Reaction of Glucose with Acids
Alcohols are very weak bases so their conjugate acids are strong.
Each of the OH groups on glucose could be reversibly protonated by strong acids just like the OH group in the methanol molecule above.
For the cyclic isomers of glucose, one hydroxy group is unique. This is the one derived from nucleophilic attack on the aldehyde functional group. The carbon bearing that OH group is also bonded to another oxygen atom.
When that OH group is protonated, the bond between that oxygen and the ring carbon can break to form water. The ring carbon then has only three bonds and a positive charge. VSEPR tells us that the carbon will have a trigonal planar geometry with an empty p orbital perpendicular to the plane. The adjacent ring oxygen atom has two non-bonding electron pairs. One of these is in an orbital that lines up with carbon's empty p orbital.
The filled ring oxygen orbital and the empty carbon p orbital overlap to form a partial pi bond. This stabilized the carbocation (positively charged carbon atom).
The positively charged carbon is a very electrophilic. Nucleophiles, such as water or an alcohol can add to carbon. When one of the oxygen atoms of another sugar molecule adds to the carbocation and a proton is lost, a disaccharide is formed.
Because the carbocation is planar, two structural isomers could be formed when a particular nucleophile adds. The nucleophile could add above the plane (as shown above) or below the plane.