ascorbic acid ionises, the hydrogen leaves as an ion (H+) and its electron remains on the molecule to form an anion, whereas when the molecule is oxidised the hydrogen leaves along with its electron to give a diketone. These two processes are quite different and ascorbic acid is a good molecule to demonstrate the difference. 

Ascorbic acid was, therefore, shown to be a y-lactone, conveniently represented as in Figure 3.5 although other tautomeric forms may exist in small quantities. The configuration at C-5 is l- (or S-, using the Gahn-Ingold-Prelog system). The acidic nature in aqueous solution derives from the ionisation of the enolic OH on C-3 (pA a 4.25), the... 

Salts of L-ascorbic acid contain the resonance-stabilised ascorbate anion formed on ionisation of the C-3-OH proton. A-Ray data reveal the anticipated lengthening and shortening of the bonds along the conjugated 0-C-3 = C-2-C-l = 0 system of the anion as compared with the neutral molecule (Table 3.2). 

Figure 3.11 shows that the concentration of [Fe(CN)g] decreases more rapidly for solutions containing a higher concentration of NaNOj, that is, the velocity of the reaction increases with the ionic strength. This is what is expected for reactions between two ions of the same charge. One mechanism proposed for the reaction involves initial ionisation of the ascorbic acid (AHj), which rapidly and reversibly forms an ascorbate anion intermediate (AH"), which then subsequently transfers an electron to the hexacyanoferrate(III) in the slowest step in the mechanism ... 

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