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1.2.3 Chemistry of Lactic Acid
ОглавлениеThe lactic acid molecule has a hydroxyl and an acid functional group, which may result in intermolecular and intramolecular esterification reactions. The first step is the formation of a linear dimer (lactoyl lactic acid). This condensation reaction can proceed to higher oligomers and is promoted by removal of water. Also a cyclic dimer, lactide, is formed in small amounts. Lactide can be formed by intramolecular esterification of lactoyl lactic acid or by breakdown of higher oligomers. All reactions are equilibrium reactions (Figure 1.2).
Due to these reactions, a solution of lactic acid at equilibrium consists of monomeric lactic acid, dimeric lactic acid or lactoyl lactic acid, higher oligomers of lactic acid, and lactide. The ratios between all substances depend on the amount of water present; for example, a 90.1% lactic acid solution (total acidity) contains about 59.3% of monomeric lactic acid and 27.3% of lactoyl lactic acid and higher oligomers [14].
TABLE 1.1 Physical Properties of Lactic Acid [14]
| Property | Value | Reference |
|---|---|---|
| CAS number | General: 50‐21‐5 | — |
| (S)‐Lactic acid: 79‐33‐4 | — | |
| (R)‐Lactic acid: 10326‐41‐7 | — | |
| Molecular weight (g/mol) | 90.08 | — |
| Formula | C3H6O3 | — |
| Melting point (°C) | 18 (racemic) | [17] |
| 53 (chiral pure) | [18] | |
| Crystal structure | (S)‐Lactic acid: orthorhombic, space group P212121 | [19] |
| Solid density (g/mL) | 1.33 (solid, 20°C) | [20] |
| Solubility in water (wt%) | 86 (20°C, monomeric (S)‐lactic acid) | [20] |
| Heat of fusion (kJ/mol) | (S)‐Lactic acid: 16.8 | [21] |
| Boiling point (°C) | 122 (at 14 mmHg) | [22] |
| Liquid density (g/mL, 20°C) | 1.224 (100% undercooled liquid) | [22] |
| 1.186 (80.8% solution in water) | [23] | |
| Viscosity (mPa s) | 28.5 (85.3% solution in water, 25°C) | [23] |
| pK a | 3.86 | [24] |
| Specific heat (J/(g K) at 25°C) | Crystalline (S)‐lactic acid: 1.41 | [25] |
| Liquid lactic acid: 2.34 | [26] |
The condensation reactions are also the reason that it is quite difficult to obtain pure, solid, and enantiopure lactic acid. This can only be achieved by crystallization [27, 28]. The kinetics of the condensation reactions determine the stability of a solution of monomeric lactic acid and have a large influence on the stability of solid lactic acid.
