The melting point of glucose is 146C. This seems quite high compared to the 6.5C melting point of cyclohexane. What functional groups are responsible for the difference? What do these functional groups due to raise the melting point

The hydroxyl  group and aldehyde group

The form polar bonds

Explanation:

Glucose has a very high melting point compared to cyclohexane. What could the secret be?

We now turn our attention to the structure of the both molecules. While cyclohexane is just an alkane, glucose contains both the -OH and the CHO groups  which form polar bonds.

In cyclohexane, the only intermolecular force is the London dispersion forces while the intermolecular forces in glucose include the London dispersion forces as well as dipole interactions occasioned by the presence of polar -OH and -CHO groups in the molecule thereby increasing the melting point of glucose far above that of cyclohexane.