Iodine tends to react with starches an not with sugars because starches are generally linear molecules that are typically made up of hundreds of glucose molecules. These molecules are covalently bonded with one another. The iodine reacts well and bonds easily with the strings of starch and thus creates a chemical reaction. Sugars on the other hand, are typically found in the form of glucose, fructose, or two sugared molecules that are not bonded in the same way that starch is. Thus, iodine does not react an bond well with the molecule.