Condensation reactions in polysaccharide formation primarily involve the hydroxyl (OH) group at carbon positions 1, 4, and 6 of glucose, while reactions at carbon positions 2 and 3 are less common. This selectivity is due to the structural characteristics of glucose and the stability of the resulting glycosidic bonds. The end OH and H groups are more reactive, facilitating the formation of these bonds during enzymatic processes, such as those catalyzed by amylase in starch and in the branching structure of glycogen. The discussion references resources that provide further insights into glycosidic bonds and carbohydrate chemistry, which can enhance understanding of these reactions.