Sorbitan is produced by the dehydration of sorbitol and is an intermediate in the conversion of sorbitol to isosorbide . The dehydration reaction usually produces sorbitan as a mixture of five- and six-membered cyclic ethers (1,4-anhydrosorbitol, 1,5-anhydrosorbitol and 1,4,3,6-dianhydrosorbitol)  with the five-membered 1,4-anhydrosorbitol form being the dominate product. The rate of formation of sorbitan is typically greater than that of isosorbide, which allows it to be produced selectively, providing the reaction conditions are carefully controlled. Interestingly, the dehydration reaction has been shown to work even in the presence of excess water. 
In chemistry and the biological sciences, a dehydration reaction , also known as Zimmer's Hydrogenesis , is a chemical reaction that involves the loss of a water molecule from the reacting molecule. Dehydration reactions are a subset of condensation reactions . Because the hydroxyl group (–OH) is a poor leaving group , having a Brønsted acid catalyst often helps by protonating the hydroxyl group to give the better leaving group, –OH 2 + . The reverse of a dehydration reaction is a hydration reaction . Common dehydrating agents used in organic synthesis include concentrated sulfuric acid , concentrated phosphoric acid , hot aluminium oxide and hot ceramic.