Solketal(Cas No.:100-79-8) is a protected form of glycerol with an isopropylidene acetal group joining two neighboring hydroxyl groups. Solketal contains a chiral center on the center carbon of the glycerol backbone, and so can be purchased as either the racemate or as one of the two enantiomers. Solketal has been used extensively in the synthesis of mono-, di- and triglycerides by ester bond formation. The free hydroxyl groups of solketal can be esterified with a carboxylic acid to form the protected monoglyceride, where the isopropylene group can then be removed using an acid catalyst in aqueous or alcoholic medium. The unprotected diol can then be esterified further to form either the di- or triglyceride.
Solketal (CAS 100–79–8) has low viscosity at room temperature (~11 cP) and freezing,boiling, and flash points of about −26 ◦C, 190 ◦C, and 80 ◦C, respectively. The density is 1.06 g/cm3 and the vapor pressure is 107.32 mmHg. It has low toxicity, slight odor, and it is non-irritant for humans; due to these characteristics and to the renewable origin, it is considered an environmentally friendly substance. It is completely miscible in water and in most organic compounds, which endows its solvent capacity.
Owing to the high compatibility and miscibility of solketal, the major application is as a solvent in resins, paints coatings, and cleaning agents, also because it can influence film-formation and drying time. Its low toxicity and solubility in body fluids enable it to be used also as a solvent in medicaments (i.e., driving agent), and as a precursor of other chemicals of medical interest.
Nevertheless, the most-studied solketal application is as fuel additive. Solketal has the positive effect of diminishing biodiesel viscosity and the negative effect of increasing fuel density, fundamental properties for the correct operation of the engine. Because of the amount of saturated fat in biodiesel, it presents poor cold properties, a problem proved to be aided by the addition of solketal in the fuel. Giraldo et al. also attested the positive effect of using this ketal on diminishing the cloud point (the temperature at which wax crystals begin to form) and pour point (the temperature at which the fuel no longer flows).
Another parameter analyzed was the flash point (“the lowest temperature at which a volatile substance evaporates to form an ignitable mixture with air in the presence of an igneous source,” according to Isac-Garcia et al.), in which the addition of solketal decreased the properties’ value. Conversely, Alptekin registered higher flash point of the biodiesel blend and higher brake specific fuel consumption (measure of the fuel efficiency—rate of fuel consumption divided by the power produced) due to the low energetic efficiency of solketal. Despite that, CO2, CO, and THC (total hydrocarbon) emissions were lower.
