Why glucose has cyclic structure?

Glucose has cyclic structure to make it more stable.Glucose molecules form rings. The first carbon atom (C₁), which is an aldehyde group (-CHO), creates a hemiacetal with the fifth carbon atom (C₅) to make a 6-membered-ring (termed a pyranose). The atoms in this cyclic molecule then arrange themselves in space to minimize the amount of strain on each of the covalent bonds.

The carbon atoms in the glucose ring each have four covalent bonds. The best, or optimum angle, between all these bonds is 109.5 degrees, which results in a perfect tetrahedron. If, for any reason, these bonds are forced into greater, or smaller angles then the molecule will be strained or stressed, and be much less stable. It follows, therefore, that the glucose molecule will be at its most stable when all the carbon atoms can arrange themselves so that their bond angles are all close to 109.5 degrees.